ISSN:0306-0012    

DOI:10.1039/CS97504FX001

出版商:RSC    

年代:1975

数据来源: RSC

摘要:

Chemical Society Reviews Vol 4 No 1 1975 Page PEDLER LECTURE Porphyrins and Related Ring Systems By A. W. Johnson 1 CENTENARYLECTURE Quadruple Bonds and other Mutiple Metal to Metal Bonds By F. A. Cotton 27 Kineticsof Reactions in Aqueous Mixtures By M. J. Blandamer and J. Burgess 55 HANDLING TOXIC CHEMICALS- ENVIRONMENTAL CONSIDERATIONS I Introducing a NewAgricultural Chemical By J. F. Newman 77 II Health Hazardsto Workers from Industrial Chemicals By A. Munn 82 IU Radioactive and Toxic Wastes :A Comparison of their Control and Disposal By A. W. Kenny 90 IV Environmental Protection in the Distribution of Hazardous Chemicals By A. E. Meadowcroft 99 Vibrational Infraredand Raman Spectroscopy in Inorganic Chemistry By I. R. Beattie 107 Olefin Metathesis and its Catalysis By R. J. Haines and G. J. Leigh 155 The Chemical Society London

ISSN:0306-0012    

DOI:10.1039/CS97504BX003

出版商:RSC    

年代:1975

数据来源: RSC

摘要:

CENTENARY LECTUW* QuadrupIe Bonds and other Multiple Metal to Metal Bonds By F. A. Cotton DEPARTMENT OF CHEMISTRY, TEXAS A & M UNIVERSITY, COLLEGE STATION, TEXAS 77843, U.S.A. 1 Introduction Almost exactly 120 years elapsed between the first discovery' (1844) of a com-pound which contains a quadruple bond, namely, Cr2(02CCH&(H20)~, and the recognition (1964) that quadruple bonds e~ist.~,~? However, following this improbably long hiatus, the field has enjoyed a decade of explosive efflorescence and now stands poised for further broad growth. The purposes of this review are to trace the developments over the past ten years, to summarize our present knowledge, and to point out the areas where further work is most needed and further progress is most likely.The planned and deliberate investigation of quadruple bonds, and other highly multiple bonds, between metal atoms began in the only way that it could -with the first conscious and explicit recognition of a multiple, specifically quadruple, bond between two transition-metal atoms. This occurred only in 1964 in my laboratory in a way that I shall now briefly describe. Our studies4-6 of rhenium(m) chloride and its derivatives, which showed that the characteristic structural unit in the chloride itself is an Re3 triangular cluster, and that this cluster persist in all complexes obtained under mild conditions, e.g. Re3C1123- and Re3Cl~(P&)3, had been essentially concluded. Dr. Neil Curtis was then a guest in the laboratory and I suggested that he examine the aqueous chemistry of rhenium with a view to finding ways of preparing ReIII compounds by reduction of Reo4- and also with the idea of preparing, if possible, some mixed clusters (an Re~OsC112~- cluster was a possibility we specifically discussed).After a short period of exploratory work, trying different reductants and various *First delivered at a meeting of The Chemical Society on 4th February 1974 at Bristol University.t1 am reminded in this connection of the similar history of transition-metal n-compIexes. The first one was discovered in 1829 (W. C. Zeise, Pogg. Ann., 1831, 21,497), but here also, almost exactly 120 years elapsed until the true nature of such complexes was recognized(M. J. S. Dewar, Bull.SOC.chim. France, 1951, C71; J. Chatt and L. A. Duncanson, J. Chem. Soc., 1953, 2979). E. Peligot, Compt. rend., 1844,19, 609. F. A. Cotton, et al., Science, 1964, 145, 1305. a F. A. Cotton, Inorg. Chem., 1965,4, 334. J. A. Bertrand, F. A. Cotton, and W. A. Dollase, J. Amer. Chem. SOC.,1963, 85, 1349; Inorg. Chem., 1963, 2, 1106. F. A. Cotton and J. T. Mague, Inorg. Chem., 1964,3, 1094. F. A. Cotton and J. T. Mague, Znorg. Chem., 1964,3, 1402. Quadruple Bonds and other Multble Metal to Metal Bonds reaction conditions, Curtis presented me one day with a sample of a royal blue compound which had an analytical composition of CsReCI4. Since the ‘CsReCld which had already been shown to be Cs~Re~Cl12was dark red, we were naturally very intrigued, and immediately began work to discover the true molecular formula and structure of this substance.Simultaneously, Dr. Brian Johnson, another guest in the laboratory, had been checking a report’ in the Russian literature in which it was claimed that ReCle3- species could be isolated by reduction of ReO4- in aqueous hydrochloric acid with hydrogen gas. These claims proved erroneous, all of the alleged M13ReC16 compounds being, in fact, MIzReCh compounds.* However, the Russian authors mentioned that a blue-green product, to which they assigned the formula KzReC14, was also obtained. We also observed this product. In view of its colour, we wondered if this too could have been incorrectly formulated; we wondered if it might be ‘KReC14’ and thus related to Curtis’ ‘CsReCld.It was soon shown that indeed the stoicheiometry had been reported incorrectly and that the substance isolated by hydrogen reduction of Reo4- is KReC14,HzO. Since the substance crystallized very nicely, an X-ray crystallographic study of it was undertaken by Mr. C. B. Harris, then beginning his doctoral research. A thorough check of the Russian literature, which was impeded by delays in getting papers translated, then revealed that several other blue or green com- pounds alleged to be rhenium(@ compounds had also been reported,g-ll among them (NH&ReC14, KHReC14, and (pyH)HReCle. With regard to the last two, the proposed presence of H appeared to lack any direct support, but was based, apparently, on the result of a determination of oxidation state by a method devised by the Noddacks in their pioneer studies of rhenium chemistry.12 According to Tronev and co-workers, the oxidation state was found to be +2.Presumably, then, in order to reconcile this result with the analytical data, it was considered necessary to introduce a hydrogen atom, as H+, into the formula. We repeated this work and found that, in our hands,13 the Noddacks’ procedure led to an oxidation number of 2.9 f0.2. While Harris was proceeding slowly with the crystal structure analysis on KReCl4,HzO, learning crystallography as he went, we received an issue of the Zhurnal strukturnoi Khimie containing an article14 on the structure of ‘(pyH)HReCl4’. S. J. Lippard, who had just completed a course in Russian, was pressed into service, and he speedily produced a translation which revealed that this compound had been found to have ‘the dimeric group [RezCl~]*-.Eight chlorine atoms constitute a square prism .. . the rhenium atom has for its V. G. Tronev and S. M. Bondin, Khim. Redkikh, Elementov, Akad. Nauk S.S.S.R., 1954,1, 40.* F. A. Cotton and B. F. G. Johnson, Inorg. Chem., 1964,3,780. V. G. Tronev and S. M. Bondin, Dokiady Akad. Nauk, S.S.S.R., 1952, 86, 87. lo V. G. Tronev and A. S. Kotel’nikova, Zhur. neorg. Khim., 1958,3, 1008. l1 G. K. Babeshkina and V. G. Tronev, Zhur. neorg. Khim., 1962,7,215. la I. Noddack and W. Noddack, Z. anorg. Chem., 1933,215, 182. 13 F. A. Cotton, N. F. Curtis, B. F. G. Johnson, and W.R. Robinson, Inorg. Chem., 1965,4, 326. l4 B. G. Kumetzov and P. A. Koz’min, Zhur. strukt. Khim., 1963, 4, 55. Cotton neighbours one rhenium atom, at a distance of 2.22 A, and four chlorine atoms at a distance of 2.43 A. It may be surmised that four hydrogen atoms are situated between C1 atoms on centres of symmetry .. . and serve to bond the [Re2ClsI4- groups . ..to each other.’ We viewed this extraordinary structure with misgivings. The state of refinement was poor and uncertain. The presence of isolated protons ‘on centres of sym-metry’ was unaccountable. The Re-Re distance seemed unbelievably short and the prismatic rather than antiprismatic array of chlorine atoms seemed inexplic- able. Finally, it was stated that severe difficulty with twinning had been encountered, and we wondered if the difficulties had in fact all been sorted out.Harris was impressed with the urgency of solving the KReC14,H20 structure, which he did in a very short time.15 MirabiZe dictu, KReC14,HzO was found to contain an RezCls entity essentially identical to that described by the Russian workers. Some apparent discrepancies between their dimensions and ours were later resolved, and the best value of the Re-Re distance is 2.24f0.01 A. The structure is shown in Figure 1. Figure 1 The structure of the RepCl,*-ion. Once assured that the structure was correct, the obvious challenge was to interpret it in terms of bonding and electronic structure. This was soon done and resulted in the proposal293 that a quadruple bond, consisting of one 0-,two x-,and a 6-component, exists between the rhenium atoms.In so doing, the ion was taken to be RezCls2-, that is, a compound of rhenium(nI), and the previously postulated hydrogen ions were dismissed as spurious. There has never, in my opinion, been the slightest reason to believe they exist, but the myth that they are, l6 F. A. Cotton and C. B. Hams, Inorg. Chem., 1965,4,330. Quadruple Bonds and other Multiple Metal to Metal Bonds or at least may be, present seems to persist in the Russian literature,16-18 with formulas such as ReC12,CH3COO(H),Ha0,16J7where the significance of (H) is left entirely to the reader’s imagination, unless the descriptionlo of it as ‘free hydrogen (H)’ is considered enlightening.The original discussion3 of the quadruple bond was Lased on qualitative considerations of orbital symmetries and rough estimates of overlap integrals. The Re-Re axis was defined as the z-axis, and the Re41 bonds were assumed to project upon the x-and y-axes. The d22-y2 orbital on each metal atom was assumed to be employed mainly in Re-Cl bonding and the remaining four d-orbitals on each metal atom were used to form the Re-Re bond. Overlap of the dz2orbitals gives rise to a a-bond; overlap of corresponding pairs of the d,, and dyzorbitals leads to formation of a pair of n-bonds. Finally, overlap of the d,, orbitals gives rise to a b-bond. The extraordinary shortness of the Re-Re distance is explained by the high multiplicity of the bond.The eclipsed configur- ation is a consequence of the b-component of the bond since that is the only component which is angle-dependent. As Figure 2 shows, the 6 overlap is maximal in the eclipsed configuration and goes to zero upon rotation by 45” to the staggered configuration. cp 1I (01 (b) Figure 2 Sketches showing (a) the maximization of 6 overlap in the eclipsed configuration and (b)how it becomes zero for a staggered configuration. lo P.A. Koz’min, M. D. Swazhkaya, and V. G. Kuznetzov,Zhur. strukt. Khim., 1967,8,1107. l7 P. A. Koz’min, M. D. Surazhkaya, and V. G. Kunetzov, Zhur. sirukt. Khim., 1970, 11, 313. P.A. Koz’min, Doklady Akad. Nauk S.S.S.R.,1972, 206, 1384. Cotton 2 Elaboration of Dirhenium Chemistry With the existence and structure of Re2Ch2-, as well as a working hypothesis as to the bonding, established, attention was next turned to the chemistry of this and related species.An immediate question concerned the ability of the Re2 unit to persist as the ligands were changed. The existence of the bromo-analogue was already kn0wn.19 It could be prepared directly,13J9 in the same manner as Re2Cls2- by using HBr in place of HCl, or it could be obtained by treating a salt of Re2Cls2- with an excess of HBr.l3 The ligand exchange was shown to be completely reversible.13 The existence of the Re2Brs2- ion, incorrectly described as RezBra4- or H2Re2Br82-, was established in a crude crystallographic study in which inaccurate Re-Re distances of 2.21 and 2.27 8, for two crystal forms were given.20 An accurate study 21 made later gave a metal-metal distance of 2.228(4) A, in excellent accord with the best value, 2.24 k 0.01 8, in Rezcls2-.The insensitivity of the Re-Re distance to replacement of Cl by the larger Br is in accord with the expected strength of the metal-metal bond. The fist non-trivial chemistry of the RezXs2- species to be di~coveredl~~~~ was their reversible reaction with carboxylic acids. When RCOzH is present in excess the reactions proceed quantitatively according to reaction (1); moreover, the reactions are completely reversible. The R~z(OZCR)~XZ compounds had previously been obtained in small yields by reaction of rhenium(v) compounds with carboxylic acids23 and much earlier, by Russian workers,24 but the latter formulated them incorrectly as rhenium@) compounds and their true identity became evident only after they had been rationally synthesized by reaction (1).Their stru~tures~5)~~ are as shown in Figure 3(a). The tetracarboxylate is a limiting stoicheiometry, and species with fewer carboxy-groups and correspondingly more halide ions can easily be envisioned. The intermediate case, Re2(02CR)zX4, is well do~urnented,~8*~~ and structures of both the trans-type [Figure 3(b)] and the cis-type have been reported. The compound Re2(02CPh)214, shown in Figure 4, provides an example of the former28 while Rez(OzCCH&CI4,H20 affords an example of the latter.29 Factors affecting the relative stabilities of the cis and trans structures are not understood.A considerable number of other ligand-substitution reaction~~~~2~J0~31 have l9 G. K. Babeshkina and B. G. Tronev, Doklady Akad. Nauk S.S.S.R., 1963, 152, 100. zo P. A. Koz'min, V. G. Kuznetzov, and Z. V. Popova, Zhur. strukr. Khim., 1965, 6, 651. 21 F. A. Cotton, B. G. DeBoer, and M. Jeremic, Znorg. Chem., 1970, 9, 2143. 22 F. A. Cotton, C. Oldham, and W. R. Robinson, Inorg. Chem., 1966, 5, 1798. 83 F. Taha and G. Wilkinson, J. Chem. SOC.,1963, 5406. 14 A. S. Kotelnikova and V. G. Tronev, Zhur. neorg. Khim., 1958, 3, 1016. 25 M. J. Bennett, W. K. Bratton, F. A. Cotton, and W. R. Robinson, Inorg. Chem., 1968, 7, 1570. 2E C. Calvo, N. C. Jayadevan, C. J. L. Lock, and R. Restivo, Canad.J. Chem., 1970,48,219. 27 F. A. Cotton, C. Oldham, and R. A. Walton, Inorg. Chem., 1967, 6, 214. 28 K. W. Bratton and F. A. Cotton, Inorg. Chem., 1969, 8, 1299. 29 P. A. Koz'min, M. D. Surazhskaya, and V. G. Kuznetsov, Zhur. slrukt. Khim., 1970, 11, 313. 30 F. A. Cotton, N. F. Curtis, and W. R. Robinson, Inorg. Chem., 1965, 4, 1696. 31 F. A. Cotton, W. R. Robinson, R. A. Walton, and R. Whyman, Inorg. Chem., 1967,6,929. 31 Quadridple Bonds and other MultQle Metal to Metal Bonds R /% 00LXI/x X/Re -Re-0I x/l\C/* R Re I oc Figure 4 The structure of ReP(04CPh)J,. Cotton been carried out on the ion, leading to products such as Re@CN)s2- and RezCl~L2, where L = tetramethylthiourea or a phosphine. The retention of the RexRe bond is shown by i.r.32 and Raman33 spectra and X-ray crystallo- graphically, as in the case of Re2Cls(PEt3)~,~~the structure of which is shown in Figure 5.The Re~(S04)4~- ion, with bridging sulphate ions, has also been n Figure 5 The molecular structure of RezCls(PEt3)z. prepared.35 It should be noted that there are also many reactions of Rezcls2-, and other quadruply bonded species, in which destruction of the ReRe unit occurs.36 A list of all published structures which contain a quadruple bond between two Re111 atoms is given in Table 1. Oxidation and reduction reactions have not yet been extensively studied, but it appears that they sometimes cause extensive structural change, expecially the oxidations.38 When Cl2 and Br2 were used as oxidants towards Re2Cls2- and Re2Brs2-, respectively, the products were Re2Xg1-J-, which appear to have the 3a C.Oldham and A. P. Ketteringham, J.C.S. Dalton, 1973, 2304. a3 J. San Flippo, jun., and H. J. Sniadoch, Inorg. Chem., 1973, 12, 2326. F.A. Cotton and B. M. Foxman, Inorg. Chem., 1968, 7,2135. 36 F. A. Cotton, B. A. Frenz, and L. W. Shive, Znorg. Chem., in the press. 86 J. A. Jaecker, W. R. Robinson, and R. A. Walton, J.C.S. Chem. Comm., 1974, 306. P. A. Koz’min, G. N. Novitskaya, V. G. Kuznetsov, and A. S. Kotel’nikova, Russ. J. Znorg. Chem., 1971,12, 861. *6 F.Bonati and F. A, Cotton, Inorg. Chem., 1967, 6, 1353, Quadruple Bonds and other Multiple Metal to Metal Bonds Table 1 Re-Re Bond lengths in variousquadruply bonded dirhenium(1rr) species Species Compound Re-Re Distance 8, Ref.Re2Clg2-Cs2Re2ChQH20 2.235( ?) 37 Re2Cb2-K2Re2CI8,2H20 2.241 (7) 15 Re~c18~-(C5HeN)2Re2Cls 2.244(15) 28 Re2Ch(H20)22-Cs2Re2Cl8,2H20 2.213( ?) 37 Re2Brs2-CszRezBrs 2.228(4) 21 RezC14(OAc)2 RezCl4(02CCH3)2,2HzO 2.224(5) 29 Re2(02CC3H7)4 Re2(02CC3H7)4(Re04)2 2.251 (2) 26 Rez(OzCPh)4C12 -2.235(2) 25 Re2(S04)4(H20)22-Na2Rez(S04)4,8H20 2.214(1) 35 Rezh(02CPh)2 -2.198(1) 28 RezCls(PEts)z -2.222(3) 34 well known confacial bioctahedral structure. Reduction of RezCls2- to Re2Cls3- can be carried out electrochemically, but the product decomposes with a rate constant of about 0.4 s-l and is thus not likely to be isolable, at least at room temperat~re.3~The one-electron reduction40 of Re2(02CCH3)4C12 has been observed to give a yellow product, Re2(02CCH3)4+ or Re2(02CCH&CI, which is stable for hours in solution.Its e.s.r. spectrum affords valuable information on its electronic structure, as will be noted later. 3 Extension to Other Metals However remarkable and interesting the quadruply bonded dirhenium compounds might be, this chemistry could scarcely have been regarded as important if it began and ended with the element rhenium. Therefore one of our earliest goals, to which effort was directed immediately after the main facts about the dirhenium compounds were established, was to see whether similar, multiply-bonded pairs of metal atoms did not also occur in the chemistry of other metals in their lower oxidation states.This work was in fact undertaken with considerable optimism since it did not seem likely that a phenomenon which was so prominent in the chemistry of one element could be entirely lacking in that of others. The first developments in extending the chemistry to other elements were completely logical ones. The basic concept of the Periodic Table, namely, that elements with analogous chemical properties occur in the same column, naturally led to early consideration of technetium. Also, it was thought probable that elements from neighbouring columns in oxidation states that would make them isoelectronic with ReIII might well behave similarly. These ideas furnished guidance but no guarantee of success, since the move from the third to the second transition series is in the direction of decreasing stability of metal-to-metal bonds, and changes in oxidation number necessarily involve differences in charge and orbital size which could oppose the formation of multiple metal-to-metal 3s F.A. Cotton and E. Pedersen, Znorg. Chem., in the press. 40 F. A. Cotton and E. Pedersen, J. Amer. Chem. Soc., in the press. Cotton bonds. Thus the position was precisely that expressed in the well-known maxim: Die Theorie leitet, das Experiment entscheidet. With respect to technetium, success came very quickly. The literature already recorded a compound with the empirical formula (NH&T~zC18,2Hz0,~~ and we were able to that it contains TCzcls3- ions essentially isostructural with Re2C1g2-.The difference in charge, implying the presence of one more electron than the eight required for the quadruple bond, was the only surprising feature. It has been shown39 that Tc2Cls3- can be reduced electrochemically to Te2Cls2-, which is diamagnetic and has a lifetime of at least 5 minutes. A more recent claim43 that compounds of the stoicheiometry MI8 [Tc2C18]3,4HzO can be prepared is entirely erroneous; the compounds in question are all M13[Tc2C18],2H20, with MI = NH4, K, and CS.~~ Simultaneously with our discovery of the TCzcls3- ion, it was found by Lawton and Mason45 that molybdenum(I1) acetate, which had been reported earlier by Wilkin~on,~~has a dinuclear structure with four bridging acetate groups, and a very short Mo-Mo distance, reported as 2.11 A but more recently47 found to be 2.093(1)A. The structure is essentially the same as that shown in Figure 3(a) but without the coaxial ligands, X.Since MoII is isoelectronic with ReIII, it was immediately obvious that a quadruple bond exists between the molybdenum atoms. In view of the close analogy of Mo2(0CCH3)4 to Rez(OzCCH3)&12, it was natural to think in terms of a reversible reaction comparable to reaction (1) above, which would lead to the molybdenum analogue of Re2Cls2-, namely Mo2Cls4-. Such a reaction can be carried out, although the conditions of temperature and concentration are critical and the MozCl84- ion is readily hydrolysed. A number of salts of the MozCls4- ion were prepared and structurally characterized48-51 by Mr.J. V. Brencic, a visitor from Jugoslavia, in 1967-1969. Since that time there has been an enormous output of new compound~5~-~~ containing the Moz4+ unit. Those which have been structurally characterized are listed in Table 2, where their Mo-Mo distances are given. Virtually any carboxylic acid will serve in the Moz(OzCR)4 compo~nds.~6@~53 Two of the most stable and easy to handle are the acetate and, particularly, the trifluoroacetate,51,55 which is soluble and volatile. It has been shown that one factor that can increase the chemical stability of the carboxylates is the ability of large R groups to impede access of attacking groups to the coaxial positions.56 Amidine anions 41 J. D. Eakins, G.D. Humphreys, and C. E. Mellish, J. Chem. SOC.,1963, 6012. 42 F. A. Cotton and W. K. Bratton, J. Amer. Chem. SOC.,1965, 87,921; Inorg. Chem., 1970, 9, 789. 43 M. I. Glinkina, A. F. Kuzina, and V. I. Spitsyn, Zhur. neorg. Khim., 1973, 18,403. 41 F. A. Cotton and L, W. Shive, to be published. 45 D. Lawton and R. Mason, J. Amer. Chem. SOC.,1965, 87, 921. 46 T. A. Stephenson, E. Bannister, and G. Wilkinson, J. Chem. SOC.,1964, 2538. 47 F. A. Cotton, Z. C. Mester, and T. R. Webb, Acta Cryst., in the press. 48 J. V. Brencic and F. A. Cotton, Inorg. Chem., 1969, 8, 7. 49 J. V. Brencic and F. A. Cotton, Inorg. Chem., 1969, 8, 2698. 50 J. V. Brencic and F. A. Cotton, Inorg. Chem., 1970, 9, 346. 51 J. V. Brencic and F. A. Cotton, Inorg. Chem., 1970, 9, 351.35 Quadruple Bonds and other Multiple Metal to Metal Bonds Table 2 Dimolybdenum species having muItlIple bonds with confirmed structures Mo2 Entity Compound Mo-Mo DistancelA Ref. (a) Bond Order 4 MozCl84- ~4MOzCl8,2Hz0 2.139(4) 48 (enH2)2MozCl8,2HzO 2.134(1) 49 (NH4)5MozCIg,Hz0 2.150(5) 50 Li4Moz(CH3)8,4THF 2.147(3) 72 M02(02CCH3)4 2.093( 1) 47 Moz(OzCCF3)4 2.090(4) 54 Moz(OzCCF3)4,2py 2.129(2) 55 M02(N2CPh3)4 2.090(1) 57 K4M02(S04)4,2H20 2.1 lO(3) 58, 59 Moz(SzCOCzH5)4,2THF 2.125( 1) 60 M0z(C3H5)4 2.1 83(2) 68 K3Moz(S04)4,3.5HzO 2.164(3) 75, 59 Moz(CHzSiMe3)s 2.167(?) 76 Moz(NMez)s 2.21 4(2) 78 In A. B. Brignole and F. A. Cotton, Znorg. Syntheses, VoI. 13, ed., F.A. Cotton, McGraw-Hill, New York, 1972, p. 87. S. Dubicki and R. L. Martin, Austral. J. Chem., 1969, 22, 1571. 64 F. A. Cotton and J. G. Norman, jun.,J. Coordination Chem., 1971,1, 161. 66 F. A. Cotton and J. G. Norman, jun., J. Amer. Chem. SOC.,1972,94,5697. E. Hochberg, P. Walks, and E. H. Abbott, Inorg. Chem., 1974, 13, 1824. 67 F. A. Cotton, T. Inglis, M. Kilner,'and T. R. Webb, Znorg. Chem., in the press. 68 C. L. Angell, F. A. Cotton, B. A. Frenz, and T. R. Webb, J.C.S. Chem. Comm., 1973,399. 6B F. A. Cotton, B. A. Frenz, E. Pedersen and T. R. Webb, Inorg. Chem., in the press. eo L. Richard, P. Karagiannidis, and R. Weiss, Znorg. Chem., 1973, 12, 2179. D. F.Steele and T. A. Stephenson, Znorg. Nuclear Chem. Letters, 1973, 9, 777. 6a E.H. Abbott, F. Schoenewolf, jun., and T. Backstrom,J. Coordination Chem., 1974,3,255. e3 J. San Filippo, jun., Znorg. Chem., 1972, 11,3140. 64 J. San Filippo, jun., H. J. Sniadoch, and R.L. Grayson, Inorg. Chem., 1974, 13, 2121. 66 J. V. Brencic, D. Dobenik, and P. Segedin, Monatsh., 1974, 105, 142. 66 G. Wilke, et al., Angew. Chem. Internat. Edn., 1966, 5, 151. 67 W. Oberkirch, Dissertation, Technische Hochschule, Aachen, 1963. F. A. Cotton and J. R. Pipal, J. Amer. Chem. SOC., 1971, 93, 5441. Imperial Chemical Industries, Ltd., 1967, B.P. 1 091 296. 70 V. I. Skohlikova, et al., Vysokomol. Soedineniya, 1968, 10, B, 590 (Chem. Abs., 1969, 70, 50602). 71 R.P. A. Sneddon and H. H. Zeiss, J. Organometallic Chem., 1971, 28, 259. F. A. Cotton, J.M. Troup, T. R. Webb, D. H. Williamson, and G. Wilkinson, J. Amer. Chem. Soc., 1974.96, 3824. 73 B. Heyn and H. Still, 2. Chem., 1973, 13, 191. 74 A, R. Bowen and H. Taube, Znorg. Chem., 1974,13,2245. 76 F. A. Cotton, B. A. Frenz, and T. R. Webb, J. Amer. Chem. SOC.,1973, 95, 4431. 76 F. Huq, W. Mowat, A. Shortland, A. C. Skapski, and G. Wilkinson, Chem. Comm., 1971, 1079. 77 M. H. Chisholm and W. W. Reichert, J. Amer. Chem. SOC.,1974,96, 1249. 78 M. H. Chisholm, W. W. Reichert, F. A. Cotton, B. A. Frenz, and L. Shive, J.C.S. Chem. Comm., 1974,480, 804. Cotton RlNC(RZ)NRl-, which are isoelectronic with RC02-, can also serve as bridges,57 as can the sulphate ion,58~~~ the ethylxanthate ion,6096l apparently also ethyl acetate,62 the trifluoromo,thanesulphonateion,62 and the thiobenzoate ion.61 There is a large derivative chemistry of Mo2Cls4-, principally involving molecules of the type Mo2C14L4, in which there are at least 16varied types of L gro~p.~~-~~ Some Mo~Br4L4 analogues were also rnade.G4 There are several organo-derivatives of the Moz4f unit.The first to be rep0rted,~6,67 Mo2(C3H5)4, has been structurally characterized;68 it has the structure shown in Figure 6, and has been shown to possess catalytic activity for Figure 6 The structure of Mo,(allyl), and Cr2(allyl)4.Hydrogen atoms are omitted. olefin69 and stereospecific butadiene70 polymerization. It appears, however, that it is the ally1 groups rather than the Mo2 unit which are the source of the cata- lytic function.The analogous Crz(allyl)4 has similar activity.71 The reaction of Mo2(02CCH3)4 with LiCH3 in ether72 produces Li4 [Mo~(CH~)~],~(C~H&O, from which derivatives with other ethers can be obtained. The structure of the THF-containing compound has been detem~ined;~z it contains the eclipsed octamethyldimolybdenum anion shown in Figure 7. The preparation of a substance with the empirical formula LizMo(CsH5)2H2,2THF has been reported.73 It would seem very probable that this compound is similar to the octamethyl one and contains the M0z(CsH5)4H4~- anion. A considerable aqueous chemistry of the Moz4+ ion has already been developed by Bowen and Ta~be.7~ They found that the M02Cl8~- ion in O.lM-HS03CF3 is hydrolysed to Mo2Ch2+(aq). The addition of an excess of K2S04 to a solution of K4Mo2CIs in 0.2M-HS03CF3 causes a pink precipitate of ICQ[Mo~(S04)4],2H~0 to form in >90 % yield.This can be dissolved in 0.01M-HS03CFs and a slight excess of Ba(SOsCF3)2 can be added to precipitate all the sulphate and leave what is presumed to be the Mo2*+(aq) ion in solution. Despite numerous attempts in various laboratories (all unpublished) no one has succeeded in precipitating this ion with a nonco-ordinating anion. The Moz4+(aq) has an electronic spectrum not greatly different from that of Mo2(02CCH3)4. 37 Qtradruple Bonds and other Mirltiple Metal to Metal Bonds c Figure 7 The structure of the MO~(CH~),~-ion. The compound Mo2(en)4CL was prepared by treating K4Mo2CI8 with neat ethylenediamine on the steam bath for 30-60 minutes followed by washing with ethanol and ether.It is the first compound containing a cationic complex of Moz4+. It has an electronic spectrum quite similar to that of M02~+(aq). The [Mo2(en)4l4+ complex undergoes aquation only slowly (ca. 30 minutes) and reacts slowly enough with 02 to permit the solutions to be handled without rigorous exclusion of air. In the process of crystallizing pink K4 [Mo2(S04)41,2HzO slowly so as to obtain crystallographically useful crystals, it was found that lavender crystals also f0rrned.~~*75X-Ray crystallography showed that they have the composition K3Mo2(S04)4,3.5H20 and contain the [Moz(S04)4]3- ion, which has a structure very similar to that of [Mo2(S04)4]4- (Figure 8).According to the accepted description of the quadruple bond, the electron lost from the 4-ion to give the 3 -ion should come from the &bonding orbital, thereby giving the latter species a 2Bzg ground state. There is considerable evidence to support this idea and thus, indirectly, to support the general picture of the quadruple bond. Thus, as Table 2 shows, loss of this weakly bonding &electron causes a lengthening of the Mo-Mo bond, by about 0.05 A. The two ions are related by an electrode potential of +0.22 V. The 3-ion is paramagnetic, with p = 1.65 BM, gll = 1.891, and 38 Cotton Figure 8 The structure of the MO~(s04)4~-entity which occurs with n = 4 in K4[Mo2(S0J4],2H20and n = 3 in K,[MO~(SO~)~],~.SH~O.There are oxygen atoms in the coaxial positions.gl= 1.903. The e.s.r. spectra show that the spin Hamiltonian has axial symmetry and that the unpaired electron is evenly distributed over two magnetically equivalent molybdenum atoms. Qualitative arguments show40 that the quadruple- bond formulation in quantitative form (See Section 4) would require that both g values be <2, as observed. Before leaving molybdenum, it should be mentioned that not only does this element seem, on present knowledge, to be the most prolific former of quadruple bonds, but it also has a marked tendency to form triple bonds. The two best- characterized examples are Mo2 [CH2Si(CH3)3]676 and Mo2 [N(CH3)2]6.77*78 The structure of the latter is shown in Figure 9, and the metal-metal distances in the two compounds are listed in Table 2.A similar compound,7Q Moz [OC(CH3)3]6, presumably has the same sort of structure. The substance [(q5-C5Hs)Mo(C0)2]2 7B M. H. Chisholm, personal communication. Quadruple Bonds and other Multiple Metal to Metal Bonds C Figure 9 The molecular structure of Mo2[N(CH,),],. The view is almost down the molecular three-fold axis. has been briefly described;8u it presumably has a triple Mo-Mo bond by analogy with { [q5-C5(CH3)5]Cr(C0)2>2.s1 Tungsten.-The great readiness of molybdenum to form M-M bonds of orders 3 to 4 naturally led to the hope that tungsten would also have a rich chemistry of similar compounds. This has not proved to be the case so far. The reaction of W(CO)s with glacial acetic acid does not appear to give Wz(OzCCH3)4, but instead some sort of trinuclear species,s21*3 possibly W3(02CCH&O.With a number of other acids, e.g. C&,C02H, CsF5C02H, C~H~COZH, and C3F7CO2H, compounds of the correct stoicheiometry have been obtained,s2 but none of them have formed crystals, and none are therefore substantiated structurally. The triply-bonded tungsten compound76 W2 [CH2Si(CH3)3 16 has been prepared and reported to form crystals isomorphous with those of the molybdenum compound. R. C. Job and M. D. Curtis, Inorg. Chem., 1973, 12,2510. J. Potenza, P. Giordano, D. Mastropaolo, A. Efraty, and R. B. King, J.C.S. Chem. Comm., 1972, 1333. Es F. A. Cotton and M. Jeremic, Synth. Znorg. Metal-Org.Chem., 1971, 1, 265. 83 T. A. Stephenson and D. Whittaker, Inorg. Nuclear Chem. Letters, 1969, 5, 569. Cotton A triply-bonded tungsten compound, W2[N(CH3)2]6, can be made but has not yet been fully purified (separated from W [N(CH3)2]6} or structurally characterized. If the inaccessibility and intractability of compounds with quadruple W-W bonds results from relative weakness of those bonds, which is not necessarily the case, we must naturally try to understand why this bond should be significantly weaker than those in the Mo24+ and Re26+ cases. The answer is not yet obvious to this writer. A decrease in stability from M024+ to W24+ might be attributed to the presence of the 4f14 shells in the latter impeding the close approach necessary for the &overlap to be effective.However, this factor would not be expected to diminish greatly on moving to Re26+; at least I do not think so. Further efforts towards the preparation of tungsten compounds would seem to be pertinent. Chromium.-Though in many other cases multiple M-M bonds found in com- pounds of the second- and third-rows have no parallel in the first-row elements, such is not the case here; there are numerous compounds containing the Crz4+ entity. Those structurally characterized are listed in Table 3. Table 3 Compounds containing multiple bonds bet ween chromium atoms which have confirmed structures Cr2 Entity Compound Cr-Cr DistancelA Ref. (a) Bond Order 4 Crz(OzCCH3)4,2H20 Cr2(02CCH3)4,2H20 2.3855(5) 84 [Cr2(CO3)414-Mg2Crz(C03)4,6H20 2.22( ?) 86 D~(C~HS)~ L~~C~~(C~HS)~,~THF 9014-1.9735) [c~~(cIG)~14-hCrz(CH3)8,4THF 1.980(5) 89 Crz(ally1)4 Cr2(allyl)4 1.97( ?) 87, 88 (b)Bond Order 3 ( [q5-Cs(CH3)5 ICr(C0)z }2 2.276(2) 81 It is fitting to begin by mentioning Cr2(03CCH3)4,2H20, first reported1 in 1844 but only in 1970 accurately described as to structure and bonding.84 The Cr-Cr distance is 2.39 A and the compound is isostructural and isoelectronic with Mo2(02CCH3)4, except for the presence of the coaxial water molecules.It is therefore reasonable to propose that it contains a quadruple bond. This would require that the compound be diamagnetic, and it probably is. Reported suscepti- bilities for this and a large number of other Cr2(02CR)4L2 compounds85 are 84 F.A. Cotton, B. G. DeBoer, M.D. LaPrade, J. R. Pipal, and D. A. Ucko, J. Amer. Chern. SOC.,1970, 92, 2926; Acra Crysr., 1971, B27, 1664. S.Herzog and W. Kalies, 2.anorg. Chem., 1964,329,83;1966,351,237and other references cited therein. 86 R. Ouakes, Y. Maouche, M.-C. Perucaud, and P. Herpin, Compt. rend., 1973,276,C, 281. 87 G. Albrecht and D. Stock, 2. Chern., 1967,7, 321. T.Aoki, A. Furusaki, Y. Tomiie, K. Ono, and K. Tanaka, Bull. Chem. SOL.Japan, 1969, 42, 545. J. Krausse, G.Marx, and G. Schodl, J, Organometallic Chern., 1970, 21, 159. J. Krausse and G. Schodl, J. Organometallic Chern., 1971, 27, 59. 41 Quadruple Bonds and other Multiple Metal to Metal Bonds always weakly paramagnetic, with apparent magnetic moments of 0.50-0.85 BM.It seems quite likely that this is due to traces of chromium(u1) compounds as impurities, although there is also the possibility that the &bond is so weak that a 66* triplet state is detectably populated. Jt is possible that this uncertainty could be resolved by an e.p.r. study. As will be seen presently, there are other structurally characterized compounds with Cr-Cr quadruple bonds, and one with a triple bond, in which the Cr-Cr distances are much shorter than that in the acetate. This simply illustrates the fact that quadruple bonds can vary in strength and that bond multiplicity is not a direct or single-valued index of bond strength. This situation prevails for bonds of other orders as well.There are, for example, wide variations in the strengths of single bonds, as illustrated by the series C-C (356 kJ mol-I), N-N (160 kJ mol-l), 0-0(146 kJ mol-l), and F-F (158 kJ mol-l). In the case of Cr2(02CCH3)4,2H20 the relatively strong binding of the coaxial ligands (Cr-0, 2.27 8,) is probably related to the relative weakness of the Cr-Cr bond. In Moz(OzCR)4 compounds, where the Mo-Mo bonds are very short, coaxial ligands are either absent or only weakly attached. The exact interplay between these two factors is not clear, but a reciprocal relationship between the strengths of the two bonds does appear to exist. An interesting compound which is similar to the acetate is Mg2Cr2(C03)4,6H20. This contains the [Cr2(C03)4(H2O)2’J4- ion, where the carbonate ions play the same bridging role as do the acetate ions in the acetate and the H2O molecules serve as coaxial ligands.86 In this case the Cr-Cr bond length is 2.22 8, and the Cr-OH2 bond length is 2.32 A, shorter and longer, respectively, than the corresponding bonds in the acetate.We turn now to several compounds in which there are extremely short Cr-Cr bonds. These ‘superbonds’ are the shortest metal-to-metal bonds known. Crz(allyl)4 has a structures7988 analogous to thaF of Moz(allyl)4 (Figure 6), with a Cr-Cr distance of only about 1.97 A. There is also a [Cr2(CH3)8I4- ion with a structuresg essentially identical to that of [Mo2(CH3)sl4- (see Figure 7); the Cr-Cr distance is 1.980(5) A. A closely related species is the [Cr2(C4H8)4I4- ion, in which there are four chelating -(CH&-units, two on each Cr atom.90 The Cr-Cr bond here is the shortest M-M bond presently known, with a length of 1.975(5) A.In addition to the structurally characterized compounds of dichromiumfu) just mentioned there are a number of other reported compounds which seem certain to contain Cr-Cr quadruple bonds. Aside from the numerous carboxylato- bridged species other than the acetate,85,91 there are several organometallic compounds. The compound Cr2 [(CH2)2P(CH3)2 kg2 presumably contains a Cr-Cr quadruple bond and eight Cr-C bonds, but it is not known whether the structure is (a) or (b) of Figure 10, or possibly the variant of (a) in which the two 91 P. Sharrock, T. Thiopanides, and F. Brisse, Canad, J.Chem., 1973, 51, 2963. 92 E. Kurras, U. Rosenthal, H. Mennenya, G. Oehme and G. Engelhardt, 2. Chem., 1974,14, 160. Cotton Figure 10 Two possible structures for Cr, [(CH2)2P(CH,)2]4. sets of chelate rings are eclipsed rather than staggered. Compounds which may contain [Cr2(C5H10)4I4- have been rep~rted.~O The compounds LizCr(U-CsH40)2, 2Ether and several similar onesg3 may well contain a [Cr2(o-C1&0)4]~- ion in which the o-CsH402- ions occupy a set of positions similar to those of bridging carboxy-groups. There is also C~(U-C~H~OM~)~,~~ which could contain dinuclear molecules, though its low solubility perhaps indicates a polymeric structure. It should be noted, to complete the picture regarding chromium(II), that there are also many compounds, including important ones, which do nut have metal- metal bonds according to magnetic or crystallographic data, or both.These include Cr12,95 CrC12,4H20,96 several with the formula M12Cr(S04)2,xH20 where MI is an alkali some M12CrC14 MI~C~BI-~(H~O)~,~~ and Cr [N(SiMe& ]2(THF)2.100 There is no evidence for the existence of a dinuclear aquo-ion (through no effort to detect it is recorded). It has been proposedlO1 that the rate-controlling step in the redox and substitution reactions of 83 F. Hein, R. Weiss, B. Heyn, K. H. Barth, and D. Tille, Monatsber. Deut. Akad. Wiss. Berlin, 1959, 1, 541. s4 F. Hein and D. Tille, 2.anorg. Chem., 1964, 329, 72. ss F. Besrest and S. Jaulmes, Acta Cryst., 1973, B29, 1560.96 H. G. von Schnering and B. H. Brand, 2.anorg. Chem., 1973,402, 159. 87 A. Earnshaw, L. F. Larkworthy, K. C. Patel, and G. Beech, J. Chem. SOC.(A), 1969, 1334. H. J. Seifert and K. Klatyk, 2.anorg. Chem., 1964, 334, 11 3. L. F. Larkworthy and A. Yavari, J.C.S. Chem. Comm., 1973, 632. looD. C. Bradley, M. B. Hursthouse, C. W. Newing, and A. J. Welch, J.C.S. Chem. Comm., 1972, 567. R. D. Cannon and J. S. Lind, J.C.S. Chem. Comm., 1973, 904. 43 Quadruple Bonds and other Multl'ple Metal to Metal Bonds Cr2(02CCH3)4(H20)2 in aqueous media is the dissociation : Cr2(02CCH3)4 2 Cr(02CCH3)2 It thus appears that complexes of the dinuclear cation Crz4+ play a very important role in the chemistry of chromium(@, but is not as dominant as is Moz4+ in the chemistry of molybdenum(I1).The latter, of course, also has an extensive cluster chemistry102 involving Moe clusters with Mo-Mo single bonds, and this has no parallel whatever in the chemistry of chromium(r1). Two compounds containing triple Cr-Cr bonds have been reported. The first was (q5-C5Me5)2Cr2(C0)4, which consists of two (q5-C5Me5)Cr(C0)2 units joined by a Cr-Cr bond which is 2.276(2)A long.1°3 On the reasonable assump- tion that each chromium atom is to have an 18-electron configuration, this should be considered a triple bond. Very recently it has been shown104 that the presum- ably isostructural (q5-C5H5)2Cr2(C0)4 forms readily on thermolysis of the highly strained (q5-C5H5)2Cr2(C0)6.*05 Heteronuclear Quadruple Bonds.-The possibility of forming mixed metal species [MM'(02CR)4], containing two Group VI metals, is very obvious, but little has yet been published.The isolation of CrMo(02CCH3)4 has been re- ported.lO6 The structure has not yet been determined, but if a band in the Raman spectrum (and also in the ix.) at 392 is assigned to Cr-Mo stretching, then the Cr-Mo force constant is only about 2/3 that for Mo-Mo in Mo2(02CCH3)4. McCarley and co-workers107 have also prepared this compound as well as several MoW(02CR)4 compounds and shown that both Moz(OzCR)4 and MoW(02CR)4 species can be oxidized by halogens to such products as Mo2(02CR)4+13- and MoW(02CR)4I. Ruthenium and Iron.-About ruthenium we know only enough to suggest that there may be significant things still to learn.In 1966 Stephenson and Wilkinsonl08 reported the preparation of a series of compounds with the unusual stoichei- ometry Ru2(02CR)4Cl, and having magnetic susceptibilities suggesting the presence of three unpaired electrons per formula unit. The true nature of these compounds was established X-ray crystallographically~~9 a few years later ;the significant portion of the structure of the butyrato-compound is shown in Figure 11. The very short Ru-Ru distance, 2.281(4)A, implies that a very strong M-M bond is present. An orbital scheme was also proposed to explain the presence of three unpaired electrons within the general framework of the quadruple-bond scheme. lo' F. A. Cotton, Accounts Chem. Res., 1969, 2,240. lo3J.Potenza, P. Giordano, D. Mastrapaolo, A. Efraty, and R. B. King,J.C.S. Chem. Comm., 1972, 1333. lo4 P. Hackett, P. S. O'Neill, and A. R. Manning, J.C.S. Dalton, 1974, 1625. lo6R. D. Adams, D. E. Collins, and F. A. Cotton, J. Amer. Chem. Soc., 1974,96, 749. lo6 C. D. Gamer and R. G. Senior, J.C.S. Chem. Confm., 1974, 580. lo' R. E. McCarley, R. J. Hoxmeier, and V. Katovic, personal communication. lo8T. A. Stephenson and G. Wilkinson, J. Znorg. Nuclear Chem., 1966, 28, 2285. lo@M. J. Bennett, K. G. Caulton, and F. A. Cotton, Znorg. Chem., 1969, 8, 1. Cotton Figure 11 A portion of the structure of RU~(O~CC~H,)~C~.The long, angular CI bridgesbetween Ru,(O&H,)~+ units are to be noted. Very recently, Ruz(OzCC3H7)4CI has been more thoroughly investigated.ll0 The magnetic susceptibility from 60 to 300 K and the e.p.r.spectrum in solution show conclusively that Ru2(02CC3H7)4+ has a quartet ground state and that the unpaired electrons are equally shared by the two metal atoms, thus ruling out any mixed-oxidation-state (RuII, RuIII) formulation. It was also shown that oneelectron reduction to Ru2(02CC3H7)4 occurs quasi-reversibly at potentials in the range 0.00 to -0.34 V, depending on solvent. The product, presumably Ru2(02CC3H7)4, appears to be diamagnetic; a crystalline specimen of this could not be obtained. Stephenson and Wilkinsonlos had also tried, unsuccessfully, to isolate crystalline samples of various compounds, e.g. Ruz(OzCCH&(HzO) Ru(OaCR)z(py)4, and Ru(O2CR)a(py)z, which appear to contain Ru in the oxidation state II.It would seem, however, that under appropriate conditions it should be possible to do so. A compound of apparent composition F. A. Cotton and E. Pedersen, Znorg. Chern., in the press. Quadruple Bonds and other Multiple Metal to Metal Bonds Ruz(02CCH3)4.5(H20)1.5 was also reported, and its structure would be of interest. Although no compounds with quadruple bonds are known, iron ranks a close second to chromium among first-row metals in forming multiple M-M bonds. Two compounds, (1) and (2), containing Fe-Fe triple bonds have been reported. Compound (1)ll1 has an Fe-Fe distance of 2.177(3) A, which is about 0.1 A shorter than the triple bond in the very similar [(q5-Me5C5)Cr(C0)~]z.In (2)112 each set of three P atoms belongs to a HC(CH2PPhz)3 ligand and the accompany- ing anion is BPhd-. The Fe-Fe distance here is 2.34 A. We have here another good example of the fact that bond multiplicity is simply a qualitative measure of the number of electron-pair interactions and not an index of bond strength (or length). In compound (2), there seem likely to be three bond components, cr + 2.n, but they are weaker than in (1). There are three compounds containing Fe-Fe double bonds. The first,l13 (3), has an M-M distance of 2.215A. Compound (4)114 is a homologue of (3) and has nearly the same distance, 2.225(3) A. Compound (5),115 with different bridging groups, has a distinctly longer Fe-Fe distance, 2.326(4) A. It is interesting to note that the latter is scarcely shorter than the length (2.37 A) of the formal single bond in (6),116 though the latter is definitely anomalous among single Fe-Fe bonds, which are generally 2.5-2.8 A long.Rhodium.-Dirhodium tetra-acetate dihydrate is a compound which contains the shortest known84 Rh-Rh bond, 2.3855(5) 8.On the basis of comparisons with many Rh-Rh single bonds, which have lengths in the range 2.68 to 2.94 &I1* it has been suggestedllg that this bond must be a multiple one, probably with an order of 3. Recently two compounds, (7) and (8), with double bonds have been 111 S.-I. Murahashi, et al, J.C.S. Chem. Comm., 1974, 563. P. Dapporto, G. Fallani, S. Midollini, and L. Sacconi,J. Amer. Chem. SOC., 1973,95,2021. 113 K.Nicholas, L. S. Bray, R. E. Davis, and R. Pettit, Chem. Comm., 1971, 608. 114 H.-J. Schmitt and M. L. Ziegler, 2. Naturforsch., 1973, 28b,508. 115 J. Calderon, S. Fontana, E. Frauendorfer, V. W. Day, and S. D. A. Iske, J. Organometallic Chem., 1974, 64, C16. 116 P. E. Baikie and 0. S. Mills, Znorg. Chim. Acta, 1967, 1, 55. 11' 0. S. Mills and J. P. Nice, J. Organometallic Chem., 1967, 10, 337. 11* K. G. Caulton and F. A. Cotton, J. Amer. Chem. SOC.,1971, 93, 1914. Cotton OC\ /4 F0 Fe =Fe OC’ ‘L’ ‘co (3) L = BU‘C=ICBU‘ described.ll9 Since these bonds have lengths of 2.46 and 2.55 A, the previous proposal receives further support. The structure of Rh2(02CCH&(PPh&, which is analogous to that of the tetra-acetate dihydrate, has been determined.120 The phosphines are fairly strongly bonded [Rh-P = 2.479(4)A] and the Rh-Rh bond is longer than in the hydrate, namely 2.449(2) A.(7) X=CO; Y=CPh2 (8) x = Y= CPh, A number of formate complexes, mostly of the type [Rh(02CH)2L]2, have been reported.121 The basis for the dinuclear formula is an unpublished X-ray crystal- lographic study of the compound with L = 0.5H20, which is said to ‘contain [Rh(OzCH)2(H20)12 and [Rh(02CH)2 12 units alternating in an infinite chain,’ but no bond distances are disclosed. The preparation of the aquo-ion Rh24+(aq) has also been described.122 This has been obtained in solution by reduction of Rh(HzO)&l+ with Cr2+; no solid compound has been isolated. It is oxidized slowly by air and reacts rapidly with various ligands, for example with acetate ion to generate what appears to be Rh2(02CCH3)4.4 Electronic Structures It is obvious that for moderately complex molecules containing atoms of very high atomic numbers, e.g. Mo2CIs4- and Rezcls2-, conventional MO calcu-113 H. Ueda, Y. Kai, N. Yauoka, and N. Kasai, 21st Symposium on Organometallic Chemistry, Japan, 1973, Abstract No. 214. laoJ. Halpern and G. Khare, personal communication. lal I. I. Chernyaev, E. V. Shenderetskaya, A. G. Maiorova, and A. A. Karyagina, Russ. J. Inorg. Chem., 1965, 10, 290, and earlier work cited therein. lZ2F. Maspero and H. Taube, J. Amer. Chem. SOC.,1968,90, 7361. Quadruple Bonds and other MuItiple Metal to Metal Bonds lations are barely, if at all, feasible.Therefore, until very recently, theoretical discussion of the electronic structures of quadruply bonded dinuclear species, and others closely related, has been of an essentially qualitative nature, and there was no important development beyond the basic qualitative proposals made by this writer some 9 years ago. Experimental approaches to questions of electronic structure have also been few. Aside from some early attempts to interpret a few electronic ~pectra~~t53J2~ the measurement of bond lengths has been the only experiment a1 met hod consis tent 1y used. In the past few years this picture has begun to change. The SCF scattered-wave Xa (SCF-SW-Xa) method of cal~ulationl~~ appears to afford a practicable avenue of theoretical approach, and experimental studies employing e.p.r.measurements of paramagnetic species generated electrochemically, as well as 0-10-Anti bonding Orbitals Sfg --20 -E 30- 0 < 40-0 2. Cr) 282, - (3 z a w 50- 5E" n- M-M Bonding Orbi to Is u' 60- 70 -282, -Ligand Orbitals and Metal -Ligand Bonding 80 -OrbitalsI I I I-90 Figure 12Aportion of the energy-leveldiagram for Mo2Cla4-according to an SCF-S W-Xor calculation by Norman and Kolari. Levels shown are those with at least 20% metal character. la3F.A. Cotton and C.B. Harris, Znorg. Chem., 1967, 6,924. la*K. H. Johnson, Adv. Quantum. Chem., 1973,7,143. Cotton more sophisticated measurements and interpretations of optical spectra, have begun to appear.The M02Cl8~- ion has been treated by the SCFSW-Xa method by Norman and K0lari.1~5 Their results strikingly confirm all of the essential features of the original proposal3 concerning the quadruple bond. Figure 12, which is adapted from the work of Norman and K~lari,~~~ shows some of their results. It is evident that a little above the metal-ligand and ligand lone-pair orbitals, and below the anti-bonding orbitals, is a set of Mo-Mo bonding orbitals, in the order (r, ?I, 6, Figure 13 The Mo-Mo u (4 A,) bonding orbital, in the xy plane, of MopCls4-accordingto the SCF-S W-Xu calculation of Norman and Kolari. la6 (a) J. G. Norman, jun. and H. J. Kolari, J.C.S. Chem. Cornrn., 1974, 303; (b) J. Amer.Chem. SOC.,in the press. Quadruple Bonds and other Multble Metal to Metal Bonds precisely as expected. The 6*-orbital is not far above the &orbital, also as expected, since this is the weakest component of the quadruple bond. Perhaps the most important result of this calculation is that the 6-,n-, and &orbitals are found to have mainly metal d character (76-93%), so that the original crude approximation3 of describing the quadruple bond in terms of Q , n-, and &overlaps of pure d-orbitals is, in essence, validated. Figures 13-1 5 show contour diagrams of the 0-, n-, and &bonding orbitals. The a-(4Alg)orbital (Figure 13) arises mainly from an overlap of metal 4dz2functions. The outer lobes and equatorial rings of the 4dz2 functions are CT anti-bonding with respect to Figure 14 One of the Mo-Mo n(SEu) bonding wavefunctions of MoZClB4--,from the SCF-SW-Xa calculation of Norman and Kolari.Cotton Mo-Cl interactions, as shown by the nodes between Mo and C1atoms, but there is some appreciable overlap with the C13p orbitals to give a bonding contribution as well. Thisa-orbital has the lowest (76 %) metal dcharacter of those forming the quadruple bond. The n-orbitals, one of which is shown in Figure 14, are obviously the result of 4dn-4dz overlap. Finally, Figure 15 shows one of four equivalent sections through the maximum electron density of the &orbital. This one has the highest metal dcharacter (93 %) and looksjust as one would expect for a 4d6-4d6 overlap.In addition to the basic description of the quadruple bond,3 which the Figure 15 A section through one of the maximalplanes (halfway between the xz and yzplanes) of the Mo-Mo 6(2Bzg)bonding orbital of Mo2Clsa-from the SCF-SW-Xor calculation of Norman and Kolari. Quadruple Bonds and other MultlIple Metal to Metal Bonds SCF-SW-Xa calculations so strikingly confirm, this writer subsequently suggested that there might also be two essentially non-bonding a-orbitals, directed outwards along the M-M axis, and formed mainly from metal s-and pz-orbitals, and that at least one of these would have an energy similar to those of the 6-and 6*-orbitals. The SCF-SW-Xa calculation does not support this sugges- tion. While this is of no importance in describing the ground state of Mo2Cls4-, or other systems in which there are eight or fewer M-M bonding electrons, it is important with regard to the excited states of these molecules and for all species such as Tc2Ch3-, Ruz(OzCR)4+, and Rhz(OzCCH&(Hz0)2, in which there are more than eight electrons.The molecular and electronic structures of these species have previously been rationalized by the writer in ways which employed at least one of these a non-bonding orbitals, but, as Norman and Kolari suggest, it may not be too difficult to rationalize them in other ways. One case, however, that at present seems to offer difficulty is Re2C14(PEt3)4, which has the structure126 shown in Figure 16. This contains two more electrons Figure 16 lXe structure of Re,Cl,(PEt,),, with the ethyl groups omitted for clarity. than Re2Cls2- or Re2Cls(PEt3)2 and is subject to considerable internal crowding that should tend to stretch the Re-Re bond.If the two additional electrons occupy the 6*-orbital, thus nullifying the &bond, as the diagram of Figure 12 would suggest, it is not easy to understand why the Re-Re distance of 2.232(6)A is not significantly longer than those in Re2Cls2- [2.241(7)A] and RezCl6(PEt& [2.222(3)A]. From the spectroscopic side, there is evidence to support the level scheme of F. A. Cotton, B. A. Frenz, J. R. Ebner, and R. A. Walton, J.C.S. Chem. Comm., 1974,4. Cotton Norman and Kolari. They have shown that the observed bands in the visible and U.V.spectra of Mo2Cls4-can be satisfactorily assigned using their diagram; in so doing they assign the lowest observed band to the 6 3 6* transition, whereas, if a non-bonding a-orbital were to lie below the 6*-orbital, this transition would be assigned to 6 -+a. For Re2Ck2- and Re2C16(PEt&, Cowman and Gray127 have offered experimental evidence which favours assigning the lowest observed band to the 6 -+ 6* transition in these species, too. The writer had previously rejected that assignment because the transition is extremely weak although a 6 --f 6* transition is orbitally allowed. Recent work in the writer’s laboratory40 involving an e.p.r. study of species with unpaired electrons that must occupy orbitals above the &orbital also gives results that cast doubt upon the earlier proposal that a a non-bonding orbital is next above the &orbital.This is not the place to pursue the interesting but currently controversial question of the detailed electronic structures of the quadruply bonded and related species. There are many other data bearing on the question. The foregoing discussion merely serves to show that this aspect of the field as well as chemical aspects remain challenging subjects for further research. la’ C. D. Cowman and H. B. Gray, J. Amer. Chem. SOC.,1973,95,8177.

ISSN:0306-0012    

DOI:10.1039/CS9750400027

出版商:RSC    

年代:1975

数据来源: RSC

摘要:

Kinetics of Reactions in Aqueous Mixtures By M. J. Blandamer and J. Burgess DEPARTMENT OF CHEMISTRY, THE UNIVERSITY, LEICESTER, LE7 1RH 1 Introduction The dependences of kinetic parameters for reactions on the composition of mixed aqueous solvents often afford complicated patterns. Several examples, involving organic and inorganic substrates and a variety of kinetic parameters such as enthalpies, entropies, volumes, and heat capacities of activation, are shown in Figures 1-3. Such patterns cannot but arouse interest and enquiry. Figure 1 Dependence of activation parameters on solvent composition: an,AG* (O),6,AH+ (01,and TijmAS* (A) fur reaction between nickel(l1) and 2,2’-b@yridyl in methanol + water mixtures at 298 K (values taken from references cited in text) In aqueous solutions, at least, it has become clear that an important aspect of the activation process is the reorganization of the solvent surrounding the reacting solute(s).l-3 Not surprisingly, such reorganizations will be profoundly l E.A. Moelwyn-Hughes, ‘The Chemical Statics and Kinetics of Solutions,’ Academic Press, London, 1971. R. E. Robertson, Progr. Phys. Org. Chem., 1967, 4, 213. E. F. Caldin and H. P. Bennetto, J. Solution Chern., 1973, 2, 217. Kin etics of Reactions in Aqueous Mixtures Figure 2 Dependence of activation parameters on solvent composition: amA Cp* for the solvolysis of t-butyl chloride in binary aqueous mixtures containing t-butyl alcohol (O), (values taken from references ethanol (A), and acetonitrile (0)cited in text) Bldndamer and Burgess 2 Xa -0.2 0.4 0.6 0 I 1 I -4 .d c8 5 s s -8 -12 OJ -16 Figure 3 Dependence of activation parameters on solvent composition: LA V* (p = 0,323 K) for the solvolysis of benzyl chloride in binary aqueous mixtures contain-alcohol (o),references cited in text) ing t-butyl (.) and DMSO (A),acetone (values taken from affected by addition of a co-solvent to these aqueous soIutions.Indeed it is now clearly recognized that the action of the non-aqueous component of a mixture is more than as a simple diluent of water or modifier of its dielectric properties. To understand the complex patterns of the type shown in Figures 1-3, it is Kinetics of Reactions in Aqueous Mixtures necessary to investigate how reactants and transition states are affected when a co-solvent is added to thz reacting system in water.In this present Review we shall firstly deal with such properties of binary aqueous mixtures and solute-solvent interactions as are relevant to our main topic. We shall then define and discuss the thermodynamic transfer functions used as the basis of our discussion of the kinetic patterns observed in binary aqueous mixtures. Having established these foundations we shall describe and discuss variations in reactivity (in terms of free energies, enthalpies, entropies, heat capacities, and volumes of activation) with solvent composition. Through- out we shall attempt to correlate the kinetic patterns with other property trends established for these solvent mixtures.We shall also examine how far the variations in thermodynamic properties of the solvent mixtures (the various thermodynamic functions of mixing) are reflected in the reactivities of the substrates dissolved in these mixtures. We restrict our attention to substitution reactions where certain key features of the mechanism are established, ranging from dissociative reactions such as the solvolysis of t-butyl chloride and complex formation from hexa-aquonickel(n) to associative processes such as the alkaline hydrolysis of organic halides and carboxylic esters. We do not deal with the complementary area where the dependence of rate constants on solvent composition is used in the diagnosis of reaction mechanism.In view of the complexities of kinetic behaviour in mixed aqueous solvents, exercises of the latter type should be embarked on, and any conclusions treated, with great ~aution.~ 2 Water and Aqueous Solutions Water.-The properties of water have been reviewed in considerable detail.5 A simple but attractive model describes5a liquid water in terms of a dynamic two-state system, (H20)b + (H20)d, comprising hydrogen-bonded (bulky) and non-hydrogen-bonded (dense) states; AH* > 0 and A Ve < 0. The latter state is formed by water molecules that occupy voids formed by intermolecularly hydrogen-bonded water molecules. The lifetime of the bulky state is between 10-10 and lo-%, just long enough for this state to be chemically meaningful, and very much shorter than the half-lives of the chemical reactions discussed in this review.Binary Aqueous Mixtures.-These can be conveniently classified6 on the basis of their thermodynamic properties, particularly their molar excess functions, X” (where X is a thermodynamic function of state, e.g. the Gibbs function G, enthalpy H, entropy S, volume V, and heat capacity at constant pressure Cp). E. Grunwald and A. Effio, J. Amer. Chem. SOC.,1974, 96,423. ‘Water -A Comprehensive Treatise’, ed. F. Franks, Plenum Press, 1973; (a) H. Frank, Vol. 1, Chapter 14; (6) G. M. Schneider, Vol. 2, Chapter 6; (c) F. Franks, Vol. 2, Chapter1 ;(d) M. J. Blandamer and M. F. Fox,VoI. 2, Chapter 8; (e) M. J. Blandamer, Vol.2, Chapter 9; cf)M. D. Zeidler, Vol. 2, p. 564; (g)F. Franks and D. S. Reid, Vol. 2, Chapter3; (h) H. Friedman and C. V. Krishnan, Vol. 3, Chapter 1. ‘Hydrogen-Bonded Solvent Systems’, ed. A. Covington and P. Jones, Taylor and Francis, London 1968; (a) F. Franks, p. 31; (b) J. B. Hyne, p. 99. Blandamer and Burgess Excess functions express the extent to which the properties of a given mixture differ from those of the corresponding ideal mixture at the same temperature and pressure. Those aqueous mixtures for which GEis positive and I TSE I >I HE I, (i.e. dominant entropy of mixing) are called typically aqueous, TA; acetone + water and ethyl alcohol + water mixtures are examples of this class. These mixtures may separate at a lower critical solution temperature, LCST, e.g. triethylamine + water.Critical solution temperatures of aqueous mixtures are very sensitive to added solutes, and the tendency should be borne in mind for a given mixture to undergo phase separation (appear cloudy) when reactants are added. Some TA mixtures, while completely miscible at ambient pressure, will undergo phase separation if the pressure is changed.5b The properties of TA mixtures are particularly sensitive to the mole fraction of the non-aqueous co-solvent,x2, but within this group, some clear-cut patterns have emerged.se-s# At low mole fractions, a TA solvent exerts a water ‘structure-forming’ action, the solvent co-spheres around each solute overlapping and mutually enhancing water-water interactions.As more co-solvent is added, the mole fraction exceeds a particular mole fraction, x2*, where there is insuffcient water to maintain a three-dimensional hydrogen-bonded network of water molecules. Localized attempts to regain this arrangement produce concentration fluctuations (i.e. an overall water ‘structure-breaking’ action), the effect being maximal at a mole fraction XZ**. At this mole fraction, the system has the largest tendency to undergo phase separation. Indeed, it would be interesting to know if the kinetics in aqueous systems close to phase separation at a critical point [i.e. near an upper critical solution temperature (UCST) or lower critical solution tempera- ture (LCST)] are in any way anomalou~.~ The two mole fractions x2* and x2** depend both on co-solvent and temperature.Further, their precise values depend on the property of the mixture under examination. Nevertheless, these values do provide a useful signpost for effects in kinetic data. At 298 K approximate values of x2* and XZ** are 5c 0.09 and 0.2 (ethanol), 0.06 and 0.18 (isopropyl alcohol),0.04 and 0.1 (t-butyl alcohol), and 0.06 and 0.35 (acetone), respectively. For tetrahydrofuran (THF), x2** fi 0.3. Dioxan is often used as an organic co-solvent because the relative permittivity of this aqueous mixture can be varied over a wide range, 2 c Er .c 78. However, the properties of this mixture sometimes conflict with, for example, the generalizations discussed above. Thus dioxan-water is a TA system but some authors* suggest that dioxan is a water structure-breaker. The latter conclusion may be consistent with the observation9 that the kinetic solvent isotope effect, k(DzO)/k(HzO), for the solvolysis of t- butyl chloride tends to unity when dioxan is added, although this behaviour is not observedlO for methanesulphonyl chloride.Aqueous mixtures where I HE I > I TSE I, i.e. enthalpy-controlled mixing, J. C. Wheeler, Ber. Bunsengesellschaftphys. Chem., 1972, 76, 308.* R. K. Mohanty and J. C. Ahluwalia, J. Solution Chem., 1972, 1, 531. W. C. Craig, L. Hakka, P. M. Laughton, and R. E. Robertson, Canad. J. Chem., 1969, @ 41, 2118. loM. L. Tennet and A. N. Hambly, Austral. J. Chem., 1970,23,2427, 2435. 59 Kinetics of Reactions in Aqueous Mixtures are called typically non-aqueous,6a TNA.In some TNA mixtures, GE is positive, TNAP, e.g. acetonitrile + water.ll However, for some TNA mixtures, GE is negative, TNAN, e.g. hydrogen peroxide + water12 and dimethyl sulphoxide (DMSO) + water.l3 In the TNAP mixtures, the co-solvent exerts a depoly- merizing effect on water (cf. the effect of carbon tetrachloride on methyl alcohol) and in this sense is a structure breaker. Such mixtures may undergo phase separation at a UCST. In the TNAN mixtures, intercomponent association occurs (cf.the effect of chloroform on acetone). This intermolecular association in, for example, DMSO + water5f leads to a breakdown of water-water inter- actions. (See also hydrogen peroxide + water14).Indeed, analysis of the kinetics of hydrolysis of acetals in DMSO + water mixtures prompted the conclusionl5 that these mixtures can be divided into three regions, 0 < x2 ;5 0.3, 0.3 5 x2 5 0.45, 0.45 5 x2 < 1. In the second region, the properties of the system are dominated by 2:l H2O :DMSO associated species (cf. solubility and thermody- namic data16). The volumetric properties of aqueous mixtures1' are important in the analysis of volumes of activation. Generally, the relative partial molar volume of a TA co-solvent has a minimum close to x2*, but that for a TNA co-solvent shows no minimum at low mole fractions.6a Solute-Water Interactions.-The low solubility of non-polar solutes [i.e. AG * (gas-taqueous solution) > 01 in water is a consequence of a large negative value for TdSe even though AH* is negative (i.e.exothermic). The current view is that in water these solutes enhance water-water interactions and so are structure forrners.5e The organization of the water molecules around a non-polar solute may resemble that found in clathrate hydrates, the enhancement of water-water interactions being responsible for the positive partial molar heat capacity of the solute, which increases as the hydrophobic content of the solute increases.18 A similar structure-forming action is attributed to large hydrophobic ions, e.g. tetra-alkylammonium ions where R is larger than ethyl. However, ions, in general, by virtue of intense ion-solvent interactions, exert a structure-breaking A.L. Vierk, 2.anorg. Chem., 1950,201,283;K.W. Morcom and R. W. Smith, J. Chem. Thermodynamics, 1969,1, 503. 1s G. Scatchard, G. M. Kavanagh, and L. B. Ticknor, J. Amer. Chem. SOC., 1952,74, 3715; A. G. Mitchell and W. F. K. Wynne-Jones, Discuss. Faraday SOC., 1953, 15, 161 ; P. A. Giguere, 0. Knop, and M. Falk, Canad. J. Chem., 1958,36, 883. la H. L. Clever and S. P. Pigott, J. Chem. Thermodynamics, 1971, 3, 221; J. Kenttamaa and J. J. Lindberg, Suomen Kem., 1960, 33B, 98. IcJ. H.Stern and W. R. Bottenberg, J. Phys. Chew., 1971, 75, 2229. Is R.K.Wolford, J. Phys. Chem., 1964, 68, 3392. laE.A.Symons, Canad. J. Chem., 1971,49,3940;D. D. MacDonald and J. B. Hyne, Canad. J. Chem., 1971, 49, 611. 17 R. Battino, Chem. Rev., 1971, 71, 5.10 D. M. Alexander and D. J. T. Hill, Austral. J. Chem., 1969, 22, 347; E. M.Arnett, W. B. Kover, and J. V. Carter, J. Amer. Chem. SOC., 1969,91,4028;R. D.Wauchope and R. Haque, Canad. J. Chem., 1972,50, 133. Blandamer and Burgess influence on water beyond a nearest neighbour layer of electrostricted water molecules.1Q 3 Analysis of Kinetic Data According to transition-state theory,20 the rate constant at fixed temperature and pressure is related to dG*,the difference between chemical potentials of transi- tion state, p*, and reactant(s), p *, in their solution standard states; thus for a first-order reaction, dG* = pf -p; where species 3 is the reactant. Standard thermodynamic operations on dG* lead to the definition of isobaric (AH* and AS* from the temperature variation) and isothermal (AV* from pressure variation) parameters.Further operations lead to the definition of a heat capacity of activationd Cp*(differentiationofdH* with respect to temperature), the isothermal compressibility coefficients of activation dKT* (calculated from the dependence of A V* on pressure), and the isochoric quantity dUv*.Ob-viously, more caution is required in the derivation of the latter three quantities than the former three2’ The various Maxwell equations provide additional links between the activation parameters.22 The experimental data comprise values of the rate constants k for a given reaction at discrete values of temperature, pressure, and co-solvent mole frac- tion, x2.The dependence of k on T at fixed p and x2 and on p at fixed T and x2 is analysed using a similar methodology to that used in the analysis of equi-librium donstants, K.Many methods have been proposed, including expressing the dependence of k (or K) on T using equations which are polynomials in T, polynomials in T -8, where 8 is a reference temperature,23a and using ortho- gonal polynomials23b or spline functions.23c Various empirical expressions have been used to represent the dependence of rate constant on pressure. When the dependence of rate constant on either T or p has been described satisfactorily, the activation parameters can be calculated at selected values of either T or p, respectively. 4 Thermodynamic Transfer Functions The calculated activation parameters AX* for a given reaction depend on x2 and are assumed to be continuous functions of x2.The change in dX* on going from a solution in water to one in a binary solvent mixture can be represented by &AX*[= dX*(x2) -dX*(x2 = O)], where 6m is the solvent operator.24 M. J. Blandamer, Quarr. Rev., 1970, 24, 169; W.-Y. Wen, J. Solution Chem., 1973, 2, 253; T. S. Sarma and J. C. Ahluwalia, Chem. Sac. Rev.,1973,2,203. lo S. Glasstone, K. J. Laidler, and H. Eyring, ‘The Theory of Rate Processes’, McGraw-Hill, New York, 1941. *l G. Kohnstam, Adv. Phys. Org. Chem., 1967,5, 121. la(a) K. J. Laidler, Discuss. Faraday SOC.,1956, 22, 88; (b) S. J. Dickson and J. B. Hyne, Canad. J. Chem., 1971,49,2394. l3(a)E. C.W. Clarke and D. N. Glew, Trans. Faraday SOC.,1966, 62, 539; (b) D. J. G. Ives and P. D. Marsden, J. Chem. SOC.,1965, 649; (c) S. Wold, Acta Chem. Scand., 1970, 24, 2321 ;J. Phys. Chem., 1972,76, 369. J. A. Leffler and E. Grunwald, ‘Rates and Equilibria of Organic Reactions’, Wiley, New York, 1963. Kinetics of Reactions in Aqueous Mixtures Because dX* depends on x2, the corresponding properties of transition and initial states must differ in regard to their own dependence on x2. If the measured rate constant is first-order, then 8mX* # SmX;, although either S,X* or SmX; may be zero. The co-solvent may change the properties of a solute (reactant or transition state) in water by direct co-solvent-solute interaction. For example, the dependence of activation enthalpies on composition for the solvolysis of benzyl chloride and derivatives in water + ethyl alcohol mixtures can be attributed to changes in specific solvent-reactant and solvent-transition state interaction^.^^ However, equally striking changes in the properties of a solute may result from modifications of water-water interactions produced by the co-solvent, i.e.an indirect effect. In addition, where the reactant and co- solvent are, to a large degree, hydrophobic, they may associate by hydrophobic bonding.26 If the measured rate constant is second-order, and the reaction mechanism simple bimolecular, the dependence of 8dX* on x2 must be considered with reference to the three quantities SmX*, SmX:, and SmX:, where dX* = X* -(X; + X:), species 3 and 4 being the two reactants.The quantities SmX* and 8mX" are thermodynamic transfer functions, representing the effect of co-solvent on a particular partial molar property. Clearly, analysis of the factors contributing towards a given &AX* requires information concerning the individual transfer functions. By definition, it is not possible to measure independently SmX*. Indeed, as the composition of the solvent mixture changes, both the energy and the nature of the transition state (e.g.position along reaction co-ordinate) may change. Such changes may produce a change in the nature of the products. Thus in the solvolysis of t-butyl chloride at 298 K, the percentages of alkene produced are 6.2 and 52 when x(DMS0) = 0.134 and 0.512, re~pectively.~~However, these observations do not necessarily require a gross change in the transition state because the product may be determined in another stage of the reaction after the transition state has been passed, i.e.in a product-determining step. It is an unfortunate aspect of this subject that so much of the discussion must necessarily centre around the nature and energy of an elusive and ephemeral entity called the transition state/acti- vated complex. However, attempts have been madeto estimateamX* by measuring SmX" for a solute which is thought to resemble the transition state. Thus value of SmV" for various salts have been measured in order to estimate 8,V* for the transition states involved in the hydrolysis of alkyl and substituted-benzyl chlorides.2* Where reactants are concerned, these conceptual difficulties do not, of course, arise.Even where a reactant is labile, measurements of certain SmX: quantities have been made, although special techniques must be used. Thus, using a specially designed calorimeter, 8mH" values for t-butyl chloride in water + ethyl 85 J. B. Hyne, J. Amer. Chem. SOC.,1960,82,5129;J. B. Hyne, R. Wills, and R. E. Wonkka, ibid., 1962,84,2914;J. B. Hyne and R. Wills, ibid., 1963,85, 3650. 26 W. Kaumann, Adv. Protein Chem., 1959, 14, 1. 27 K. Heinonen and E. Tommila, Suomen Kem., 1965,38B,9. 28 I. Lee and J. B. Hyne, Cunud.J. Chem., 1968,46,2333; 1969.47, 1437. Blandamer and Burgess alcohol mixtures were determined before significant hydrolysishad 0~curred.~~p~0~ A method for estimating instantaneous values of SmV* for reactants in aqueous mixtures has been described31 In other cases, the properties of the reactants can be conveniently measured because the conditions (e.g.pH, temperature) have been so adjusted that no or negligible reactions occur. Where measurements either cannot or have not been made, it is often possible to draw tentative conclusions concerning the trends in SmX" for a reactant from the behaviour of solutes which resemble the reactants. Thus in the analysis of solvent effects on alkyl halides, which are generally hydrophobic solutes, information concerning the solubilities of hydrocarbons and other gases in pure solvent^^^^^^ and in aqueous mixtures33p3* can be most useful.Hydrophobic solutes are more soluble in an organic co-solvent than in water. Consider the case of ethane in ethanol + water mixtures.33 The change in chemical potential for ethane, 6,p e, on going from pure water to pure ethanol is negative, although the transfer is endothermic, &He > 0. Nevertheless, SmSe is > 0, the change in entropy being more dramatic, i.e. T6mS" > SmHe.The data show another important feature. Thus in the aqueous mixtures, amp" changes almost regularly with increase in x2, with only small deviation from a linear interdependence. However, SmHe and SmSe for ethane deviate considerably from such a simple pattern, e.g. SmH" is much larger in the water-rich aqueous mixtures than might other- wise be predicted.At this stage it is possibly unwise to advance a firm prediction. However, we might tentatively conclude that when an organic co-solvent is added to a hydrophobic reactant in water, there will be a tendency for LAG* to increase (i.e. rate constant to decrease) because this reactant will be stabilized, i.e. amp; < 0. Further, this behaviour will contribute to a more marked change in TsmAS* than in S,AH*. We will return to these considerations in Sections 7 and 8. Thermodynamic transfer functions for salts from water to other pure solvents have been calculated for a wide range of systems. Unfortunately, in the analysis of kinetic data, transfer functions for single ions are usually required, and some assumption must be made before these can be obtained from the values for salts.5h Single ion transfer functions have been calculated35 for seven pure co- solvents (using as a reference the tetramethylammonium ion) and so provide some indication of the change expected in the properties of ionic reactants and transition states between the extremes xz = 0 to x2 = 1.For the large hydro- IoE. M. Amett, W. G. Bentrude, J. J. Burke, and P. M. Duggleby, J. Amer. Chem. Soc., 1965, 87, 1541; E. M. Amett, P. M. Duggleby, and J. J. Burke, ibid., 1963, 85, 1350; E. M. Arnett and D. R. McKelvey, ibid., 1965, 87, 1393; 1966, 88, 5031. ao 'Physico-Chemical Processes in Mixed Aqueous Solvents', ed. F. Franks, Heinemann, London, 1967; (a)E. M. Arnett, p.105; (6) D. Feakins, p. 71. )* H. S. Golinkin, I. Lee, and J. B. Hyne, J. Amer. Chem. Soc., 1967, 89, 1309. )lR.Battino and H. L. Clever, Chem. Rev., 1966, 66, 395; E. Wilhelm and R. Battino, ibid., 1973,73, 1. M. Yaacobi and A. Ben-Naim, J. Solution Chem., 1973,2,425; J. Phys. Chem., 1974,78, 175. arA.Ben-Naim and S. Baer, Trans. Faruduy Soc., 1964, 60, 1736; A. Ben-Naim, J. Phys.Chem., 1965,69, 3245. M. H. Abraham, J.C.S. Faraday I, 1973, 69, 1375. 63 Kinetics of Reactions in Aqueous Mixtures phobic ions, e.g.Bun4N+ and PU-, 8weis negative for transfer from water into N-methylformamide, methanol, ethanol, DMSO, acetonitrile, DMF, and acetone. Transfer of small anions, e.g. chloride, bromide, and iodide, is distinctly unfavourable,amp > 0, but 8mp for small cations depends on solvent and ion, being nevertheless much smaller in magnitude. The entropies of transfer for small ions, e.g.K+ and C1-, are generally negative, but those for large hydro- phobic ions, e.g. Bun4N+ and PU-, are positive. Of course, in the present context, it is preferable to consider the transfer function for ions as a function of co-solvent mole fraction. These have been measured for certain salts in selected aqueous mixtures, and different methods of calculating single ion values have been examined.30b In methyl alcohol and water mixtures, 8w* is positive for small anions, e.g. Cl- and OH-,36 and negative for small cations,30* e.g. H+, although there is some disagreement between reported values for alkali-metal ions.37 Similar trends are reported for ions in dioxan, acetone, and ethyl alcohol aqueous mixtures.38 Plots of 6,p* for salts against xz do not show, in general, marked extrema (for one example of an exception see hydrogen iodide in methanol + water30b). However, these are clearly seen in the plots of the corresponding enthalpy and entropy quantities (e.g.hydrogen chloride in aqueous methanol and aqueous ethanol39). The corresponding plots of amp* for ions against x2 again show no marked extrema as a general rule, although there are exceptions, e.g. H+in methanol + water3' and Ph4B- in acetone + ~ater.3~~ The partial molar entropies of small ions (e.g. Li+, Na+, C1-, Br-) increase when small amounts of methanol are added to aqueous solutions of these ions, but then decrease steadily, to values more negative than in water, as the mole fraction of methanol increases.These deviations from a simple linear inter- dependence between entropy and mole fraction are largest when SEfor the solvent mixture is most negative.40 The possibility of linking the properties of a solute in a binary aqueous mixture with the thermodynamic excess function will be mentioned again in Section 7. Information concerning thermodynamic transfer functions for alkali-metal and halide ions can be obtained from n.m.r. shift measurements on the respective nuclei; this technique may prove important in the present context.41 Again it is probably too simplistic to draw general conclusions from the pattern identified here for ions.Nevertheless some predictions can be made. For example, in alkaline hydrolysis reactions, addition of an organic co-solvent would appear to destabilize OH-ion, and this will contribute to a decrease in 6dG* and thus an increase in rate constant. In contrast, the rate of a reaction involving H. Rochester, J.C.S. Dalton, 1972, 5. 37 C. F. Wells, J.C.S. Furaday I, 1973, 69, 684; D. Feakins and P. J. Voice, ibid., 1972, 68, 1390; A. L. Andrews, H. P. Bennetto, D. Feakins, K. G. Lawrence, and R. P. T. Tomkins, J. Chem. SOC.(A), 1968, 1486. 38 (a) H. P. Bennetto, D. Feakins, and K. G. Lawrence, J. Chem. SOC.(A), 1968, 1493; (b)D. Bax, C. L.de Limy, and A. G. Remijnse, Rec. Trav. chim., 1972,91,965, 1225. 8B J.H. Stern and S. L. Hansen, J. Chem. and Eng. Data, 1971,16, 360. C. M.Criss, R. P. Held, and E. Kuksha, J. Phys. Chem., 1968, 72, 2970; F. Franks and D. S. Reid, ibid., 1969, 73, 3153. I1 A. K. Covington, K. E. Newman, and T. H. Lilley, J.C.S. Faraduy I, 1973, 69, 973. Blandamer and Burgess a complex ion where the ligands are hydrophobic will tend to fall because the co-solvent stabilizes such ions. Such conclusions must be drawn carefully. It is noteworthy, for example, that &He for a given solute in a particular mixture often bears little relation to the sign and magnitude for ampeof the same solute, so that &He alone is of little value in predicting trends in the rate constants.42 Finally, it is important when using these thermodynamic transfer functions in the analysis of kinetic data to pay close attention to the definition of the solution standard states.5 Analysis of Solvent Effects in Kinetics So far we have attempted to establish that in coming to an understanding of the patterns shown by &dX* quantities for reactions in aqueous mixtures, the often unique properties of both aqueous solutions and aqueous mixtures should be borne in mind. For example, in aqueous systems, we have seen that the entropy changes are extremely important, controlling in some cases the sign and magnitude of the change in Gibbs function. We have also noted that in inter- preting a given 8,dX*, some information is required concerning the corres- ponding changes in the initial and transition states.sb These generalizations are now illustrated by reference to examples taken from quite an extensive literature.We start our discussion with an examination of trends in dG* because this is directly calculated from the primary observable, the rate constant, and because interpretation of &dG* illustrates most of the points made above. 6 Rate Constants and SmdG* dG*is generally a 'well-behaved' function -it usually changes smoothly and gradually as the solvent composition changes. This behaviour contrasts with those of dH* and AS*; these quantities often vary in, at first sight, an erratic manner but usually in such a way as to minimize changes in dG*.The depen- dence of dG* on solvent composition has been reported for a wide range of organic and inorganic reactions in a variety of mixed aqueous solvents. However, the distribution of such mixed solvent systems between the classes discussed in Section 2 is highly biased towards TA mixtures; only one TNAN co-solvent (DMSO), and only one "NAP co-solvent (acetonitrile) have been extensively used in kinetic studies.The lack of any real success in correlating rate constants with dielectric properties of solvent mixtures prompted the development of the more generalized concept of solvent polarity.43 Thus solvent Y values,44 defined in terms of solvent effects on rates of SN~solvolysis of t-butyl chloride, have been used to correlate effects of solvents on rate constants for other more or less related reactions.Such correlations have also been used to probe the mechanisms of a E. M. Arnett and D. R. McKelvey, J. Amer. Chem. SOC.,1966, 88,2598. 43 C. Reichardt, Angew. Chem. Internat. Edn., 1965,4,29. E. Grunwald and S. Winstein, J. Amer. Chem. SOC.,1948, 70, 846. Kinetics of Reactions in Aqueous Mixtures variety of reactions, in inorganic (e.g. solvolysis of halides of phosphor~s,~5q sulphur,450 and b0ron45~) and organ~metallic~~ as well as in organic chemistry. These correlations involving Y values are examples from the many linear free- energy relationships in chemistry. Underlying the Y value concept is the require- ment that solvent and solute interact through a single mechanism.24 Although the use of solvent Y values has permitted the correlation of kinetic results in certain areas of chemistry, further generalization and codification of reactivities in mixed aqueous media are difficult.Nonetheless some regular patterns can be discerned. Thus rates of S"1 solvolysis of organic halides44 and of ld aquations of cobalt(nI)-ammine or -amine halide complexes47 always decrease as the proportion of a TA non-aqueous component increases. The same is true for the TNAP co-solvent acetonitrile,48,49 and even for the TNAN co- solvent DMS0.27Only for the inorganic TNAN co-solvent hydrogen peroxide is there an increase in rate (for ButCl solvolysis) with increasing amount of non-aqueous co-~olvent.~O The aquations of chromium(Ir1) and of rhodium-(111)~~~chloride complexes, arguably at least partly associative in character, follow the same pattern in TA and TNAP solvents (their behaviour in "NAN solvents remains to be investigated).Examples of reactions whose rates increase with increasing proportion of non-aqueous component are much rarer ;51they include SN~decarboxylation of some cyanoacetates51a (in mixtures containing the TA co-solvents ethanol or dioxan), and the SN2 aquation of Me3NSO3 [aqueous acetone (TA) and DMSO (TNAN)].51b Patterns of kinetic behaviour for the solvolysis and base hydrolysis of esters and related compounds are varied -most of the (few) examples of maxima or minima in k vs. x2 plots come from this type of reaction.52 There is surprising variation in kinetic pattern with the nature of the leaving group for the aquation of the iron@) complexes of various substituted 1,lO-phenanthrolines.48~53 In aqueous t-butyl alcohol and in aqueous ethyl alcohol mixtures (TA co-solvents), the rate of aquation of the 5-nitro-complex increases markedly with increasing x2, in aqueous acetonitrile (TNAP) the rate increases less markedly, whilst in aqueous formic acid the rate decreases with increasing 46 (a)E.W. Crunden and R. H. Hudson, J. Chem. SOC.,1962,3591;(6) 0.Rogne, J. Chem. SOC.(B), 1969, 663; (c) J. R. Lowe, S. S. Appal, C. Weidig, and H. C. Kelly, Inorg. Chem.,1970,9, 1423. 40 See e.g. F. E. Smith and I. S. Butler, Canad. J. Chem., 1969, 47, 131 1 ;W.J. Bland, J. Burgess, and R. D. W. Kemmitt, J. Organometallic Chem., 1965,15, 217; 1969, 18, 199.(a) C. H.Langford, Znorg. Chem., 1964, 3, 225; (b) J. Burgess and M. G. Price, J. Chem. SOC.(A), 1971, 3108. J. Burgess, J. Chem. SOC.(A), 1970,2351. 4D R. E. Robertson and S. E. Sugamori, J. Amer. Chem. SOC.,1969,91, 7254; Canad. J. Chem., 1972,50, 1353. M. J. Blandamer and J. R. Membrey, J.C.S. Chem. Comm., 1973, 514. s1 (a) A. Thomson, J. Chem. SOC.(B), 1970, 1798; (b) J. H. Krueger and M. A. Johnson, Inorg. Chem., 1968, 7, 679. st See e.g. (a) E. Tommila, A. Koivosto, J. P. Lyrra, K. Antell, and S. Heimo, Ann. Acad. Sci. Fennicae, MI, 1952, 47, 3; (6) E. Tommila, Suomen Kem., 1964, 37B, 117; (c) R. K. Wolford, J. Phys. Chem., 1963, 67, 632; (d)R. L. Foon and A. N. Hambly, Ausfral J. Chem., 1970,23,2427, 2435. 63 J. Burgess, J.Chem. SOC.(A), 1968,1085; 1969, 1899. Blandamer and Burgess x2. In contrast, the rate of aquation of the 4,7-dimethyl complex [like aquation rates for, e.g., cobalt(rrr) or chromium(rrr) halide complexes] decreases with increasing x2 no matter what co-solvent. It is interesting that in t-butyl alcohol + water mixtures, the effects of ligand substitution and counter ion on reactivi- ties are small below x2* but rapidly become more marked beyond that composi- tion.53 Having established these rough generalizations, we now wish to review how these variations can be accounted for. For the most part our analysis must be qualitative because much of the information required for a quantitative analy- sis is not available. The increase in &dG* with xz for hydrolysis of alkyl halides when organic co-solvents are added (e.g.ethyl alcohol, t-butyl alcohol, THF, or acetonitrile to t-butyl chloride in ethyl alcohol to benzyl chloride or derivatives in ~ater,~5 dioxan to alkyl esters of benzenesulphonic acids,55 and acetone to alkyl halides in water56) can be attributed in part to the increased solubility of these hydrophobic solutes in the mixture, i.e. Smpe < 0. Indeed, solubility data show that for t-butyl chloride in ethanol-rich mixtures, ampeis negative and accounts for more than one-half of the increase in dG*.54 Similarly, in the bimolecular (SE2)reaction between tin tetra-alkyls and mercuric chloride in methanol-rich aqueous mixtures, the larger part of the increase in dG* with increase in x2 can be attributed57a to a stabilization of both reactants.A similar conclusion57b was reached for the Menschutkin reaction of trimethylamine with methyl iodide, but in the case of the reaction between lead tetra-awls and iodine, changes in the chemical potential of the transition state are import- ant.57 However, here interpretation is not straightforward because there is some ambiguity over the structure, i.e. open or cyclic, of the transition state. In some cases, the chemical potentials of the reactants may change in quite different ways when the co-solvent is added. The chemical potential of OH- in water is dramatically increased when DMSO is added58 so that, despite an expected stabilization of benzyl chloride (see below), the rate constant for alkaline hydrolysis of benzyl chloride increases when DMSO is added.59 A similar explanation may account for the increase observed in the rates of alkaline hydrolysis of methyl iodide60a or of fluorobenzene and derivatives,60b and in the reaction between thiosulphate and benzyl chloride,60C anions being generally destabilized by added DMSO.61 In other cases the effect of added co-solvent is determined by the change in the chemical potential of the transition state.Thus I4S. Winstein and A. H. Fainberg, J. Amer. Chem. SOC., 1957, 79, 5937. s6 E. Tommila and E. Merikallio, Suomen Kem., 1953, 12B,79. s6 E. Tommila, M. Tiilikainen, and A. Voipo, Ann. Acad. Sci. Fennicae AZI, 1955, 65, 3. 87 M. H. Abraham, (a)J. Chem. SOC.(A), 1971, 1067; (6)J.C.S. Chem. Comm., 1969, 1307; (c)J.C.S. Perkin 11, 1972, 1343. s* A. K. Das and K. K. Kandu, J.C.S. Faraday I, 1973, 69,730. 6D E. Tommila and I. P. Pitkainen, Acfa Chem. Scand., 1966, 20, 937. 6o (a) J. Murto, Suomen Kern., 1961, B34,92; (b) J. M. Murto and A. M. Hirro, ibid., 1964, B37, 177; (c) K. Kalliorinne and E. Tommila, Acta Chem. Scand., 1969,23,2567. 81 J. Courtot-Coupez, M. L. DtmCzet, A. Laauenan, and C. Madoc, J. Elecfroanalyt. Chem., 1971, 29,21. Kinetics of Reactions in Aqueous Mixtures the addition of DMSO, acetonitrile, or dioxan to ethyl acetate in water produces a stabilization of the ester,O2 i.e. ampe c 0. In dioxan + water, ampe (ester) is approximately -1.5 kJ mol-l at x2 = 0.5. However, amp* for the hydroxide ion is positive,3sa approximately +5.54 kJ mol-l when x2 = 0.5.Thus, taken to- gether, these require that the rate constant should increase. However, the reverse trend is observed, dmdG* = +0.19 kJ mol-l, and so amp* for the transition state is approximately +4.2 kJ mol-1. It appears that while ampe(OH-) is usually positive when a co-solvent is added, it is not sdliciently large (except in the case of added DMSO) to determine the change in rate constant except in co-solvent-rich mixtures. Thus the complicated pattern referred to above emerges,52 depending on whether the trends in initial or transition states are dominant. For example, in contrast to the effect of DMSO, added acetone59 or dioxan63a result in a fall in the rate constant for the alkaline hydrolysis of benzyl chloride.Similarly, the rate constant for the alkaline hydrolysis of methyl acetate decreases when methanol is added63b even though amp "(OH-) is again positive in this mi~ture.3~ The marked enhancement of the rate of alkaline hydrolysis of esters by added DMSOhas attracted considerable attention.5lbp52bv64 Thermodynamic results indicate that e(H+) is generally negative when organic solvents are added to H+ in water. However, the use of this information in rationalizing reactivity patterns for the acid-catalysed hydrolysis of estersy62b*65 is complicated by the two-stage nature (rapid equilibrium of ester and proton followed by rate-determining dissociative or associative solvolysis of the pro- tonated ester) of the reaction mechanism.Another pattern which emerges from the available data concerns a possible link between GE for the binary mixture and the trend in 8dG*. It is noteworthy, for example, that in the hydrolysis of t-butyl chloride, SmdG* at a fixed mole fraction in the TA aqueous-rich mixtures increases as GE increases at that mole fraction. Results for solvent mixtures containing the TNAP solvent acetonitrile can be included in this pattern but those for mixtures containing DMSO, a TNAN system, cannot. If for this reactant the increase in SmdG* can be solely attributed to a stabilization of the initial state, a clear-cut correspondence be- tweenamdG* and GEwould only be expected if the solubilities of t-butyl chloride in the co-solvents were the same.If the mixture were ideal (i.e. GE = 0), then amp * (ButCl) and thus &dG* would be linear functions of x2. In fact the solu- bilities of hydrocarbons, e.g. ethane and methane, in alcohols, whilst markedly different from those in water, are, within themselves, very close. Further, an extra-thermodynamic analysis indicates that, at least qualitatively, deviations from this ideal behaviour can be related to Pfor the binary mixture.66 Thus if Oa B. G. Cox, J.C.S. Perkin ZI, 1970, 607. O3 (a) H. Sadek, F. M. A. Halim, and E. Y. Khalid, Suomen Kem., 1963,36B, 141; (b) E. Tommila and S. Maltamo, ibid., 1956, 28B, 73. O4 (a)E. Tommila and M.L. Murto, Acta Chem. Scand., 1963,17, 1947; (6) E.Tommila and I.Polenius, ibid., 1963, 17, 1980. Or, (a)P. T. McTigue and P. V. Renowden, Austral. J. Chem., 1970, 23,297; (b) E. Tommila and A.Hella, Ann. Acad. Sci. Fennicae AIZ, 1954,53, 3. ee J. P. O'Connell and J. M. Prausnitz, Ind. and Eng. Chem. (Fundamentals), 1964,3, 347. Blandamer and Burgess GE is positive the solute should be more soluble in the mixture than predicted from the solubilities in the two pure solvents, and several examples for aqueous mixtures bear this This simple analysis, by being applied to a hydrophobic initial state, may account in part for the fact that the increase in dG* when a monohydric alcohol is added to t-butyl chloride has the order ButOH > PrQH > EtOH and also why, when H202 is added, dG* decreases.Similarly, dG* for the hydrolysis of methyl trifluoroacetate and of chloromethyltrichloroacetate increases more rapidly when either acetone (TA) or acetonitrile (TNAP) are added than when DMSO (TNAN) is added.68 Again dG* for the hydrolysis of benzyl chloride increases more markedly when acetone, ethyl alcohol, or dioxan are added than when DMSO is added.69 There are insufficient results for other reactions and other solvent systems to press this point further. However, the reactions of aquonickel(1x) with 2,2-bipyridyl in ethano1,'Ob and t-butyl alcohol70b mixtures seem to show a similar correlation between LAG* and GE (though we have too few values at present to be certain). Current work in our laboratories indicates that rate constants for the dissociative aquation of the Fe(S-NO~phen)3~+ cation in a wide range of aqueous mixtures conform to this pattern, This is an interesting aquation reaction because when TA co-solvents are added, 8,d G* decreases as GE increases.The quite different behaviour from t-butyl chloride hydrolysis can nevertheless be explained in a very similar fashion. For the iron complex, however, the transition state is more hydrophobic than the initial stateY7l so that here the transition state is stabilized by added co- solvent in the same way that the initial state of t-butyl chloride is stabilized. These qualitative links between &dG* and P,indeed between SmdX* and Pin general, have received support from a more quantitative analy~is.~ Under-lying the difficulties of relating kinetic data, e.g.&AX, to the properties of the binary aqueous mixture, e.g. P,is the awareness that the properties of the two components and, consequently, of the mixture will be modified where reactants are added. A quantitative method has been ~uggested,~ therefore, of transforming the derived activation parameter dG* for reaction in the binary mixture to an endostatic quantity dG,*. The latter is calculated for a system (binary mixture + reactants + transition state) in which the ratio of the thermodynamic activities of the two components, a1/a2, remains the same as in the binary mixture. Further expressions are obtained for related endostatic activation quantities, e.g. AH^* and d V,*, which involve the appropriate partial molar properties of the two components of the binary aqueous mixture.This analysis has only been applied to one system so farY4namely the hydrolysis of t-butyl chloride in ethanol-water, so it is too early to judge the efficacy of this analytical method. It is not clear, for example, why or what significance can be read into the observation that the 67 (a) T. T. Herskovits and J. P. Harrington, Biochemistry, 1972, 11,4800; (b) C.L.de Lignyand N. G. van Deer Keen, Rec. Trav. chim., 171,90,984. N.J. Cleve, Suomen Kem., 1972,45B, 235,285. ge E. Tommila, Acta Chem. Scand., 1966,20, 923. 70 (a)H.P. Bennett0 and E. F. Caldin,J. Chem. SOC.(A), 1971,2207;(b)P.K.Chattopadhyayand J. F. Coetzee, Inorg. Chem., 1973,12,113. 71 M.J. Blandaer, J.Burgess, and S. H. Morris, J.C.S. Dalton, 1974, 1717. Kinetics of Reactions in Aqireoris Mixtures dependence of AG,* on mixture composition is more complicated than that for dG* but that of AH,* is less complex than AH*. Nevertheless the proposals warrant serious attention in the context of both organic and inorganic reac- tants. 7 Trends in 8mdH* and SmdS* As noted above, the quantities SmdH and Tam&* often have similar sign and magnitude. Of course, they are rarely equal, otherwise the rate constant would be independent of x2. Again, this type of behaviour is not confined to kinetics.72 In the hydrolysis of neutral organic halides, the increase in dG*with increase in x2 generally stems from a dominant change in TSmAS*, the latter decreasing with increase in ~2,~~ Indeed, if e.g.alkyl halides in acetone + water mixt~res.~e the rate constant were a function of dH* only, then, for example, the rate constant for the hydrolysis of t-butyl chloride would increase when t-butyl alcohol was initially added. This dominant role of entropy changes is often a characteristic of aqueous solutions. In fact, following on from the importance of the changes in the properties of initial states in determining SmdG*, the quantity 6mS" for such states is expected to play an important part. For neutral solutes in water TASe (gas phase -water) is large and negative whereas in non- aqueous solvents, AHe is usually the important term. Thus as x2 increases, * should increase more rapidly than 8mH3 for the initial state, so con-tributing to an increase in S,AG*. However, for the decarboxylation of car- boxylic acid derivatives, bothdH* and AS* decrease as x2 increases when either ethyl alcohol or dioxan are added, but now AH* decreases more rapidly so dG* decrea~es.5~~In contrast, AH * increases as x2 increases for the hydrolysis of t-butyldimethylsulphonium ions in ethyl alcohol + water mixtures, with a maximum at x2 = 0.2, but no maximum is observed in t-butyl alcohol + water mixtures.25 However, &AH* shows a minimum for the hydrolysis of aryl- sulphonic esters in water containing methyl alcohol, ethyl alcohol, or isopropyl alcohol.73 A convenient summary of the data is obtained by plotting AH* against dS*.54970a Where the points fall on a single straight line, the slope is called the isokinetic temperature, but this behaviour is rarely observed over a wide range of x2 values. Often the points for the aqueous-rich mixtures fall close to a single straight line when x2 -c x2*, e.g.t-butyl chloride in alcohol + water mixture~.~Q Similarly, dV* appears to be a linear function of AS* for the hydrolysis of benzyl chloride in aqueous alcohols over this range.74 A linear dependence of AH* on dS* is observed on either side of the mole fraction at which AH+ is a minimum in the alkaline hydrolysis of ethyl Nevertheless a statistical 7a See e.g. W. Van der Poel and P. J. Slootmaekers, Bull. SOC.chim., belges, 1970, 79,223; 1971, 80,401. 73 J. B. Hyne and R. E.Robertson, Canad. J. Chem., 1956, 34, 931. 7b H. S. Golinkin and J. B. Hyne, Canad. J. Chem., 1968, 46, 125. Blandamer and Burgess analysis of trends in SmdH* and SmdS* is still required to ensure that these trends are not illu~ory.~5 As noted above, entropy quantities are important in aqueous solutions, so attention should be concentrated on 6mdS*, SmSt, and 6mS*. By their very nature, entropies are not directly measurable but are calculated from two other properties, the appropriate Gibbs and enthalpy functions. However, enthalpy data are often more readily obtained, so it is often more convenient to analyse trends in &AH*. In most systems, an extremum in SdS* occurs close to the mole fraction where SmdH* is an extremum. However, this is not always the case.Extrema in activation parameters, 6mdX*, do not always occur at the same mole fraction for a given reaction in a particular mixture. Thus an explanation for an extremum is &AH* cannot be immediately used to account for extrema in SmA V*, 8mdCp*, Smd Uv*,although the causes are often related because these usually stem from the same source, the properties of the solvent mixture. 8 Activation Enthalpies and SmdH* We have so far stressed that, in the analysis of activation parameters, the changes in the initial state cannot be overlooked, and that information concerning such changes can often be gained from other non-kinetic experiments. A striking example of both points concerns the analysis of &AH* for t-butyl chloride in ethyl alcohol + water mixtures.29~30a In this mixture, 8mHe for neutral solutes and salts increases when ethyl alcohol is added to a solution in water, passing through a maximum when x2 N 0.1 (cf.x2*) and then decreasing when x2 > x2*.The sharpness of the maximum and the extent of the endothermic shift depend on the solute. When t-butyl alcohol is used instead of ethyl alcohol, the trend is more dramatic, the endothermic maximum coming near xz N 0.04 (cf. x2*). Indeed, extrema in SmAH* (and SmdCp*) are especially marked in this mix- ture.49 With reference to t-butyl chloride in ethyl alcohol + water, at least 95 % of the minimum in 6mdH* near x2 = 0.1 stems from a maximum in SmHe for t-butyl chloride. Indeed the calculated 6mH* (= &dH* + *) is close to zero over the range 0 < xz < 0.4.The fact that SmH* is zero for this transition state is itself largely fortuitous. In another case, SmdH* might be zero because 8mH3e21 a&*, and this is thought to be the case in the hydrolysis of butyl- dimethylsulphonium ion in water + ethyl alcohol. Clearly a small change in SmdH* can mask quite marked changes in the enthalpies of initial and transition states. This behaviour is also observed following analysis of volumes of activa- tion (see below). Similar positive extrema in 6mH" for ethyl acetate in aqueous mixtures (cf. the behaviour of carboxylic acids76) may contribute to the negative extrema in 6,dH* for the alkaline hydrolysis of ethyl In the latter system, the intensity of the extremum decreases through the series t-butyl alcohol, isopropyl alcohol, acetone N ethyl alcohol, methyl alcohol, ethylene glycol, and increases 7b 0.Exner, Progr. Phys. Org. Chem., 1972, 10, 411 ; Coll. Czech. Chem. Comm., 1972, 37, 1425; 0. Exner and V. BerBnek, ibid., 1973, 38, 781; S. Wold and 0. Exner, Chemica Scripta, 1973, 3, 5. L. Avedikian, J. Juillard, J.-P. Morel, and M. Ducros, Thcrmochim. Acta, 1973, 6, 283. Kinetics of Reactions in Aqueous Mixtures as the hydrophobic nature of the ester increases.77 However, a detailed analysis requires information concerning amHe (OH-), which is unlikely to be zer0.3' The extent of the extrema in SmdH* for the reaction of nickel(I1) with 2,2'-bipyridyl in aqueous alcohols also decreases along the series t-butyl alcohol, ethyl alcohol, methyl alcohol; in each case the extremum occurs when x2 N x~*.~O (Parallel behaviour is shown by 6,dS*.) Extrema in 6mdH* are often much smaller in TNA than in TA mixtures.For example, extrema in &AH* and &dS* for hydrolysis of benzyl chloride in acetone + water mixtures are almost lost when acetone is replaced by DMS0.6D Similarly, the minimum in &AH* for the hydrolysis of methyl trifluoroacetate becomes less marked through the series of co-solvents acetone, acetonitrile, DMSO; in the latter case only a very shallow minimum is observed, near x2 = 0.3. However, a minimum in SmdH* for the hydrolysis of t-butyl chloride in acetonitrile + water mixtures is still clear-cut, although slightly smaller than when ethyl alcohol, THF, or t-butyl alcohol are co-solvents.The distinction between TA and TNA co-solvents is more obvious in 6mdCpf (see below). It is noteworthy that the extrema in &He for solutes in ethanol + water do not occur at the mole fraction where HEis a minimum77 nor where the relative partial molar enthalpy of ethanol is a maximum. Similarly, extrema in 6mdS* occur at a lower mole fraction than indicated by the minimum in SE.74 Although the reality of the endothermic maxima for solutes in ethanol + water is well established, its explanation is not obvious in view of the observation that nearly all solutes are affected in the same way. It appears that when a hydrophobic structure-former is the solute, the amount of water structure which can be enhanced by this solute decreases as more ethanol is added, soSmHefor the solute is positive.Alternatively, the amount of water structure which can be broken by a structure-breaker increases with increase in x2, and 60 6mH3e is again positive. Only in the limit that the solute exerts little effect on the structure of the system (cJMe4N+ Cl-), then 8mH" -+0. In all cases, this unique behaviour of the aqueous mixture disappears when xz > x2.* 9 Heat Capacity of Activation and 6mdCp* Although the dependence of dCp* on x2 has been measured for only relatively few systems, quite marked extrema have been observed (compare Figure 2). For example,4Q when t-butyl alcohol, isopropyl alcohol, ethanol, or THF (which form TA mixtures) are added to t-butyl chloride in water, 6mdCp* decreases initially with increase in x2, reaches a minimum when x2 21 x2*, and then increases.In contrast, when acetonitrile (TNAP) is added, Smd Cp* increases for the hydrolysis of t-butyl chloride49 and a series of sulphamoyl chl0rides.~8 A fairly clear-cut pattern emerges in the hydrolysis of methyl trifluoroacetate,68 where the addition of acetone (TA) produces an initial decrease in dCp*, 77 R. F. Lama and B. C. Lu, J. Chem. and Eng. Data, 1965, 10,216; J. A. Boyne and A. G. Williamson, ibid., 1967, 12, 216. 7sE.C. F. KOand R.E. Robertson, Canad. J. Chem., 1972, 50, 946; J. Amer. Chem. SOC., 1972, 94, 573. Blandamer and Burgess addition of acetonitrile produces an initial increase (extremum near x2 = 0.2), whereas addition of DMSO (TNAN) results in a more gradual increase in dCp* (see also hydrolysis of chloromethyl dichloroa~etate~~, 79).The complexity of dCp* quantities makes the interpretation dficult. It is noteworthy, however, that CpEvalues for alcohol + water mixtures show positive extremaso near x2 N x2*, these becoming more marked and at lower mole frac- tions along the series methanol, ethanol, isopropyl alcohol, t-butyl alcohol, (but CpE is much smaller for DMSO + water13). A large negative dCp* for the hydrolysis of t-butyl chloride in water is attributed2 to the S,l mechanism, which requires significant breakdown of water-water interaction around the solute in order to stabilize the developing cation.Thus with dCp* = Cp* -Cpe, in this case Cp* 21 0 and Cpe > 0. This is consistent with the idea concerning AH*, where it is argued that an important contribution to the activation process is the need to break down water-water interactions. This contribution will decrease with increase in temperature. Addition of a structure-forming co-solvent will produce a more thermally labile initial state, so that 8mCpe > 0, and then SmdCp* < 0 until x2 2 x2*. Addition of a structure breaker (cf: acetonitrile) produces the opposite effect, namely SmdCp* > 0. This interpretation places a great deal of emphasis on the initial state and, as in the analysis of &AH* for t-butyl chloride, may also rely on the particular properties of the ionic transition state for this reaction.If indeed GrnCp*N 0, the trend is for 8,CP of a hydro- phobic solute over the range 0 < xz < 1 to decrease because dCp (gas -+ water) for organic solutes > 0 but dCp (gas -+organic solvent) N 0. Conse-quently, 8mACp* should increase (i.e. dCp* change from negative to zero) and then the negative extrema in the aqueous systems reflect the unique character of these solvent mixtures. There is little information concerning the dependence on x2 of partial molar heat capacities of neutral hydrophobic solutes in aqueous mixtures. However, the partial molar heat capacities of large alkylammonium ions in water increase to a positive extremum at x2 N 0.4 when t-butyl alcohol is added81 (seealso behaviour for sodium tetraphenylb~ronate~~).This trend is a consequence of the hydrophobic character of these ionss2 and promotion of water-water interactions by the added alcohol. By way of contrast, heat-capacity data for these salts show that dioxan,8 ethylene glycol, and urea, break water- water interactions.8l In view of the extensive information available concerning aqueous solutions of urea,83 kinetic information for reaction in these solutions should prove interesting, although specific solute-urea interactionss4 may add new complications. N. J. Cleve and E. K. Euranto, Suomen Kem., 1964, 37B, 126. R. Arnaud, L. Avedikian, and J. P. Morel, J. Chim. phys., 1971, 45. *l R. K. Mohanty, S. Sunder, and J. C. Ahluwalia, J.Phys. Chem., 1972,76,2577;B. Chaula and J. C. Ahluwalia, ibid., p. 2582;J.C.S. Faraday I, 1973, 69,434. 8* Compare heat capacity of alkylammonium ions in non-aqueous solvents; C. de Visser and G.Somsen,J.C.S. Faraday I, 1973, 69, 1440. 83 H. S. Frank and F. Franks, J. Chem. Phys., 1968, 48,4746; E.G.Finer, F. Franks, and M.J. Tait, J. Amer. Chem. SOC.,1972,94,4424. 84 W.-Y. Wen and C. L. Chen, J. Phys. Chem., 1969,73, 2895. Kinetics of Reactions in Aqueous Mixtures 10 Volumes of Activation and SmA V* The extrema in Pand, more particularly, in the partial molar volumes of the co-solvent in a TA mixture indicate that extrema are to be expected in 6md V* quantities. Indeed in ethanol + water mixtures, AV* for the hydrolysis of benzyl chloride, which is negative over the range 0 < x2 < 0.4, has a minimum at x2 = 0.3 (323.4 K).S5 Through the series of TA co-solvents methanol, ethanol, isopropyl alcohol, and t-butyl alcohol, the minimum in dV* becomes more marked and at lower mole fractions, e.g.at x2 = 0.1 for t-butyl alcoho1.85 The latter minimum is, for the most part, determined by a maximum in SmV" for the initial state. A similar conclusion is drawn from direct measurements of solution densities for the extrema shown by 6md V+in the other alcohol + water mixtures. A comparison of the SmA V* and SmVe values for solvolysis of p-chlorobenzyl chloride, benzyl chloride, and t-butyl chloride shows that the pattern shown by 6md V* depends considerably on both 8mV* and 8mV*, which in turn depend on the mechanism of the reacti0n.~5 For example, a mini- mum in 8md V* for p-chlorobenzyl chloride in ethanol-water at x2 2: 0.2 is a consequence of maxima in 8mV* and am?'*, the latter being more marked.Maxima in SmV* and SmV* appear to be characteristic of non-ionic solutes and minima of ionic states. The minimum in SmV* for t-butyl chloride is similar to that observed for Me4Nf C1-. The extrema in these volume parameters correspond closely to the mole fraction XZ**. Thus for benzyl chloride, Vi -Vi is negative (Vi = molar volume of pure benzyl chloride) in ethanol-water when xz < x2**, indicating that the solute is accommodated to some extent in the clathrate-like cavities in the aqueous solution. But when x2 > XZ**, V: > V;, and indeed a sharp positive extremum in dd V*/dp is observed at x2 2: 0.04 for benzyl chloride in t-butyl alcohol-water system,22a although when x2 = 0, ddY*/dp = 0, which means that the compressibilities of transition state and hydrophobic initial state are equal.When acetone is added to benzyl chloride in water, SmdV* has an extremum near x2 = 0.2, but when DMSO is added, SmAV*, &V*, and 8mV3 * are much smaller. We are unable to extend this discussion of SmAV* to include inorganic reactions because relevant results are singularly rare. 11 Isochoric Activation Parameters: d Uv* The dependence of these quantities on the composition of an aqueous mixture has not been investigated in any great detail.86 The quantity dU,* for the hydrolysis of t-butyl chloride increases when ethanol is added to water, then decreases, and eventually increases at higher alcohol mole fractions.A similar ebJ. B. Hyne, H. S. Golinkin, and W. G. Laidlaw, J. Amer. Chem. SOC.,1966, 88, 2104; H. S. Golinkin, I. Lee, and J. B. Hyne, ibid., 1967, 89, 1307; M. J. Mackinnon, A. B. Lateef, and J. B. Hyne, Canad.J. Chem., 1970,48,2025; D. D. MacDonald and J. B. Hyne,ibid., 1970, 48, 2494. BE B. T. Baliga and E. Whalley, Canad. J. Chem., 1970, 48, 528; B. T. Baliga, R. J. Withey,D. Poulton, and E. Whalley, Trans. Furaday Soc., 1965, 61, 517; B. T. Baliga and E. Whalley, J. Phys. Chem., 1967, 71, 1166. Blandamer and Burgess pattern is shown by &dS,'. The extrema here are roughly one-half those observed in the isobaric activation parameters AH* and AS*,and do not occur at the same mole fraction.However, in two other cases, viz. acid-catalysed hydrolysis of methyl acetate in acetone + water and hydrolysis of benzyl chloride in ethanol + water, less striking extrema relative to those in isobaric quantities are observed in the isochoric quantities. At this time there seems to be no simple way in which the behaviour of isochoric quantities can be explained. Thus the concept of activation at constant total volume is simple to understand, but the implications on molecular processes accompanying the conversion of initial state into transition state are not. 12 Conclusions We have attempted to highlight a number of themes in this review. First, the complex patterns in activation parameters often reflect the sometimes unique features of aqueous systems. Second, that in explaining the observed trends, the changes in activation parameters reflect changes in both transition and initial states, the latter being dangerous to ignore. Finally, it is possible to analyse the kinetic data by using thermodynamic properties of solutes in aqueous and non- aqueous systems. We hope that this review will encourage chemists to measure kinetics of reaction in a wide range of aqueous mixtures as functions of x2, temperature, and pressure.

ISSN:0306-0012    

DOI:10.1039/CS9750400055

出版商:RSC    

年代:1975

数据来源: RSC

摘要:

HANDLING TOXIC CHEMICALS -ENVIRO-NTAL CONSIDERATIONS* I Introducing a New Agricultural Chemical By J. F. Newman I CI PLANT PROTECTION LTD., JEALOTT’S HILL RESEARCH STATION, BRACKNELL, BERKS., RG12 6E7 Agricultural chemicals, like the more traditional implements of agriculture, are designed to produce biological effects, usually by killing insects, or fungi, or higher plants. While, from the human point of view, the distinction between insect and pest, fungus and disease, or plant and weed is often very clear, the physiological distinction may be much less clear, and it is evident that an agricultural chemical may well produce some effects upon living things other than the pests, diseases, and weeds at which it is aimed. While quite remarkable progress has been made in chemicals having selective activity, such as the hormone weed-killers, which will destroy broad-leaved weeds in a cereal or grass crop, it is broadly true that the introduction of more effective pesti- cides, and in particular more persistent pesticides, has intensified the environ- menta1 problems.It is clear that no new biologically active chemical should be introduced without intensive experimental work to establish its likely impact upon the environ- ment. This is demanded by public opinion, recognized by the chemical industry, and ensured by the increasing weight of regulatory law relating to the registra- tion of pesticides. The types of technical problems involved are well illustrated by a consideration of environmental effects of two types of agricultural chemicals which have been in use for many years.The organochlorine insecticide DDT was introduced into agriculture in the 1940s, following earlier and spectacularly successful use in the control of insect disease vectors during the war years. Understandably, this development had gone on with minimal attention to possible environmental effects. DDT is a new chemical addition to the environment, in that it did not exist until it was synthesized in a laboratory. DDT can now be found practically everywhere, in soil, in water, in air, and in living things. The levels of DDT in these places are generally low and without physiological significance, and the fact that they can be determined with precision is a tribute to the advances in analytical methods.This situation arises from the chemical and physical properties of *These papers were originally presented at a symposium of the Environment Group of the Industrial Division of the Chemical Society at the Scientific Societies Lecture Theatre, Savile Row, London W1, on 13th November 1973. Handling Toxic Chemicals -Environmental Considerations. Part I DDT. It is chemically stable, so that when applied to the soil it maytake between 10 and 20 years for 95 % of the applied dose to disappear. Its relatively low solu- bility in water, of the order of 0.001 p.p.m., together with a ready solubility in oils and fats, makes it possible for animal life to accumulate higher residue loads of DDT from a relatively low contamination level in the environment.This effect can be particularly marked in animals which have specialized feeding habits, such as small birds specializing in earthworms in sprayed orchards. While DDT is not a particularly poisonous substance, in the acute sense, to higher animals, there is evidence that high residue levels in some birds can pro- duce indirect effects in enzyme systems, causing the birds to lay thin-shelled eggs and so decreasing their reproductive capacity. It is probably true that DDT became widely distributed in the environment before we had any idea of what constituted an ecologically acceptable level, and while all the indications are that a tolerable level has not been exceeded, the experience of the problems which have arisen in 30 years of DDT usage must be taken into account in the design of environmental research work on new chemicals before their introduction.Useful information also arises from a consideration of the use of mercury compounds as agricultural fungicides. While the use of mercury as a preservative has long been known, the most important agricultural use today is as a seed-dressing on cereal and other seeds for the control of seed- and soil-borne diseases. This became possible with the discovery of suitable organic mercury compounds early in this century. The use of, for example, a phenylmercury acetate dressing on cereal seed, at a rate of about 5 g of mercury per hectare, will give complete protection from diseases formerly very damaging.Unlike DDT, mercury compounds are not new additions to the environment. Mercury occurs naturally in the rocks of the earth’s crust. Erosion and evapora- tion from such natural resources and subsequent distribution by water and air movements ensure that mercury is present everywhere in Nature, although generally in minute concentration. Some organic mercury compounds can accumulate into animals, and such compounds can be formed in Nature by microbiological methylation of inorganic mercury. Any additional mercury introduced into the environment through human activities must therefore be considered against this background. If the quantity introduced is small in relation to natural background levels, it is reasonable to believe that the situation is satisfactory. While the ingestion of any mercury is probably not beneficial, there is little point in avoiding a modest use of mercury in pesticides if they are beneficial in crop protection.The development of a new agricultural chemical starts from the synthesis of a few grams in the laboratory and proceeds eventually to the design and erec- tion of a chemical plant to produce many tons of it per year. The initial step of synthesis is relatively inexpensive, while the final stage is extremely costly. From the point of view of the manufacturer it is of the greatest importance to know of any adverse environmental effects which may limit the application of the chemical, and to know of these effects at as early a stage as possible, and certainly prior to undertaking any major development expenditure.In favour- Newmn able circumstances the time from point of discovery to point of sale is likely to be a minimum of five years. Such a time scale may pose problems for the environmental scientists concerned with amassing sufficient information on the probable environmental impact of the chemical. Such matters as the accumulation of the chemical into large predatory animals, and possible effects upon the reproductive efficacy of slow-breeding forms of life, cannot be studied in the field on a small scale, since such information could only rise from the treatment of large areas of land with correspondingly large amounts of chemical. Decisions are needed, however, at a time in the early part of the development period when only small quantities of chemical are available and when large-scale field treatments would not in any event be desirable.The problem which the industrial ecologist has to face is that of devising small-scale experiments from the results of which reasonable deductions may be made of the large-scale environmental behaviour of the chemical. An approach to this in the Environmental Science Group at Jealott’s Hill has involved the treatment of 6 m x 6 m field plots, some at application rates near to those likely to be used in commercial practice, and others at grossly excessive rates, perhaps 100 times the expected rates. Observations are then made of the behaviour of the pesticide residue in the soil, in terms of penetration down the soil profile and rate of decomposition, and careful and detailed observations are made on the effects of the treatments upon various small forms of life in and on the soil.Effects upon microbiological activity in the soil are investigated by taking soil samples from the field plots and setting them up in the laboratory to measure such things as the total respiration (as a measure of overall activity), the rate of conversion of ammonia into nitrate (a function of some importance in agriculture), and the rate of breakdown of soil organic matter. Organic matter breakdown is measured by adding 14C-labelled organic matter, and, to separate soil samples, 1%-labelled organic fractions such as sucrose, starch, cellulose, urea, and fatty acids, and then trapping the evolved 14C02 over a period of several weeks.Measurements are also made of total microbiological biomass in the soil, by estimates of the ATP content by a luminscent biometer and of the DNA content by a specific staining reaction in soil dispersions in an agar film. The microarthropod soil fauna in the plots is sampled by taking soil cores to various depths and extracting the arthropods by a wet-sieving and differential- wetting technique. The arthropod fauna, principally mites and Collembola, are identified to species and counted, and the information is used to monitor changes in populations and species diversity at various times after application of the pesticide. The earthworm fauna is sampled by a combination of digging and expelling them by the use of a 0.2 % formaldehyde solution applied to the soil.In appropri- ate temperature and humidity conditions, particularly in spring and autumn, reasonable earthworm population estimates are attainable. In addition to information on earthworm populations and species diversity, pesticide residue measurements can be made. The earthworms may be dissected and residue Handling Toxic Chemicals -Environmental Considerations. Part I analyses done on the gut content and on the remaining tissues. Since the earth- worms feed on soil organic matter, or, in the case of some species, on surface organic matter, and are themselves an important source of food for birds and small mammals, the earthworm is an important link in food chains from the soil.Worms are particularly useful, therefore, as a means of obtaining an early warning of the possibility of an accumulation effect in a food chain. Biological work involving small plots uses small areas of land and little chemical, and can therefore be done early in the research and development programme, and yet it can provide a lot of information on the likely biological impact. If, after a year of such work, no major environmental hazards have become apparent, development may reasonably proceed to larger areas of land. When areas of 5-10 ha of crops are treated, ecological investigations can pro- ceed to studies of the effects upon larger forms of surface invertebrate life, such as beetles and spiders.These forms are sampled by trapping techniques, using a grid of 20 small pitfall traps set into the ground, each with a small depth of preservative liquid in the bottom. The catch of small animals may then be collected at intervals of a few days. While such traps do not give absolute popula- tion estimates, and, owing to differences in trapping efficiency, cannot be used to compare populations of different species, they can give useful information on the effects on the populations of the same species of different pesticide treatments in the same crop. Insects able to fly are better sampled by other methods of trapping, such as liquid or sticky traps, or by the use of a mechanical suction trap. The measurement of effects in the field on vertebrate animals, and particularly on birds, necessitates the use of even larger areas of treated land.This work is of particular importance when a chemical is to be used as a seed-dressing. A seed-dressing is one of the most economical ways of using a pesticide, in that the chemical is placed in close proximity to the seedling, where it can provide the maximum protective effect in the vulnerable young state of the plant, and yet the whole bulk of the soil is not unnecessarily contaminated. Seed-eating birds and mammals, however, may collect the seed and so acquire a relatively large dose of the pesticide. For work on small birds a treated area of about 50 ha is desirable. The fields in this area are drilled with the treated seed and observations are made upon the number and extent of the territories occupied by the breeding pairs of birds, using the census techniques employed by ornithologists.This involves regular visits to the area by a skilled bird observer over a period of several weeks in the spring. Nests may be found and a measure obtained of breeding success. Samples of common seed-eating birds may be taken, under appropriate licence, and analyses made to determine pesticide residue levels in the birds. This information may then be compared with that obtained in labora- tory toxicological tests, in which birds are fed known quantities of pesticide, so that it becomes clear if, in the field situation, the birds acquire a level of pesticide which might do them harm. By the application of these procedures, together with the related work on toxicology and on the persistence and ultimate fate of chemical residues in soil, Newman in water, and in plants and animals, it is possible to build up a body of knowledge on the likely environmental behaviour of a new agricultural chemical before it goes into commercial use.In experimental work, however, it is clearly not possible to cover all living species and soto recognize all specific peculiarities and suscepti- bilities. For this reason, it is desirable that ecological observations should continue for some years as a new material goes into increasing commercial use. The primary aim of environmental investigations during the development period is to provide the necessary information to meet the requirements of the various national registration schemes. In recent years, requirements have multi- plied in various countries, with rather little international collaboration. Since the cost of environmental investigations is a substantial part of the total research and development cost, it is important that registration requirements should be reasonably related to the proposed uses of the chemical, so that new materials are developed not only for widespread, major, crop uses but also for small and specialized applications where different environmental considerations might apply.

ISSN:0306-0012    

DOI:10.1039/CS9750400077

出版商:RSC    

年代:1975

数据来源: RSC

摘要:

I1 Health Hazards to Workers from Industrial Chemicals By A. Munn ICI ORGANICS DIVISION, P.O. BOX 42, HEXAGON HOUSE, BLACKLEY, MANCHESTER M9 3DA. Our affluent society could not have evolved had we not been an industrial society. Chemistry and the chemical industry have made a substantial contribu- tion to our economic progress, with its constant efforts not merely to develop entirely new products, but also to find improved methods of making existing products better and more cheaply. The blessings thus conferred upon mankind, however, require, by the very nature of the industry, that workmen continue to work with materials that may be toxic, foul-smelling, corrosive, dermatitic, carcinogenic, powerfully staining, allergenic, and offensive in every conceivable way.Increasing public awareness of the problems of toxic chemicals is thus reflected not merely by interest in ecological considerations, but also by changing social attitudes in respect of health hazards to workers. Unfortunately, this desirable trend is not always based upon proper understanding, and the purpose of my remarks this morning is to throw some light upon what is commonly described as ‘industrial toxicology’. Toxicology is the study of poisons and poisoning. It has several facets, each of which involves different criteria. Thus, the toxicology of drugs and medi- cines involves different considerations from forensic toxicology, which again involves different considerations from the toxicology of chemical additives to food, or to food-packaging materials.Industrial toxicology is different yet again. It may be defined as ‘the study of chemicals used in industrial processes with regard to their liability to produce adverse effects upon the health or efficiency of workers from industrial conditions of exposure’. Experimental toxicology is the tool which is common to all of these different aspects of toxicology, but it should be noted that industrial toxicology is primarily about people who work in factories and not about animals in experimental laboratories. The latter are simply a means to an end; they do not constitute an end in themselves. The artificial laboratory situation, where measured quantities of chemicals are fed to experimental animals, or deliberately introduced into their bodies by injection, does not, in any way, replicate industrial conditions to which workmen are exposed.The fundamental concept of all toxicology is that any chemical will exert toxic effects if it enters the body in sufficient quantity, even salt or sodium bicarbonate, and the art of industrial toxicology is concerned not only with the nature of the toxic effects that a specific chemical will induce, and the quantity of it that is required to induce them, but also with those factors which influence the extent to which that chemical will enter the workman’s body. If one excludes Munn local effects on skin and eye, it must be self-evident that no toxic chemical can exert its toxic effects upon a workman unless and until it has been absorbed into his body.There are three routes by which chemicals may enter a workman’s body from industrial exposure: (1) they may be ingested, i.e. taken into the mouth and swallowed. This route is of little importance in industrial toxicology. Workmen do not customarily eat the chemicals with which they work, and the requirements of the Fac- tories Acts prevent the workmen from eating where noxious dust or fumes are given off, so there is little opportunity for their food to become in- advertently contaminated. (2) they may be absorbed through the skin. This is a far more important route than generally realized, and many important industrial chemicals are readily absorbed through the skin, e.g. aniline, nitrobenzene, other nitro- and amido-derivatives of benzene, phenol, nicotine, stilbestrol, parathion, HCN, etc. It is mainly a question of high lipid solubility associated with some water solubility, but not all the factors are fully understood.Compounds of high molecular weight are seldom absorbed through the skin. (3) they may be absorbed by inhalation. This is the commonest route by which industrial chemicals gain access to the body. It implies that they must be airborne, as a dust, fume, mist, or vapour. When inhaled, they may have a local effect on the respiratory tract, e.g. sulphur dioxide or phosgene, but they may also be readily absorbed and exert a systemic effect, i.e. they may pass into the blood-stream and be distributed throughout the body, e.g.HCN, carbon monoxide, HzS, or lead dust or fume. There are three separate though related matters which are commonly con- fused-sometimes even by toxicologists, regrettably -and I would like to ensure that you all understand the difference between them. They are: (a) The toxicity of a compound. (6) The toxic hazard of that compound. (c) The toxic hazard of an industrial process in which that compound is used. The toxicity of a compound is something that is capable of being measured by animal experiment. There is frequently, of course, considerable variation in species response, both qualitatively and quantitatively, but this is not the place to enlarge on that problem. The purpose of the experiment is not to determine whether a compound is toxic or non-toxic, but to determine what is the nature of the toxic effect, and what dose has to be absorbed to induce the toxic effect.Both acute and chronic toxicity studies may be necessary, i.e. investigations into the short-term effects of large doses and the long-term effects of small doses. The toxic hazard of a compound is only partly a function of its toxicity (as measured by experiment), but also a function of the ease with which it is absorbed into a workman’s body -i.e. with compounds of similar toxicity, a compound readily absorbed through the skin will be more hazardous than one which is not, a volatile compound (i.e. one that is readily inhaled) will be more hazardous than one which is not volatile. Sodium cyanide and hydrogen Handling Toxic Chemicals -Environmental Considerations.Part II cyanide provide an excellent example. They are of virtually the same toxicity, the toxic effect resulting from the circulation of the cyanide ion within the body. However, although of similar toxicity, they present very different degrees of toxic hazard. Hydrogen cyanide is a volatile liquid, readily absorbed through the skin, and wherever it is used it presents a most serious toxic hazard. Spillage will result in volatilization, and when a workman inhales the vapour, if it is there at more than a very low atmospheric concentration he will die. Similarly, quite a small splash on the skin will be absorbed very quickly, and, again, the workman will die.Sodium cyanide, on the other hand, is commonly used in a variety of industries with very much less care than is necessary for HCN. It is a crystalline solid, usually compressed into ‘eggs’. It is not volatile, and there is little, if any, airborne dust. It is not absorbed through the skin to a significant extent, and although it has a very high degree of acute toxicity it cannot exert its toxic effects if it has not been absorbed into the workman’s body. The toxic hazard of an industrial process in which a specific compound is used depends not merely upon the toxic hazard presented by the compound itself, but also upon the circumstances of its use, as it is the latter which deter- mines the extent to which the noxious agent -if indeed it is noxious -con-taminates the environment of the workpiace.The most toxic chemical in the world will not poison a process operator if it remains totally enclosed within a sealed reaction vessel. Thus, before commenting on the toxic hazard of a par-ticular industrial operation, it is essential to know -and to understand -the nature of that operation. It is essential to know what is involved in filtration procedures, in stove-drying, in spray-drying, in grinding, milling, and blending, and in discharging a product into drums or sacks. Regrettably this obvious and simple rule is not always observed, and profound statements of gloom -and impending doom -are from time to time made by individuals unprepared to take the trouble to go and see what actually happens in the factory.The lead hazard provides a simple and obvious example. Should a workman spend his entire day handling lead ingots, he is unlikely to develop any adverse effects on his health (unless he drops one on his toe or strains his back). However, if he puts these lead ingots into an open furnace, heats them until they melt, and main- tains the molten lead at an elevated temperature, unless adequate arrangements are made to deal with the lead fume which is given off in such circumstances, he will inhale the lead fume, and if this is repeated day after day will undoubtedly be at risk of developing lead poisoning. I mentioned that the toxicity of a compound can be measured by animal experiment. The customary yardstick of acute toxicity is the LD50,i.e.the dose level (administered as a single dose, and expressed in terms of weight of com- pound per unit body weight of the animal, e.g. mg kg-l), which kills 50% of the experimental population thus treated. An LD50of less than 1 mg kg-1 represents an extremely toxic substance, and anything with an LDSOof less than 50 mg kg-l is generally considered to be highly toxic -though not necessarily highly hazardous. Such measurements are of only limited value. They do give an indica- tion of the order of magnitude of the acute toxic effect, and in respect of highly toxic compounds this is sometimes important. It must be borne in mind, how-ever, that acute industrial poisoning is very rare. Most industrial poisoning results from repeated exposures to a toxic agent, with repeated absorption of very small quantities, leading to chronic poisoning.For example, by far the commonest industrial poisoning in this country is lead intoxication. The rele vance of the LD50 to chronic toxic hazards, and therefore to most industrial toxic hazards, is virtually nonexistent, but unfortunately this easily understood term has become a piece of fashionable jargon, and is being used for a variety of purposes, usually legislative, where it is simply not meaningful. More of that later, when I come to the Robens’ Report. One of the important pieces of information derived from the LDm experiment, apart from the determination of the actual figure, is that it provides useful information regarding the nature of the toxic effect.This can be a valuable piece of evidence with regard to deciding the need to carry out subacute or chronic toxicity studies, but too much weight must not be placed upon the qualitative effect, however unpleasant, without taking quantitative considera- tions into account. It can sound quite frightening to discover that, in poisoning by such and such a compound, the liver simply shrivels up or the testicles drop off, but if it takes dose levels of 10 gm kg-1 day-1 for several weeks to bring this effect about, it is of no consequence as far as industrial toxicology is concerned. From time to time I find it necessary to take issue with Government departments over the transport of so-called ‘toxic substances’, and on more than one occasion it has been my pleasure to point out that although ‘ringing in the ears, with some loss of hearing, nausea and vomiting, accompanied by profuse perspiration and severe thirst, dizziness and drowsiness progressing to delirium, hallucinations, convulsions, and coma, with death the inevitable outcome in severe cases’, may sound very frightening, it is simply a description of aspirin poisoning, and that there is not a single recorded case of occupational poisoning in dockers, or indeed in any transport worker, from the handling, in transit, of packages of aspirin tablets.Another piece of toxicological jargon that has crept into everyday parlance is the term ‘Threshold Limit Value’. This is a level of atmospheric concentration of potentially hazardous gases, vapours, or dusts, to which it is believed that workers may be exposed eight hours a day, 5 days a week, 50 weeks a year, without adverse effects on health or efficiency.The figures are not generally exactly the same as Maximum Allowable Concentrations, except in a few in- stances designated as Ceiling Values, insofar as they represent a time-weighted average, and small swings above the T.L.V. are permitted for limited periods provided they are compensated by equivalent swings below the level. They represent informed opinion on safe conditions, but they have no statutory significance (although they are intended as guidelines of good practice). They do not constitute scientific fact in the way that boiling points or vapour pressures constitute scientific fact, and as they are based on limited evidence, it is not surprising that from time to time they are altered -usually in a downward direction.It should be noted that T.L.V. figures do not represent a yardstick Handling Toxic Chemicals -Environmental Considerations. Part II of hazard. For example, phenol has a T.L.V. of 5 p.p.m., whereas benzene has a T.L.V. of 25 p.p.m. This does not in any way indicate that phenol is more toxic or more hazardous than benzene -on the contrary. In fact, most of you will know that where benzene is used on an industrial scale it requires consider- able effort to maintain the atmospheric concentration below the T.L.V., whereas with phenol, a much less volatile compound, there is really no problem.There are few aspects of chemical toxicity which have aroused more emotion or led to more muddled thinking than carcinogenicity. In recent years there has been greater awareness of the problem (though little increase in the general understanding of it), not merely because the epidemiologists have demonstrated a number of new occupational cancer hazards, not merely because of the publicity arising from litigation, but also because increasing animal experi- mentation has shown that many common industrial chemicals are ‘carcinogenic’, and because modern sophisticated analytical techniques have shown the pres- ence of carcinogenic impurities in other industrial products not themselves generally believed to be carcinogenic.The two latter points are worth elaborating in a little more detail. Firstly, experimental evidence of carcinogenicity. It is frequently assumed that substances that induce tumours when deliberately introduced into the bodies of experimental animals by a variety of routes, often at very high dose levels, will necessarily do so in workmen exposed in industrial conditions. It is scarcely necessary to point out that this is a non-sequitur.Carbon tetrachloride is carcino- genic to the mouse, the hamster, and the rat; chloroform produces liver tumours in the mouse, but despite widespread exposure there is no evidence at all to suggest that either carbon tetrachloride or chloroform has caused cancer in man. Tannic acid also induces liver tumours in the rat, whereas the drug Isoniazid induces lung tumours in the mouse.Does anyone seriously believe that these compounds present a carcinogenic hazard to man? The only assumption that should be made on the basis of such experimental evidence is that it might represent a hazard, and therefore all available evidence ought to be critically evaluated both qualitatively and quantitatively. You may not all appreciate quite the number of chemicals involved. In the 1972 List of Toxic Substances published by NIOSH,* 645 different industrial chemicals were listed as either ‘carcinogenic’ or ‘neoplastic’, though these terms were not clearly defined. The figure 645 indicated that a large number of chemicals is involved, but the magnitude of the problem becomes more apparent when careful inspection (of the NIOSH list) reveals that a-naphthylamine is not described as either carcinogenic or neoplastic.This suggests that a detailed survey might well reveal other known carcinogens to be missing. The second point mentioned earlier as requiring some elaboration is concerned with trace impurities of known carcinogens in products not themselves carcino- genic. There is a widespread, though by no means universal, view that there is no such thing as a safe dose of any carcinogen. This is not a school of thought *US.National Institute for Occupational Safety and Health. Munn to which I myself subscribe. It is a speculative view, unsupported by either experimental or epidemiological evidence, derived from statistical considerations unrelated to reality, and furthermore it is a view which most of us reject in our normal lives.For example, the first recognized carcinogen was soot, the cause of chimney sweep’s cancer of the scrotum, first described by Percival Pott in 1775. Few of us would hesitate to clean out a fireplace each morning at home simply because a small amount of soot is present. Those of us who do not clean out the fireplace ourselves do not feel it necessary to give our wives warnings of the dangers of scrota1 cancer -or indeed of any cancer which might be caused by soot. Similarly, although many doctors prescribe coal-tar ointments to be used by their patients with skin disease, I have never known any doctor so prescribing to warn his patient that the medicament might cause cancer -for coal tar was one of the first experimental carcinogens. I can only conclude that most doctors share my view that for this carcinogen also there is a safe dose.Similar considerations apply to sunlight. It is well known that exposure to sunlight is responsible for many cases of skin cancer on the exposed areas of the body. Yet this undoubted fact does not inhibit those of us who can affordit from dashing offto the Mediterannean to expose ourselves to the maximum amount of sun- shine which we find tolerable. On a more scientific note, it can be stated that not a single carcinogen has been described in respect of which it is experimentally impossible to find a dose which will not cause tumours in a finite experimental population. In practical terms, for workmen exposed to small quantities of carcinogens, a ‘safe’ dose can be defined as one which does not bring about a statistically significant increase in tumours, beyond the normal incidence in a population not so exposed.Consider the rubber antioxidant phenyl- P-naphthylamine (PBN). This is not itself carcinogenic, but analysis by gas-liquid chromatography revealed that until about two years ago, commercial PBN generally contained a P-naphthyl- amine impurity in the range 20-50 p.p.m. Despite the widespread use of PBN in the rubber industry throughout the world for many years, no excess tumour incidence has been attributed to it. Three recent epidemiological surveys (Veys, 1973; Parkes, 1972; Department of Employment, 1972) in the U.K.all indicate that in workmen who joined the rubber industry only after Nonox S was aban- doned in 1949 (Nonox S was the antioxidant whose extensive use in the rubber industry led to an occupational bladder tumour hazard in workers who joined the industry prior to 1950), the incidence of bladder tumours is not greater than in the population at large. Indeed, an expert committee (whose members included the Senior Medical Inspector of Factories and the Medical Adviser to the T.U.C.) recommended two years ago that rubber workers exposed to such products with carcinogenic impurities should not be subjected to urinary screening, as opposed to workers known to be at risk of chemically-induced bladder tumours.This apparent absence of hazard with PBN (containing up to 50 p.p.m. of fJ-naphthylamine) enables important inferences to be drawn in respect of other compounds with carcinogenic impurities. Its importance cannot be over-emphasized. Handling Toxic Chemicals -Environmental Considerations. Part 11 The real question which has to be considered is not whether such and such a compound is carcinogenic, but whether its manufacture and use presents a carcinogenic hazard to workmen; and, if so, whether adequate precautions can be introduced to obviate that hazard. This involves making a judgement in respect of each single carcinogen, indeed in repect of each single process in- volving a carcinogen.Making such judgements is not easy, but it is certainly not impossible. Indeed it is essential, because without them there can be no justification for continuing to manufacture or use on an industrial scale any of the hundreds of experimental carcinogens. The basis of the judgement can be made under four headings: (1) Critical evaluation of the experimental evidence, both qualitative and quanti- tative. (2) Epidemiological evidence -when available. Epidemiology is the study of the incidence of disease, and is an essential tool in identifying cancer risks -or non-risks. (3) Physico-chemical properties, insofar as they influence absorption in condi- tions of industrial exposure. (4) Chemical relationship to other compounds of known hazard, e.g.methylene bis-(o-chloroaniline) (MOCA) is related to aromatic amine carcinogens. The second part of the question concerns whether adequate precautions can be introduced to obviate the hazard. This is primarily a question of chemical and engineering techniques, associated with biological and environmental monitoring, which need not be discussed here. Nevertheless, by the application of appropriate measures, it has been found possible to wipe out the incidence of lung and nasal cancer associated with the Mond process for refining nickel, and the cancers associated with isopropyl alcohol manufacture: it has been found possible to use carcinogenic X-rays for purposes of medical diagnosis; and to use, for industrial purposes, highly hazardous radio-isotopes of undoubted carcinogenicity.The Robens’ Committee made far-reaching recommendations for the control of toxic substances, and the Department of Employment has recently published consultative proposals regarding the implementation of these recommendations. Many of the proposals require discussion at length, but I intend to mention only one this morning. It is concerned with the notification of new substances, or those coming into commercial use for the first time, to the proposed Advisory Committee on Toxic Substances. The criteria which have been suggested are that notification should be required only where the oral LD50 in the rat is less than 200 mg kg-l*, or where the percutaneous LD50 in the rat (i.e.the LD50 by absorption through the skin) is less than 4000 mg kg-l. Now most industrial poisoning, as I told you earlier, has nothing to do with acute toxicity. Most industrial poisoning arises from chronic exposure, from the repeated absorption of small quantities of a toxic agent over a period of weeks, or months, or years. *The figure of 200 mg kg-I in the Consultative Document was a misprint. It has sub- sequently been corrected to 2000 mg kg-l. The comment in the penultimate paragraph is, therefore, perhaps less relevant than was the case when it was made in October 1973. Munn The criteria suggested would permit substances as hazardous as P-naphthylamine, or toluene di-isocyanate, to be introduced without any reference to the Advisory Committee, thus defeating the whole object of the exercise.In my view, the only way in which the Robens recommendations can be properly implemented is by notification of all new substances, together with essential chemical, physical, and basic toxicological data about them. The basic toxicological data would simply be an oral LD50 figure, together with data regarding irritant effects on skin and eye. Armed with this information, the expert committee can then decide whether any further testing is necessary, e.g. percutaneous toxicity, inhalation toxicity, sensitizing potential, carcinogenicity studies, and the like. One may or may not believe the Robens recommendations to be sensible, but this is the only way of implementing them. Anything short of this is simply window-dressing. If it is worth doing at all, it is worth doing properly. Mr. Chairman, that is all I wish to say. We all have a duty to pay a great deal of attention to health hazards to workers, and I am grateful for the opportunity to express my views.

ISSN:0306-0012    

DOI:10.1039/CS9750400082

出版商:RSC    

年代:1975

数据来源: RSC

摘要:

I11 Radioactive and Toxic Wastes: A Comparison of their Control and Disposal By A. W.Kenny DEPARTMENT OF THE ENVIRONMENT, 2 MARSHAM STREET, LONDON SW1 The control and disposal of radioactive wastes is based firmly on radiological principles. That statement may appear to be a truism, something which need not be said. But an Act of Parliament is not just a statement of scientific principles with a legislative structure for applying them; it incorporates also the traditions of control in related fields and the views of elected representatives and the views of sectors of the public (as expressed by relevant associations), which will have been sought in preliminary consultation. The end-product, the Act of Parliament, is some compromise in which the technical principles are not necessarily dom- inant.However, the technical principles (as enunciated in a White Paper which was widely supported) of the Radioactive Substances Act 1960 are indeed dominant, with but little modification by political principles. The technical principles, not so clearly accepted in 1958 when the proposals for legislation were being considered as they now are, following ICRP and other publications, may be stated for our purpose in fairly simple terms, though each principle, as with other technical principles, is not nearly so simple as it looks and might be elaborated without undue difficulty into a volume of discussion and refinement and criticism. For effective legislation, simple, clear principles are essential ; each reservation, however well based, will probably require a Section in the Act and ultimately, if too many are included, a clumsy, cross- referenced Act will probably emerge.Unless the reservation is a major one it will probably have to be sacrificed in the interests of writing clear, effective legislation. The principles are: (a) that the permissible levels recommended, by the ICRP, which are expressed in terms of doses of radiation either to the whole body or to an organ of the body, are a proper basis and should not be exceeded irrespective of cost ; (b) that doses of radiation (but not necessarily amounts of waste disposed of) should be reduced as far as is readily achievable, economic and other factors being taken into account. From these simple principles several deductions can be made: (i) Only irradiation of man is to be taken into account :the effect on animals would be considered only in so far as man is affected.(ii) Similarly, so long as the irradiation of man is not affected, environmental effects per se are not taken into account. (iii) For any disposal of radioactive waste a derived working level may be Kenny deduced by straightforward (in principle) calculation :from the observed environmental behaviour of the radionuclide and the observed habits of man, utilizing the known radiation of the radionuclide and the known characteristics of man, one may calculate that level of disposal which just does not produce the permissible level. Thus the control of all disposals of radioactive waste is based in principle on a logical set of assumptions and deductions applicable to all the toxic substances which fall in the sub-set of radioactive substances in so far as their toxicity is attributable to the ionizing radiations they emit.The discussion of the validity of the control can concentrate then on the validity of the assumptions, one of which must be mentioned for our present purpose. Our knowledge of the effects of ionizing radiations on humans derives for the most part from observation of the effects of single relatively large doses. In order to deduce a permissible level applicable to perpetual or life-long irradiation it is necessary to deduce the effect of the latter from that of the former, guided by animal experimentation; that is, a mathematical relationship has to be assumed for extrapolation (a) from high does to low dose, (b) from acute exposure to chronic exposure.(Again, I am simplifying the issues.) The extrapolation used by ICRP assumed that the former is linear and that the latter is an equality; deliberate, conservative assumptions. It is sometimes asserted that because a linear relationship is assumed to repre- sent the data at high doses and is used to extrapolate to low doses, then it is legitimate to carry the linear extrapolation further to zero, or rather indefinitely small dose; and then it would follow that any dose of radiation, however small, would produce some effects, implying that no irradiation should be tolerated, however small, unless for very good reason.It is important to be clear that the argument is false. The conclusion is not, however; as a matter of common prudence and following well-established principles in the public health field, any pollution which may affect man’s health -and perhaps that means any pollution -should be avoided, if practicable. The real problems and the art of public health control follow from the balancing of the two, the risk vs. benefit. But this attitude and this problem are not peculiar to radioactive waste control. One other factor in radioactive waste control has to be noted, the extreme sensitivity of the analytical techniques. One may be permitted to speculate whether such low permissible doses would have been specified if the analytical techniques for measuring them had not been possible; as it is, some of the derived working levels are on the edge of measurability, and ‘as low as readily achievable’ or ‘reasonably practicable’, or whatever phrase is used, seems to be interpreted sometimes as ‘just measurable’.A detailed account of how the U.K. system of control of radioactive waste disposal under the provisions of the Radioactive Substances Act 1960 operates in practice has been pub1ished;l the essentials may be summarized as follows: 1 A. W. Kenny and N. T. Mitchell, ‘Management of Low-and Intermediate-Level Radio- active Wastes’, International Atomic Energy Agency, Vienna, 1970, p.69. Handling Toxic Chemicals -Environmental Considerations.Part III (a) All users of radioactive substances, except those exempted by Order, must register with the Secretary of State. Since there is power to refuse registra- tion, there is in effect power to ban a particular use or any use by a par- ticular user for reasons related to waste disposal. That power clearly must be exercised only by the central government. (b) All disposals of radioactive waste, except those exempted by Order, must be authorized by the Secretary of State. (c) Disposal of radioactive waste by conventional methods of waste disposal, whether operated by local authorities or private individuals, is encouraged, but (d) For those wastes which cannot be so disposed of, a National Disposal Service, based on the facilities developed for their own activities by the U.K.Atomic Energy Authority and British Nuclear Fuels, is operated by the central government, the service being provided at cost. The control of radioactive wastes has been approved by the Royal Commission on Environmental Pollution,2 the caveat in respect of stored fission products being non-pertinent to the controlling Act. It is then an acknowledgedly success- ful system of control; one Act for all radioactive waste, whether solid, liquid, or gaseous. By contrast, the disposal of other wastes is controlled by three series of Acts, one for each of the phases, and in the three phases domestic waste control is mixed up with the toxic waste control. Before going further it is well to chew over the phrase ‘toxic waste’, without, however, breaking our teeth or choking on it.It is not of course an antonym of ‘radioactive waste’ or of ‘domestic waste’ and it is not a synonym of ‘industrial waste’. If the comparison of the control of radioactive and toxic waste disposals is to be of value, the phrase must be restricted in meaning to wastes containing or contaminated with substances which in the normal way are considered to be dangerous. Yet the legislation must control a wider range of substances along with or parallel with toxic wastes, and to some extent the two get mixed up. The present situation is somewhat as follows: (i) In regard to wastes (dusts, mists, and gases) discharged to atmosphere, Orders made under the general enabling provisions of the Alkali Acts have specified the classes of industrial premises to be brought under their control.Levels geared to what is practicable, having regard to the methods of treatment available and the nature, including age, of the plant have controlled existing emissions and ensured that new factories use the best practicable means for treatment. (ii) In regard to wastes discharged to sewers and rivers, the local authority and the River Authority, respectively, have the power to specify limits for constituents of the discharges. There is no regard necessarily to what is practicable or what is economic, the interest of the sewerage system (including the treatment works) and the river, respectively, being para- s Royal Commission on Environmental Pollution, Third Report: ‘Pollution in Some British Estuaries and Coastal Waters’, HMSO Cmnd.5054, London,September 1972. Kenny mount, but the authorities do, of course, have regard to what is practi- cable, and in the last resort an appeal to the Secretary of State may be made. (iii) In regard to solid wastes discharged to land or to sea, until quite recently there was no controlling legislation, and even now the control is quite loose. Proposed legislation would give the Secretary of State the power to specify wastes which would become subject to an authorization pro- cedure enforced by the local authorities. This very brief summary omits the contribution of civil law and private Acts, and also omits the effect of Acts designed to control smoke emission from private dwellings, which of course is not ordinarily regarded as industrial waste disposal. The law of nuisance, riparian rights, local Acts imposing conditions on waste tips (especially in the Home Counties), and in some cases planning controls; all these have made notable if sometimes local contributions to the control of waste disposal.It would take us too far if we were to pursue this topic; it is noted merely to warn the reader that the summary is given in order to develop the comparison which is our present purpose, and is in no sense complete. The logic of separately treating non-radioactive wastes in the three phases of matter is fairly clear. When such a waste is treated, its toxic or offensive nature is destroyed; dust is trapped and no longer inhaled, organic wastes are oxidized, acids are neutralized, and so on.Though there are exceptions (arsenic, for example) it is broadly true that toxic substances may be destroyed. With radio- active wastes, treatment merely transfers the toxic element to another phase, in which either it may be safely disposed or it may be contained, either perma- nently or until sufficiently decayed. One may deduce that toxic wastes whose toxicity cannot be destroyed by treatment are probably best controlled by com- prehensive legislation applicable to the waste, in whatever form it may arise. The next point we may take is the emphasis in the control of radioactive waste on reducing irradiation of people to the lowest reasonably practicable level.I have argued that this does not follow from any technical aspect but is mere prudence. Indeed it can never be possible to prove that any substance has no effect when inhaled or ingested in vanishingly small concentration for a lifetime; it is just not possible to perform the experiment. So to take a case at random, we cannot be certain that the trace metals in drinking water have no effect what- ever on the population supplied with the water. There is the same reason for restricting disposal of non-radioactive waste or, to be more accurate, for keeping environmental contamination to the lowest reasonably practicable. In the control of emissions to atmosphere under the Alkali Acts, this philos- ophy of best practicable means has been followed, and in the control of dis-charges to rivers one has seen the same process of gradually tightening the permissible limits not specifically because the former limits are thought to be toxic but because there is a widespread feeling that what was provided originally by Nature, namely the environment in which man has evolved, is probably a good target to aim for.A similar feeling perhaps is behind the argument that Handling Toxic Chemicals -Environmental Considerations. Purt III sewage discharged to sea should be treated even though there may be no visible contamination and despite assurances that there are no medical hazards. The concept of what is reasonably practicable is subjective and required some balancing of risk versus benefits.Some approach to the evaluation of risk has been made in the radioactive field (but it is subject to the assumptions previously discussed in extrapolating the data); with toxic substances one cannot begin to make the assessment, so meagre are the data. The benefit accruing from the use of a substance cannot usually be expressed quantitatively. Finally, in the assess- ment, the balancing of risk against benefit, one is comparing two things which cannot be expressed in any units and certainly not in the same units. In the ultimate, if one concludes that the risk from waste disposal is not justified, the use may have to be banned. The power to do so has been taken in the legislation controlling radioactive substances but it has not in fact been necessary to use the power.On the other hand, the uses of two non-radioactive substances have been controlled, if not actually banned (although no legislative power of control exists) by voluntary industrial action, viz. hard synthetic detergents and polychlorobiphenyls (PCB’s). The former are substances that were used in every household, which, when not biodegradable, caused unpleasant foaming at sewage works and in rivers and passed into drinking water, where they were present in amounts which, though not toxic, were undesirable. The latter were widely dispersed in our environment. There is still no evidence of harm to man but some reason to sus- pect harm to certain birds. Their use is now confined to those situations where no adequate substitute exists -a deliberate application of the cost-benefit principle.A ban on the use of a substance, however achieved, must be nationwide; the power to ban must rest with the central government and could not be given to local government. Even a national ban may not be effective because imports cannot be wholly controlled in practice, even if within the system of world-wide agreements on trade it is in fact possible to make effective international agree- ments to limit or ban these substances. There is always the possibility that a national ban will be undermined by imports from other countries who, legitimately making their decisions under the cost-benefit principle, reach a different conclu- sion from ours.The benefit seen depends on the state of economic development of the country. The foaming of synthetic detergents may be tolerable in a country where sewage treatment works are not widely provided, and where water is drawn from wells. The risk, such as it is, from heat-treatment cyanide that has been discarded in the countryside may be tolerated in a country which sees the economic burden of treatment or sea disposal as money best spent elsewhere. Thus the effect of a national ban may be to drive the industry elsewhere, while still not achieving the complete ban on use which was the object of the legislative ban. In the radioactive field, principally one supposes because the toxic effects of all the substances are based on a common effect and may be evaluated on the basis of an agreed set of assumptions, there is a wide international agreement, Kenny which, for example, has enabled the United Nations SCEAR Committee to publish internationally acceptable conclusions on the hazards of fall-out from nuclear tests, despite the emotive content of the subject. While this does not completely remove the possibility of nations objecting unilaterally on other than technical grounds, at least there is agreement on the technical issues, and the political arguments can be debated in the appropriate forums.The recommendations of the ICRP are accepted internationally, with a con- sequent international basis for international banning of prescribed uses of radio- active substances if that were thought desirable.There is no similar organization for other toxic substances, though the world monopoly or semi-monopoly status of international companies can operate in substitution, and perhaps just as effectively. At present one can merely note that a banning of use, though a desirable and probably essential element in any legislative system of control of waste disposal which aims to be comprehensive, is not necessarily as effective as one might hope. Complementary to this notion is the banning of a particular method of disposal for a particular substance. In the long run there are but three methods of disposal: (a)to dilute and disperse; (6) to concentrate and contain; (c) to treat and destroy; with the added bonus in the case of radioactive substances that containment affords time for decay and then in effect is a treatment process.The same bonus could come from unstable chemical substances. Of these methods, the first is environmentally most sensitive, and where disposal to the seas is involved, internationally most sensitive. In the U.K. our industry is so close to the sea that it may often be the preferred sector of the environment for getting the dispersion needed for safe disposal; and the sea is most liable to international control, though the atmosphere is an obvious close second. Clearly, any international ban affecting dispersion in the seas, as in the Oslo and London Conventions, has repercussions forcing the adoption of other methods of disposal.In the case of radioactive substances, the third method is not available, except in so far as decay is so accounted, and the inevitability of producing super-lethal amounts of fission products as by-products of nuclear power has compelled the development of the second method. Thus the U.K. nuclear industry has comprehensive and adequate facilities for waste disposal based on the first two methods, and a keen interest in preserving the first in order to reduce the cost and potential danger of storage involved in the second, an interest which will be warmly supported as obviously desirable nationally. Since this industry is and always has been nationalized in effect, the facilities are national. Consequently there exists in the radioactive field a fall-back position; the national facilities can be made available for disposal of any radioactive waste whose disposal by conventional methods (i.e.to atmosphere, rivers, unsuper- vized land, or the seas) cannot be authorized. In the legislation, this was given effect by empowering the Secretary of State to set up a National Disposal Service, which he has done through the agency of the nationalized bodies. Thus in the radioactive field there is always some method of disposal; if the cen- 95 4 Handling Toxic Chemicals -Environmental Considerations. Part III tral government bans a suggested method of disposal by refusing authorization, it provides an alternative method. There is not the same urgency for a National Disposal Service in the non- radioactive field because the third method of disposal normally provides an adequate alternative to the first and because the absence of large quantities of toxic by-products means the absence of special containment facilities.Even when treatment for destruction is not practicable, as with arsenic waste, the quantities which arise can be dealt with by the first method without recourse to the second. The principle of requiring an authorizing body to specify, if not actually to provide, an alternative method of disposal for a waste whose suggested method of disposal it is not willing to authorize is surely sound. Authority without re- sponsibility is fraught with danger. For non-radioactive wastes, there was not until the last couple of years, when the UKAEA at Harwell instituted a compre-hensive service in this field as part of a general plan of diversification from radioactive work, a national disposal service, though several private firms offered what was virtually a comprehensive service.Of course, there is no real parallel with the radioactive field since, except for certain toxic substances used for military purposes, there is no national industry manufacturing or using toxic substances and therefore no disposal service on which to base a national disposal service. The Harwell service is to some extent a development of the techniques used for disposal of radioactive wastes, with the emphasis on specially toxic substances, for which the techniques are well suited.In the radioactive field the encouragement to use conventional methods of disposal is based on two principles: (a) it is likely to be the cheapest method, (b) the National Disposal Service should not be cluttered with trivia. There is an economic element also in the latter. Since a virtually comprehensive disposal service exists in the non-radioactive field, the same principles should hold. The encouragement of this policy, in fact its enforcement by central government with the co-operation of local government through statutory consultation, is effected through the authorization procedure. The authoriza- tion procedure need not necessarily be operated through central government, though some mechanism for near uniformity of standards is probably desirable.The contrast here between the radioactive and non-radioactive field is stark. There is a considerable body of knowledge about the environmental behaviour of radionuclides in waste, much of which is applicable to inorganic or perhaps toxic metal wastes; but most of the knowledge relates to inorganic substances, and practically no information at all exists about the behaviour of organic substances such as arise in wastes from the modern chemical industry. Moreover, the be- haviour of inorganic substances as deduced from radioactive waste disposal is not too well known outside the radioactive field. Lack of knowledge will inhibit the application of the principle of local disposal, where applicable, since the authorities responsible for our water supplies, a valuable asset, will not be likely to take chances.Meanwhile, industrial wastes Kenny may be transported long distances to sites known to be safe for fear that nearer sites known to be probably safe may not in fact be so. There is an urgent need for research both in the laboratory and in the field to throw some light here and to guide future policy. The research has been initiated by the central government in respect of disposal to land where this lack of knowledge is most acute. With all legislative systems a difficulty arises in exempting small amounts. The normal procedure is to state the principle in the Act itself, and then by Regulations or Orders to exempt from the restriction of the Act those substances which contain less than a specified amount or concentration of the toxic con- stituent. Our lack of knowledge in this field makes it extremely difficult to envisage Orders based on universally accepted levels such as have been made under the Radioactive Substances Act 1960 (though even these have their arbitrary aspects).The notification of deposit required under the provisions of the Deposit of Poisonous Waste Act 1972 was subject to such an exemption procedure. The Regulations made under it specified a wide range of wastes which, provided they did not give rise to an environmental hazard, could be deposited without notice. By ignoring (in the interest of simplicity and ease of operation) amount and concentration as parameters determining the toxicity of the waste, a rough and ready classification for this purpose was made.In practice the classification has been used to specify wastes which may be deposited anywhere (i.e. the non-notsable) and those which may be deposited only at specified safe sites (i.e. the notifiable). Whilst this has actually meant a more relaxed attitude to the disposal of cer- tain industrial wastes with domestic waste on local authority tips, it has meant a tightening up on the attitude towards notifiable waste. The hope must be that with more knowledge we can transfer wastes from the notifiable to the non- notifiable class. In the past there has been no duty of local authorities to dispose of industrial refuse, though they had a discretionary power to do so.Some private waste- disposal firms have used the potential absorbing capacity of decayed domestic waste for industrial waste disposal, and many local authorities have in fact taken industrial wastes. Their experience, supplemented by the research results, may lead to wider public acceptance of this method of disposal. Before summarizing this study of how far one may apply the principles of control of radioactive wastes to the control of other toxic wastes, a word of caution is in order. Though the successful control of radioactive wastes is a tempting start for formulating a comprehensive system of control for other toxic wastes, it is not the only possible approach. By any standards the striking improvements in our urban atmospheres and in our river waters in the last decade or so are also promising lines along which to proceed.These lie outside the present study but have to be explored in order to formulate the proper control for toxic wastes. A second factor briefly referred to previously is the non-technical or what may loosely be called the political factor. This is not just a matter, or not even Handling Toxic Chemicals -Environmental Considerations. Part 111 primarily a matter, of the compromising or give-and-take that is normal to the balancing of interests which play some part in most legislation. However technically perfect a system of control may be, it cannot be successful if it is not widely accepted or if it is not accepted by those most affected and those who have to enforce it.Then we may summarize the results of our enquiry into how far the principles of control of radioactive wastes may be more widely applied as follows : (a) For chemical substances whose toxicity cannot be destroyed by chemical or biological treatment, or by extension cannot readily or reasonably economically be destroyed, effective control of disposal is possible by legislation similar to that of the Radioactive Substances Act. (b) In general, there does not appear to be a need for a National Disposal Service for toxic substances similar to that for radioactive wastes, and there is not the same economic reason for providing it because there is not a (virtually) nationalized industry already operating a comprehensive disposal service for its own needs. (c) By extension of the principle, however, it might be desirable for a firm placed in a similar position to the nationalized radioactive industry (because of a near-monopoly, say) to provide a similar service for wastes containing its toxic product.But however desirable, any attempt would probably founder on the oft-expressed reluctance of manufacturers to do other than manufacture, and on the reluctance to take responsibility for an ill-defined and possibly dangerous substance. (d) It is probably not desirable to develop comprehensive legislatory control of disposal of toxic wastes, in whatever phase of matter, as obtains for radioactive waste, though an exception might be made as in (a). (e) As an ultimate control of waste disposal, ban on use may be necessary, though it may not be completely effective unless an international ban can be imposed. (f)The principle of authorizing disposals, a key aspect of the control of radioactive waste disposal, is applicable to toxic waste disposal but to be effective and workable and realistic (i) there is an urgent need of more knowledge of the environmental behaviour of these wastes, so that wastes containing small concentrations can be exempted from the authorization procedure and so that conventional (and therefore cheap) methods of disposal may be used wherever practicable, and (ii) there must be an ultimate method of disposal available when normal methods of disposal cannot be authorized.

ISSN:0306-0012    

DOI:10.1039/CS9750400090

出版商:RSC    

年代:1975

数据来源: RSC

摘要:

IV Environmental Protection in the Distribution of Hazardous Chemicals By A. E. Meadowcroft MONSANTO EUROPE S.A., PLACE MADOU 1, B-1030 BRUSSELS, BELGIUM Whether it is in a chemical manufacturing unit, a user factory, or in part of the distribution process of transport and storage, any incident or accident is likely to receive some degree of public attention. Previous speakers have outlined some of the work going on to improve protection of factory workers when handling chemicals and of measures to protect us all against ill effects from waste products. What is less well known, except to those directly involved, is the considerable effort that has been going on, in recent years, to improve the safety aspects of the movement and handling of chemicals as such.I suggest that this work has significant relevance to eiwironmental protection in that its primary underlying concepts are, firstly, retention of chemicals within their conveying units and, secondly, operational systems to deal quickly and properly with such escapes as may occur. Because of the international nature of the Chemical Industry, much of the work is carried out under the auspices of International agencies, such as the United Nations Economic Commission for Europe (ECE), the European Economic Community (EEC) and the Intergovernmental Maritime Consultative Organization (IMCO), as well as the various National Authorities. To all of these the Chemical Industry contributes its expertise, as well as to those pro- cedures which it adopts internally and voluntarily.Basic to International agreement on the handling and transport of hazardous substances is the definition of this term by defined parameters for each class of hazard, e.g. flammability, toxicity, corrosivity, etc., and the recognition that within each hazard class there can be varying degrees of greater, medium, or lesser hazard. Hopefully, this will eventually lead to equivalent standards of hazard recognition throughout Europe and the maritime countries and thence to uniform requirements and measures of control. The safety aspects can be summarized into two general groups: prevention and cure. Measures to prevent accidents include those concerned with: (1) Packaging -The United Nations Group of Experts package-performance tests, related to operational requirements, already adopted by IMCO and now the basis for U.K.approval for marine shipment ;constructional specifications such as for International Road and Rail (ADR/RID) now being re-examined and possibly to be replaced by standards based on the U.N. tests; codes of Handling Toxic Chemicals -Environmental Considerations. Part IV practice for packaging, for conveyance by road in the U.K.; systems of certifica- tion of compliance with test standards; package identification labelling systems, including after loss overboard at sea. (2) Tanks and Containers -Constructional and operational standards existing for road, rail, and sea use, and full intermodal standards are near agreement.Intermediate bulk container (IBC‘s) requirements in draft for U.K. domestic use and expected to be considered internationally before long. (3) Vehicles -Constructional and operational requirements for international road and rail which have been considerably tightened up, while U.K. domestic legislation deals in detail with some hazard groups and others are under discus- sion. IMCO standards which now exist for construction of bulk ships and opera- tional requirements for tanker washing to minimize pollution. (4) General Instructions -Stowage segregation and unit size restrictions which already apply, while designated routing and controlled parking are under discussion. Marking systems including harmonized danger symbols and product identification.Codes of practice for loading and offloading bulk vehicles are normal in the industry, with driver training increasingly required. (5) Handling Methods -Reduction of damage, where modern handling methods use pallets, unit loads, and containers, has correspondingly reduced the proportion of chemical loss to the environment. Codes of practice exist for loading freight containers and are in preparation for handling packaged and bulk chemicals in ports and harbours. Nevertheless, and even if all standards and codes were fully applied everywhere, it is an axiom that accidents will occur from time to time, so measures to mini- mize their ill effects include: (6) Safety Instructions -Detailed instructions in writing on product hazard, identification, safety, and emergency procedures to deal with spillage and hire.The International Chamber of Shipping’s bulk cargo product sheets and the European Chemical Industries (CEFIC) Tremcards cover a wide range of products, with manufacturers using similar formats for their speciality products. (7) Hazard Coding -Abbreviated coding systems for use by Emergency Services are near adoption for primary identification of hazard (Kemler code) or action (U.K. Hazchem or U.S.A. HI). A common international scheme does not appear likely at this time. (8) Emergency Aid -Major companies’ individual schemes, some of Pan- European scope and now linking voluntarily into national mutual-aid systems such as the U.S.A. Chemtrec and the U.K. Chemsafe schemes, for emergency advice and assistance. This list is by no means comprehensive, but even so it would be impossible in the time available to deal with each item thoroughly.So I have selected a few, as examples of the extent of their scope and the detail involved. Firstly, packaging standards. The aim of the U.N. Rapporteurs is to ensure that package contents are retained despite relatively poor handling. Hence the new standards are those of operational efficiency, not of detailed construction. Meadowcroft Packages are tested for sealing and pressure retention, resistance to dropping and stacking, and must resist reaction with or attack by their contents. Product type, density, and atmospheric temperature extremes must be taken into account and drops must normally be made in at least two specified positions, including those stressing weak points of construction. The standard drop test is 1.2 metres -the height of the back of a lorry -with 1.8 and 0.8 metres for products of greater or less hazard than standard.Already this has entailed constructional improvements in the standard steel drum to meet the drop requirements. A stamp of type and test approval will be required on each package, which should then assure acceptance of the package internationally. U.N. tests have been accepted by IMCO for marine shipment, and the Depart- ment of Trade is already applying the standards to U.K. exports. U.K. home trade will have to conform to codes of practice issued by the Home Office under the ‘Conveyance by Road Regulations’, setting out types of package permitted for each listed product, and the next step will presumably be to adapt the U.N.test standards for packages for domestic transport, where handling standards are already better then average. Proposals have been made for restriction of marine stowage and size of packages for certain highly pollutant substances, and the packages may be re- quired to resist sea immersion for a period long enough to permit recovery, if lost overboard. Marine labelling standards are likely to be set, requiring adherence to packages for a minimum of three months under immersion con- ditions, and affecting, therefore, the adhesive, the material of the label, the ink used, and the surface to which it is applied.Bulk shipments, by land or sea, are generally presumed to present the larger hazard. U.K. domestic legislation for inflammables and corrosives sets out design requiremcnts for road-tanker construction on such matters as shell thickness and position, type and protection of venting, valves and other fittings, as well as labelling and operational matters. Similar regulations for peroxides and toxic substances will follow. In general, U.K. regulations are intended to fit in with international requirements to facilitate movements by common stan- dards, in what is a very international industry. The Chemical Industries Associa- tion manual ‘Safe Transport of Hazardous Chemicals’ provides a review of legislative requirements as well as a code of practice on other related aspects.Control over parking of road tankers is also in hand. There is a shortage of suitably isolated parking places so, additionally, certain types of enclosed prem- ises and approved places will be allowed. Other measures include keeping vehicles in sight when temporarily parked. German efforts to force chemical movement onto rail are well known. For practical reasons of rail facilities, this is not readily acceptable to most other European countries but, in the U.K., a survey is in hand of product quantities, distances, and routes to see what transfers are feasible. Some companies already move goods by rail when long-distance, direct-route journeys are involved, but in the U.K. generally, transfer to rail is difficult because of the shortage of Handling Toxic Chemicals -Environmental Considerations.Part I V sidings and of transfer equipment and the many cross-country routes involved. In any case, some form of route control is likely. Already this is the practice of some companies, while designated routing of all heavy lorries is common around many continental towns and could well be applied here, not just for chemical products. The Design and Use regulations for portable tanks -IS0 large tanks down to small sizes over 450 litres -are included in ADR/RID from 1974, while IMCO recommendations for various types of tanks are already agreed and to be imple- mented by DOT. Design features included designated minimum shell thickness (related to product vapour pressure at elevated temperatures), with additional thickness required for specially hazardous substances.Bottom outlets will be forbidden for the most toxic and corrosive products, and special attention is paid to protection of fittings, attachment to carrying vehicles, and resistance to G forces in transport. At present, standards for land and sea are somewhat different, and the U.N. Rapporteurs are attempting harmonization of the two standards to produce truly intermodal tank requirements. Similar standards for glass-fibre tanks are in draft for international transport, and EEC have in mind a Directive to establish this as the standard for domestic transport. Intermediate Bulk Containers, commonly called IBC‘s, are widely used for the carriage of 4-3 tons of solid and liquid chemicals. If these are pressure vessels they come under portable-tank regulations, but non-pressure types, whether metal, plastic, or expandable fibreboard, etc., are under examination now for the establishment of suitable standards of design and transport.Bulk ships, too, are subject to IMCO standards. Hazardous chemicals and others liable to cause pollution, whether through toxicity or general contamina- tion risk, are designated to ship type. Type I and Type I1 ships set standards of tank size, tank equipment design, and collision absorption spaces between hull and tanks, as well as careful positioning of tanks and equipment in relation to engine room, crews quarters, and the like. Operational requirements to mini- mize pollution include controlled disposal of ship tanks’ washings, either by landing for disposal at the user’s site or a special disposal unit in the case of certain dangerous or difficult products, or by controlling the method and quantity of disposal at sea to minimize environmental impact for the majority of listed products.Inspection and certification methods are included to enforce the control. However much care is taken with design of vehicles, tanks, and packages in transport we are, in the end, dependent on the skill and behaviour of people. ADR/RID have long required suitable instructions, in writing, to be given to drivers. Revised U.K. regulations will require this too for U.K.domestic traffic. In addition to the product name, hazard symbol, and supplier’s name and address, ‘instructions in writing’ require details of the nature of the hazard, safety measures to be taken, and action -including what not to do -in the case of fire or spil-lage. Additionally, it is the policy of the Chemical Industry to include a tele- phone number to contact in emergency. The instructions may be in the form Meadowcroft of separate sheets given to drivers, or, for packaged goods transported in the U.K., the information will be acceptable on the package labels, provided the driver is told about them and understands them. For bulk vehicles, information cards like the CEFIC Tremcards are a good way to meet these requirements.Several hundred cards are now available, and the international working party will complete the list of 600-700 chemicals known to move in bulk fairly soon. Hazard symbols are well known. The U.N. Diamond style, sometimes with additional words, is now common for sea, air, road, and rail transport -ADR/RID having recently changed over from the old black on yellow rectangles to this style. There will thus be a uniformity of hazard recognition which will be publicized by the Government shortly, with the general warning to the public -Danger -Keep Away -Ring 999. Improved handling also helps safety. Codes of practice for handling bulk and packaged chemicals in ports and harbours are being prepared to include aspects such as separation of vehicles parked at ports, prior notification to port officers of the arrival of hazardous products so that proper provision can be made, and improved standards of safety and training in those areas. Vehicle equipment such as self-sealing connectors, composite bursting disc/ relief valve fittings, enclosed loading and discharge systems to and from storage tanks, antistatic equipment and techniques, level and failure alarms are in increasing use as part of the Industry’s general safety programmes. The now widespread use of mechanical equipment and unitization of packaged goods considerably reduces damage and hence spillage, especially at ports or transfer points.Pallet covers, strapping, and shrinkwrap can further increase resistance to casual damage in handling.The use, for good commercial reasons, of roll-on, roll-off vehicles, especially to Europe, and of through-containerization worldwide (containers also have a joint CIA Chamber of Shipping code of practice for hazardous goods), minimizes package handling and can limit it entirely to sender and receiver, both of whom should have good standards of handling under their direct control. Segregation of hazardous products from food and clothing and from other chemicals likely to interact is already required for sea, air, and land transport, and similar practices are commonly applied in warehouses to minimize danger and facilitate the work of the emergency services. Packaging itself can help, studies showing, for instance, that shrinkwrapping reduces the rate of fire spread in stacks of packages. It is increasingly common practice on the Continent for warehouse operators to require ADR or IMCO classification of products before acceptance, so they can be suitably stored (and charged for accordingly).Chemical warehouses are being built to better standards with wide aisles, product separation, and fu-e-hose or sprinkler systems. The bigger companies set quite high standards -encouraged by insurance costs as well as by safety policies. Some manufacturers already co-operate with local fire services by pro- viding information on products and on emergency hazards and by setting apart Handing Toxic Chemicals -Environmental Considerations. Part I V designated storage areas, clearly marked, for different types of hazardous products.So far I have spoken of regulations and practices to prevent hazardous chemicals causing problems. These may be summarized as better design, to eliminate failure of containers in collision, and good practices, to minimize careless handling. Some of the things I have spoken about will work both ways: Segregation of Chemicals Establishment of fire equipment Packages and labels resistant to exposure or immersion to facilitate subse- quent recovery and identification etc. Incidents will occur, however much we try to prevent them. CIA'S other efforts are toward preventing a minor incident becoming a major accident. In the event that an incident does occur, actions have to be taken, usually in emergency conditions, to limit the effects.Especially for bulk vehicles in transit, urgent recognition of the nature and hazard of the product is essential. Marking the vehicle externally with the name of the product and the hazard symbol is a first step only. Having Tremcards or other instructions in writing with the driver is fine, provided the driver is not injured or otherwise incapable of delivering the in- structions to the would-be rescuer. CIA recommends that Tremcards should also be on the outside of the vehicle in at least two places. Several companies already placard their tankers with large permanent plates carrying emergency information. German practice is to put the Tremcards in special holders behind the compulsory orange plates, front and rear.However, even if available, written instructions may be difficult to understand correctly under difficult visual conditions and the extent of the emergency may require immediate action. Current thinking, therefore, is toward a system of marking vehicles with a simple code, in letters large enough to be read under poor conditions and understood by the emergency services at once, so that preliminary action can be taken to get the incident under control. The U.S.A. has come up with HI, a coding system indicating by two digits the action which should be taken in the first 15 minutes. In that time, and with the facilities available to the Police or Fire Brigade, obviously there are only limited actions which can be taken, so that complicated or obtuse instructions would be useless.HI therefore offers only a choice from a limited range of actions. The London Fire Brigade's Hazchem system can be condensed on to a pocket card giving the code for action. This scheme can cope with a wide range of pro- ducts, not only hazardous chemicals, but again restricts actions to those readily available. Unfortunately, for political reasons the ADR amending Committee could not agree on either the HI or Hazchem system, and they have come up with a third code -Kemler Number -which condenses information about the product hazard but leaves the actions to individual judgement. So for international traffic on the Continent, the symbol, U.N. number of the product, and Kemler Code will be required. In the U.K., the symbol, Meadowcroft U.N.number, and modified Hazchem number will be required. Maybe har- monization will come eventually, but not yet. In either case, however, a large vehicle plate seems likely, of defined size, lettering, and colour, to simplify the problem of the Emergency Services in the first few minutes. Even when a coding system is in use there will still be a need for expert advice and information from Industry, whether on subsequent actions for highly danger- ous products, or action for products not hazardous enough to warrant classifica- tion but unpleasant enough in an emergency. A dozen major companies have internal emergency arrangements linking offices and factories to provide advice and assistance in transport/storage emer- gencies, and various moves are in hand to link these into more comprehensive schemes.In the U.S.A. the major companies under MCA set up the Chemtrec office two years ago in Washington, giving 24 hour availability of first-help advice and with communication links to all major suppliers for follow-up. An area system also exists in Canada. In the U.K., informal intercompany area arrangements have existed for some time on a good-neighbour basis, but CIA has now, with the support of Government Departments and the emergency services, arranged the con- solidation of these into a countrywide emergency-call system -Chemsafe -whereby an initial 32 factories with 24-hour telephone cover undertake to give advice on a range of products and to provide as much help as they can.Other companies are being brought into the area contact scheme. Harwell will act as a backup centre for the Emergency Services, for which a data bank will be organ- ized to include many unclassified, as well as classified, hazardous chemicals. The aim is to have on file, for rapid retrieval, fire, spillage and first-aid advice like that on Tremcards for as many products as possible, with the name and telephone number of the supplier. The scheme, together with CIA manuals and codes on the labelling of hazard- ous chemicals, use of freight containers, safe transport of hazardous chemicals, etc., shows that Chemical Industry members recognize the need for good and improving standards of practice, and that they are willing to share their know- ledge and expertise with other chemical manufacturers. Detailed regulations are not the whole answer.Much of the present adverse publicity is due to the past slowness of the U.K. to introduce regulations (despite ADR/RID being in existence for years as a potential groundwork) and the detail which had to be argued out in almost every case. It could save time in future if legislative emphasis is placed more on definitions and described parameters of hazard, on general requirements for design and operational safety, and clearly placed onus of responsibility, but if it leaves the detail to codes of practice that are given the backing of the law and flexible enough to take account of technical change and progress.I hope that what I have said will show that, far from the gloomy picture some- times painted, there is from much of the industry a substantial expenditure of effort, expertise, time, and money on distribution safety, and hence on en- vironmental protection. The problem that remains is not so much that of estab- Handling Toxic Chemicals -Environmental Considerations. Part IV lishing suitable standards, though of course much remains to be done for some groups of products. It is rather that the Chemical Industry as a whole should bring itself up to the standards of design and operational practice already established, and should use its commercial leverage to ensure that transport, package, equipment, and chemical suppliers also do so when dealing with hazard- ous chemicals. Publicity within the Chemical Industry is one way to encourage compliance with the codes that already exist, which is why we regard meetings such as today’s as extremely important, so that CIA’S work in establishing good standards of distribution safety will be followed more quickly and closely by the Industry as a whole.

ISSN:0306-0012    

DOI:10.1039/CS9750400099

出版商:RSC    

年代:1975

数据来源: RSC

摘要:

Vibrational Infrared and Raman Spectroscopy in Inorganic Chemistry By I. R. Beattie DEPARTMENT OF CHEMISTRY, THE UNIVERSITY, SOUTHAMPTON SO9 5NH 1 Introduction A brief survey of recently published papers in inorganic chemistry indicates that, where i.r. and Raman spectra are reported, one of the principal interests lies in the use of these data as an aid to structural assignment. It is therefore important to consider from the outset how vibrational spectroscopy compares with other methods of structure determination for solids, liquids, and gases. It is also worth noting that one diffraction or microwave study may be of far greater fundamental significance than a collection of several papers using vibrational spectroscopy in a qualitative manner. Table 1 summarizes several techniques of importance in structure determina- tion.1-16 Various less important methods have not been included, for example See, for example, M.M. Woolfson, ‘X-ray Crystallography’, C.U.P., Cambridge, 1970. G. E. Bacon, ‘Neutron Diffraction’, O.U.P., Oxford, 1972; G. E. Bacon, Adv. Struct. Res. Diffraction Methods, 1966,2, 1. L. S. Bartell, in ‘Physical Methods of Chemistry’, Part I11 D, Volume 1, ed. A. Weissberger and B. W. Rossiter, Wiley, New York, 1972; also K. Kuchitsu in ‘Molecular Structures and Vibrations’, ed. S. J. Cyvin, Elsevier, Amsterdam, 1972; S. H. Bauer and A. L. Andreassen, J. Phys. Chem., 1972,76, 3099; R. L. Hildebrandt and R. A. Bonham, Ann. Rev. Phys. Chem., 1971,22,279. A. J. Careless, M.C. Green, and H. W Kroto, Chem. Phys. Letters, 1972,16,414. K. Kimura, K. Katada, and S. H. Bauer, J. Amer. Chem. SOC.,1966, 88,416. C. Glidewell, A. G. Robiette, and G. M. Sheldrick, Chem. Phys. Letters, 1972,16,526. J. Kraitchman, Amer. J.Phys., 1953,21, 17; C. C. Costain, J. Chem. Phys., 1958,29,864.* C. H. Townes and A. L. Schawlow, ‘Microwave Spectroscopy’, McGraw-Hill, New York, 1955;T. M. Sugden and C. N. Kenney, ‘Microwave Spectroscopy of Gases’, Van Nostrand, New York, 1965. See, for example, G. Herzberg, ‘Infrared and Raman Spectra’, Van Nostrand, New York, 1945. lo G. Herzberg, ‘Electronic Spectra of Polyatomic Molecules’, Van Nostrand, New York, 1966; D. M. Gruen, in ‘Progress in Inorganic Chemistry’, ed. S. Lippard, Wiley, New York, 1971, Vol.14. l1 J. A. Pople, W. G. Schneider, and H. J. Bernstein, ‘High Resolution Nuclear Magnetic Resonance, Spectroscopy’, McGraw-Hill, New York, 1959; J. W. Emsley, J. Feeney, and L. H. Sutcliffe, ‘High Resolution Nuclear Magnetic Resonance Spectroscopy’, Pergamon, Oxford, 1965. l2 B. F. G. Johnson, J. Lewis, I. G. Williams, and J. M. Wilson, J. Chem. SOC.(A), 1967, 341. l’ R.T. Grimley, D. W. Muenow, and J. L. La Rue, J. Chem. Phys., 1972,56,490. E. L. Muetterties, Znorg. Chem., 1965,4,769; S. E. Schwartz, J. Chem.Educ., 1973,50,609. l5 P. Diehl and C. L. Khetrepal, in ‘N.M.R. Basic Principles and Progress’, ed. P. Diehl, E. Fluck, and R. Kosfeld, Springer-Verlag, Berlin, 1969; see also A. Pines, M. G. Gibby, and J. S. Waugh, J.Chem. Phys., 1972,56, 1776. l6 A. Weiss, Angew. Chem. Internat. Edn., 1972,11,607. i$6'kTable 1 Comparison of some physical techniques for structural studies sTechnique Nature of the Efect Information Interaction timea Sensitivity@ Comments &X-Ray Scattering, mainly by Electron density 10-l8 s but crystal Location of light atoms or Q diffraction electrons, followed by map of crystal averaged over ca. cm3 distinction between atoms interference vibrational of similar scattering factor 2j(A = 0.l-lOA) motion difficult in presence of heavy atoms1 x Neutron Scattering, mainly by Vector internuclear 10-l8 s but crystal Extensively used to locate ? diffraction nuclei, followed by distances averaged over cu. 1cm3 hydrogen atoms.May give $ interference(A = 1 A) vibrational additional information due 8motion to spin & on neutron leading t: to magnetic scattering2 s Electron Diffraction (atom or Scalar distances 10-l8 s but 1Torr Thermal motions cause 8 diffraction molecule) mainly by due to random averaged over blurring of distances. s9nuclei, but also by orientation vibrational Preferably only one (small) fi' electrons(A = 0.1-1 A) motion species present, Heavy n%atoms easy to detect3-6 3 a Classical time-scale. In the absence of a transition probability factor apparently violates the uncertainty princip1e.l' Gb 1 Tom = 3 x 10lemol ~rn-~,mean free path 0.1 nm. Microwave Absorption of radiation Mean value of r2 10-los 10-4Torr Mean value of r2does not due to dipole change terms; potential occur at re even for during rotation function.' harmonic motion.Dipole (A = 0.1-30 cm; moment necessary. Only 300-1 GHzin one component may be frequency) detected. Analysis difficult for large molecules of low symmetry8 Vibrational Absorption of radiation Qualitative for 10-13s 1 Torr Useful for characterization. infrared due to dipole change large molecules Some structural informa- during vibration tion from number of bands, (A = 10-1-10-4cm) position and possibly isotope effects. All states of matter9 Vibrational Scattering of radiation Qualitativefor s 100 Torr C Useful for characterization. Raman with changed frequency large molecules (v4 dependent) Some structural informa-due to polarizability tion from number of bands, change during a position, depolarization vibration ratios, and possibly isotope (A = visible usually) effects.All states of matter c Resonance Raman 1 Tom. Table 1 (continued) Electronic Absorption of radiation due to dipole change during an electronic transition (A = 102-103A) Nuclear Interaction of radiation magnetic with a nuclear resonance transition in a magnetic fieId (A = 102-107 cm; 3 KHz to 300 MHz) Mass Detect ion of fragments spectrometry by charge/mass Qualitative for 10-15 s large molecules Number of 10-1-10-9 s magnetically equivalent nuclei in each environmente Mass number, plus -fragment ation pat terns Torrd 10Torr (IH) Torr S$ s. 3 Useful for characterization.a. Some structural idorma- stion from number of bands 2 and position. All states of mat terlo & Applicable to solutions and 3@ gases! In conjunction with 9 molecular weight measurements may be F zpossible to choose one from 3 several possible models11 2 Useful for characterization of species in a vapour, 3 complicated by reactions in 93 spectrometer. Does not differentiate isomers g. directly. Important for 0 detecting hydrogen in a $ moleculel2J3 C'9 d Allowed transition (in absorption). For separate resonances to be observed TA > (WA -WB))-~ < TB where (wa -WB)/2T is the chemical shift difference (cycles per second) and TA and TB are mean life times on sites A and B. For multiplet structure TA > J-'AB < TB where JABis the spin-spin coupling constant.f Geometrical data are available from liquid-crystal ~tudies,'~ also solids. l6 Beattie Mossbauer,l7 photoelectron,l* nuclear quadrupole resonan~e,~~resonance fluorescence,20 and e.s.r.21 spectroscopy and molecular-beam electric deflection.22 In addition, ancillary measurements such as molecular weight, dipole moment, conductance, and chemical analysis are important. For the solid state, where single crystals are obtainable, X-ray crystallography is the definitive technique. For solutions and melts n.m.r. spectroscopy may give unambiguous assignment of the molecular shape. In the gas phase, electron diffraction and microwave spectroscopy are of great value.Used in a qualitative manner the major value of i.r,, Raman, and electronic spectroscopy is the ability to span all phases. Certainly, the less one is able to apply X-ray crystallography (owing to inability to grow single crystals for example), n.m.r. spectroscopy (owing to lack of mag- netic nuclei or to exchange processes), electron diffraction (owing to the presence of several species in the vapour or large thermal amplitudes of vibration), and microwave spectroscopy (owing to lack of a dipole moment or the difficulty of analysis of data) so the more important becomes the qualitative approach. Another feature of interest is the level at which detection is possible in a routine study. In this respect mass spectrometry is unique. Double-resonance experiments using resonance fluorescence may result in detection levels below 10-8 Torr.In-cavity studies using laser techniques can lead to greatly improved detection levels in absorption experiments, while photoelectron spectroscopy, with detection levels of the order of Torr, is becoming increasingly important for studying high-temperature species.23 In principle, it is possible to obtain quantitative information from the ‘qualitative’ techniques given in Table 1. Detailed information on molecular geometry may be obtained from rotational analysis of (i) a rotational spectrum (microwave), (ii) a vibrational band (i.r. and Raman), and (iii) a vibrational band of an electronic transition (electronic). The difficulty of the analysis is fre-quently iii > ii > i.There are also experimental problems. The natural linewidth is given by24 5--AE 1 -Av+ h 27rr V. I. Goldanski and R. H. Herber, ‘Chemical Applications of Mossbauer Spectroscopy’, Academic Press, New York, 1968. Is C. R. Brundle, Appl. Spectroscopy, 1971, 25, 8; J. P. Maier and D. W. Turner, J.C.S. Faraday II, 1972, 68, 71 1;D. G. Tinsley and R. A. Walton, J.C.S. Dalton, 1973, 1039. Is E. A. C. Lucken, ‘Nuclear Quadrupole Coupling Constants’, Academic Press, New York, 1969; W. van Bronswyk, Structure and Bonding, 1970,7, 87. 2o R. F. Barrow, I. R. Beattie, W. G. Burton, and T. Gilson, Trans. Faraday SOC.,1971, 67, 583. 21 A. Carrington and A. D. McLachlan, ‘Introduction to Magnetic Resonance’, Harper and Row, London, 1967.22 A. Biichler, J. L. Stauffer, W. Klemperer, and L. Wharton, J. Chem. Phys., 1963,39,2299; A. Buchler, J. L. Stauffer, and W. Klemperer, ibid., 1964,40,3471. 23 T. P. Fehlner and D. W. Turner, J. Amer. Chem. SOC.,1973, 95, 7175; J. Berkowitz, J. L. Dehmer, and T. E. Walker, J. Chem. Phys., 1973, 59, 3645. 21 See, for example, A. Carrington, ‘Microwave Spectroscopy of Free Radicals’, Academic Press, London, 1974. Vibrational Infrared and Raman Spectroscopy in Inorganic Chemistry where 7is the mean life in a given state, dE is the spread of energies in that state, and dv+is the half-width of the spectral band. The lifetime for an isolated molecule in an excited state is given in terms of the Einstein A coefficient for spontaneous emission : 1 3hc3 7=---A 64n4v31pp (where p is the appropriate electric transition moment).In the microwave or molecular rotation region (v = 1010 Hz, h = 3 cm) this gives a value ca. lo-* Hz for dv+,which is negligible. Even in the near U.V. (A = 4000 A, 25 0o0 cm-l)dv+is still only of the order of 10-4 cm-1.* However, Doppler broadening occurs due to the variable velocity component, v, of the random thermal motion of the molecules (relative to the detector). At room temperature for a molecular weight of 100 the average molecular velocity is ca. 2.5 x lo4cm s-1. 21In the microwave region this leads to a value of ca. 10 kHz for dvt.(11 -.v)C whereas at 4000 A it is ca. 1 GHz (or, dividing by the velocity of light, 0.03cm-1) which is becoming serious for high-resolution studies.Formally the averaged effect on the linewidth for Doppler broadening is given by Av., = 7.15 x 10-7(~/~)*~ where T is the absolute temperature and M is the molecular weight, The me of collision broadening is more complicated. It may be thought of as arising because a molecule emitting (or absorbing) a wave train undergoes a collision which destroys the phase coherence of the wave so that dE becomes correspondingly greater. Alternatively it may be considered as knocking a molecule from one quantum state to another, r again becoming shorter anddE correspondingly larger. Using elementary collision theory one molecule in a gas of mol. wt. 150 at 700 K and 1 atm. pressure will suffer on average one collision every nanosecond.Using the approximate relationship dV+T = 1 this gives an uncertainty in the energy of 1 GHz or 0.03 cm-l. A rough rule is that at room temperature pressure broadening is ca. 10 MHz Torr-l. It must be remembered that broadening mechanisms will be different for i.r. and Raman bands and that Q-branches may be affected differently from rotational branches. The klystron provides tunable radiation in the microwave region with the frequencies capable of being measured to the order of 1 part in lo7, leading to precise determination of molecular geometry (e.g. ‘bond lengths’ to a precision of k 0.001 A, the limitation lying in the model used, not the data). The increased availability of tunable, narrow-width lasers of relatively high power is potentially capable of transforming high-resolution spectroscopy from the U.V.through the visible to the i.r. and even into the microwave region. A variety of sophisticated experiments is possible and a few of these will be outlined to illustrate the power of such techniques. The most obvious application *Inline with commonpractice, v is used for both frequency (c/A) and wavenumber (I/& 112 Beattie is an absorption experiment with the laser tuning used, instead of a conventional grating, to vary the wavelength. Thus commercial dye lasers are available, pumped with an argon laser and continuously variable through the visible region with linewidths of < 1 A and powers of the order of hundreds of mW.The linewidth can be reduced by use of an etalon in the cavity. Similarly, pulsed tunable dye lasers, pumped by a nitrogen laser, with bandwidths of c 0.004 A have been described.25 However, much more exciting experiments are possible, making use of the very narrow linewidth, high power, and directional properties of lasers. In particular, Doppler broadening effects can be drastically reduced by the use of saturation phenomena, two-photon phenomena (whereby using two laser beams in opposite directions the component of the Doppler velocity cancels out) or molecular-beam techniques. Saturation experiments, such as the Lamb dip phenomenon, can be used to reduce effective linewidths if they are Doppler broadened.26 Consider the laser radiation, in a multipass system, interacting with molecules in the gas phase.This may be in an ‘in-cavity’ experiment or a ‘saturating beam’ may pass in one direction while the (weaker) analysis beam passes in the opposite direction. Either side of an absorption maximum the molecular absorption frequency will equal the (narrow, monochromatic) laser frequency for molecules with velocity components v k dv in either direction along the laser (photon) axis (where dv refers to the energy spread of the laser beam, not the Doppler-broadened absorption under study). By contrast, when the laser is tuned precisely to the centre of an absorption maximum, only waves Ymax f dv (i.e. one group of molecules, not two groups as before) are at the laser frequency.This means there is a sharp dip in the absorption curve at the band maximum if the first pass has already saturated the molecules of Vmax f dv. As this is the centre of the Doppler-broadening band it corresponds to a Doppler velocity of v = 0 (f dv) where the molecules are moving transversely to the light beam. Thus using saturation by the intracavity field, locking a maser on the 3.39 pm rotation- vibration line of methane [P(7) of v3] gave a repr~ducibility~~ of better than 1 in loll (2.5 orders of magnitude better than the primary standard of length) and a linewidth of ca. 150 kHz--i.e.dv+/v = 10-9. Similarly, using a magnetically tuned He/Ne maser, the Stark effect in methane (which has no permanent electric dipole) can be observed on one component at 2947.802 cm-1 of this P(7) line.28 2 Vibrational Infrared and Raman Spectroscopy Although vibrational spectroscopy is a relatively poor technique for structural studies it is important principally because it is a ‘fast technique’ in terms of stereochemical non-rigidity of molecules and because of its wide application.T. W. Hansch, Appl. Optics, 1972,11,895. B6 See, for example, W. Demtroder, ‘Laser Spectroscopy Topics in Current Chemistry’, No. 17, Springer-Verlag, Berlin, 1971; see also J. E. Bjorkholm and P. F. Liao, I.E.E.E.J. Quantum Electronics, in the press. *’ K. Shimoda, Quantum Electronics Conference, Montreal, 1972. K. Uehara, K. Sakurai, and K. Shimoda, J.Phys. SOC.Japan, 1969,26,1018. Vibrational Infared and Raman Spectroscopy in Inorganic Chemistry It may be applied, for example, to open- and closed-shell compounds, to ions, to solids, liquids, solutions, and gases, to sorbed species and to species isolated in a matrix.Formally the amount of information present in a vibrational spectrum is considerable-it is the inability to interpret it that is at fault. For chemically interesting molecules the interpretation of the vibrational spectrum is essentially at a 'balls and springs' level. Intensity, half-width, band contour, and band position are frequently not understood in sufficient detail. Even for diatomic molecules the relationship between bond strength, force constant, and bond order is at best qualitative. Nonetheless, the design of i.r.and Raman spectrometers to cover the frequency range 4000-50 cm-l has now reached the stage that such investigations are almost routine. The only obvious advances yet to come appear to be the intro- duction of fully tunable lasers and an extension of interferometric techniques. It is evident that vibrational spectroscopy is likely to be even more extensively used in the future and it is reasonable to attempt to assess the value of this technique and its future application. For a vibration to be observed in the i.r. spectrum the molecular dipole moment must change during the vibration under study. Put in a more formal way choosing real wavefunctions, integrals of the type must be non-zero where (pz)a3is the transition moment, p.Iz = (8pz/aQk)ois the magnitude of the x-component of the dipole moment derivative with respect to the appropriate normal co-ordinate Qk, and i and j refer to two vibrational states; dT is a volume element in configurational space and integration is over the whole of this space.(In general for an integral of the type fAfB.dr to be non-zero the integrand must be invariant to all symmetry operations of the symmetry group to which the molecule belongs.) If i is the ground state, $"f is totally symmetric. For the direct product Qk.#vj to contain the totally symmetric representation both Qk and $,j must belong to the same irreducible representation. Clearly, for this to be so, $,,j must have the same symmetry as the normal co-ordinate. Further, i and j must differ so that dZI = 1, for simple harmonic eigenfunctions.Neglecting rotational quantization, the i.r. absorption intensity of a particular fundamental mode of vibration i is given by where N is the number of molecules per unit volume and vz is the (harmonic) frequency of the fundamental. Accurate values of absolute absorption intensity are difficult to obtain in the i.r. effect largely owing to the problem of finite slit width, leading to a beam which is not monochromatic. Two problems arise in the use of the above equation. One is that of sign due to the squared term. The other, more fundamental problem is that an 'accurate' force field is necessary to Beattie yield the form of the normal co-ordinate Qi. The principal interest in such measurements and calculations, from the chemist’s point of view, lies in dis-cussions of the degree of covalency in a m01ecuIe.~~ The criterion for Raman activity is more difficult to describe pictorially because it involves tensors. Whereas the i.r.effect is conventionally studied in transmission (although both emission and reflectance measurements can be made under appropriate conditions) the Raman effect is essentially a scattering phenomenon observed at a frequency different from that of the incident light. It is useful to look at the scattering phenomenon classically. The oscillating electric vector of the incident (monochromatic) light induces an oscillating dipole in the molecule under investigation. This dipole can then re-radiate with light of the same frequency as that of the exciting light (elastic or Rayleigh scattering). If the molecule belongs to one of the cubic groups (x = y = z) then as can be seen from Figure 1 the induced dipole is in the same direction as that of the Td or Ohmolecule observed Rayleighscattering incident radiation/1 x Figure 1 Polarization of Rayleigh scattering for a molecule belonging to the cubic groups inducing field.In considering this it is important to realize that during the time- scale of the observation the molecule may be considered to be fixed in space. If the excited state were long-lived (as occurs in resonance fluorescence processes for example) then classically the molecules (and hence the dipoles) would re-orientate during the time of the observation.Consider a less symmetrical molecule HgClz where x = y z. If we carry out $@ D. Steele, Quart. Rev., 1964, 18,21. Vibrational Infiared and Raman Spectroscopy in Inorganic Chemistry the experiment illustrated in Figure 2 it is reasonable to suppose that the indud dipole lies along the z-direction. We may write Pz = cllzzEz where Pzrefers to the induced dipole, Ez to the electric field of the incident light, and aezto the component of the polarizability in the zdirection. t :J’ observed Rayleigh scattering X Figure 2 Polarization of Rayleigh scattering for an oriented molecule of HgCI, Suppose we now incline the HgClz molecule relative to our fixed laboratory co-ordinate system. The molecular axes will be defined by x, y, z: the laboratory axes will be defined by X,Y,2.The change in co-ordinate axes may be carried out by a similarity transformation (see also p. 128): O~XYZ= T~~zyzT-l where Tis a transformation matrix (note also that because the axis systems are orthogonal the inverse of Tis also the transpose of T).The components of Tare the directioncosines of the two axis systems. To simplify the algebra, consider rotation of the HgCh molecule by an angle 8 about laboratory Y.We then obtain the position shown in Figure 3. The first row of the transformation matrix is then formed by the direction cosines : Xon x Xon y Xonz which becomes cos e 0 cos(90 -8) 116 Beattie Figure 3 Continuing in this way, writing cos 8 = c8, cos (90 -8) = sin 8 = s8 and noting c(90 + 8) = -so, we obtain ce 0 Thus for this molecule, where axx= alyy= a f azz= b, Px ce 0 Pz which on matrix multiplication yields Px ac28 + bs28 0 (6 -a)c8 Py=( 0 a 0 PZ (b -a)c8.s8 0 as28 + bc28 Note the appearance of the off-diagonal XZ terms.If 8 = 0" or 90" the off- diagonal terms disappear and we are left with the matrices Vibrational Infrared and Raman Spectroscopy in Inorganic Chemistry a00 bOO Or(: ; a) k ; 3 respectively. Thus in the discussion of Rayleigh and Raman scattering it is extremely important to distinguish laboratory fixed axes and molecular fixed axes. Pictorially it is convenient to discuss the polarizability of a molecule in terms of a triaxial ellipsoid.Perhaps the easiest way of understanding this is to approach it via classical mechanics and the momental ellipsoid. Consider a rigid molecule composed of masses rnt and weightless connecting rods. The moment of inertia about any axis of rotation through the centre of mass is given by I = crntri2, I where rc is the perpendicular distance of the mass mi from the axis of rotation. By plotting 1-4 aZong the axis of rotation from the centre of mass for all possible rotation axes a closed surface is generated. Using the symmetry-dictated Cartesian axes* as x, y, and z the surface corresponds to a triaxial ellipsoid of equation kx2 + Zy2 + mz2 = 1 where k,Z, m are constants. In an exactly similar manner the polarizability-which may be regarded qualitatively as the ease with which the electrons can be made to redistribute in the molecule-can be represented by a triaxial ellipsoid: assx2 + aygp + azz.22 = 1 To obtain this ellipsoid consider the polarizability a~ along the direction of the incident electric vector E.Thus PE = ~E.E If, as with the momental ellipsoid, CXE-~is plotted for all directions of E the polarizability ellipsoid is obtained. If the Cartesian axes are also the principal axes, no off-diagonal components arise. Note also that the shortest axis refers to the greatest magnitude of at$. It is essential to realize that the polarizability ellipsoid of (1) (like the momental ellipsoid) is in every respect identical with that of (2). * For molecules of no symmetry the chosen axes are those along which 1-4 is a maximum and a minimum, together with a third axis mutually perpendicular to them.Beattie This classically accounts for the different selection rules for pure rotation (rigid rotor) in the i.r. and Raman effects. In the i.r. effect a rotation about a line perpendicular to the (permanent) dipole axis results in the vector coming into an identical position once only per revolution. This gives the selection rule dJ = 0, k1. For the Raman effect as we have seen the polarizability ellipsoid assumes an ‘identical’ position after half a revolution.* The frequency of change of aspect is thus twice that in the i.r. effect and the selection rule is dJ = 0, k2. If we now allow the molecule to vibrate, in essence the harmonic oscillations of the molecule are superimposed on the sine form of the incident electric vector. This combination of sine terms leads to the appearance of both sum and difference frequencies.For small nuclear displacements, expanding in a Taylor series, the polariz- ability at any instant in time for the vibrating molecule is given by together with similar terms for axyt,a&, etc. Here axxorefers to the undisplaced molecule and subscript 0 to displacements tending to zero. We also havet Ez = Exocos (27rvt) etc. so that Px= (axxoExo+ axyoEyo+ axzoEzo)cos 2vvt o and similarly for Pyand Pz.(It is convenient to replace rz)by a’xy).Note that although for Rayleigh scattering atj terms are zero (i # j),it does not follow that a’ij terms will be zero.The first term refers to elastic (Rayleigh) scattering and the second term to inelastic (vibrational Raman) scattering, giving rise to light of frequency (v + Vk) or (V -Vk) where Vk is a fundamental mode of vibration of the molecule. Because quantum mechanically the (v + vk) terms correspond to depopulation of an excited state the ratio of intensities for (V + vk) to (v -vk) follows a Boltzmann distribution. In conventional Raman spectroscopy it is the more intense (V -vk) terms which are recorded. The criterion for (vibrational) Raman activity is that there is a change in polarizability * For spherical tops there is no rotational Raman spectrum. + Coincidence of molecular and laboratory axes has been assumed here for simplicity.This formally means aff’ = 0 for this case. Vibrational Infrared and Raman Spectroscopy in Inorganic Chemistry with normal co-ordinate (a'if # 0). Alternatively, taking note of the dipole- dipole nature of Raman scatterings, terms of the form must be non-zero. The expression is closely similar to that used for the infrared effect. Again a selection rule of dv = f 1 is operative for harmonic functions. Where a molecule retains the same symmetry for the polarizability ellipsoid and the same directions of the axes for the polarizability ellipsoid in the un- displaced and the displaced (during a vibration) positions, the Raman scattering tensor, like the Rayleigh tensor, is diagonal. However, an important distinction arises when this is not so, giving rise to off-diagonal terms in the Raman scattering tensor such that, using molecular fixed axes, light polarized in the z-direction may give rise to scattered light polarized in the x-direction (an term in the tensor).This may be considered for theng deformation of HgzClz where, if the xz-plane is the plane of the paper (Figure 4), clearly the x-and z-. H I c1I8" Figure 4 Polarizability e1lQmoid.v for the symmetric deformation of Hg,CI, at the undis- placedposition and at either extreme of the vibration (Note that Raman activity occurs via an of-diagonal term because the directions of the ellipsoids I and I1 difer although they are otherwise identical) axes of the displaced polarizability ellipsoid differ in direction from those of the undisplaced ellipsoid. Only the y-axis remains unaltered in direction.This gives rise to an dZzterm such that light introduced polarized along the z-direction will appear as scattered light polarized along thex-direction. Clearly this vibration is doubly degenerate (x = 7)so that by rotation of Figure 4 by 90"about the z-axis an term is obtained. The Raman tensor for theng deformation of HgzCla is thus of the form Beattie where a’zz= a’z2for normal Raman scattering. Note that the sum of the diagonal terms (a’t~)is zero in this case. This is always true for a non-totally symmetric vibration. In general, diagonal terms arise (a) for totally symmetric modes (where the full symmetry of the molecule is retained during a vibration); (6) for non-totally symmetric modes in the non- cubic groups where there is a polarizability change in the xy-plane perpendicular to an axis of higher than two-fold symmetry, when x2 -y2 (i.e.alzz = -dug) terms may occur; and (c)in the cubic groups, where for non-totally symmetric modes 2ze -x2 -y2 (i.e. -dzz = -dug= 2dZz)may occur. A related important feature which arises from the tensor relationship in Raman scattering is the possibility of obtaining anisotropic information from homogeneous fluids. The random orientation of molecules in a fluid averages the oscillating dipoles (vectors) to zero. By contrast a tensor averaged over all orientations is not zero. Consider Rayleigh scattering: the invariants of the tensor are (i) a spherical portion, = 1/3(air + a55 + am) (ii) an anisotropic portion, y2 = *[(aft -~Yjj)~4-(a5j -(lIkk)2 4-(azs -Qkk)’ + 6(a2ij i-a2tk 4-a25k)1 The values of these two functions are invariant with respect to the orientation of the laboratory co-ordinate axes relative to those of the (molecular) polariz- ability ellipsoid.From the discussion of the momental ellipsoid and the polariz- ability ellipsoid it is also clear that were the natural axes to be used there would be no off-diagonal terms. The measured intensity depends on squared terms, so that an oscillating dipole p = p cos 2TVt emits per solid angle in the X-direction Remembering that P = aE = p, for the experimental arrangement shown in Figure 1 with plane-polarized radiation incident along the X-direction, 2rr3v4 IL = -azx2Eo2cs 2~4I,, = -c3 azz2Eoa Again the off-diagonal term QZX arisesbecause the laboratory frame of reference is not aligned with the principal molecular axes.To obtain a relationship between Vibrational Infrared and Raman Spectroscopy in Inorganic Chemistry ~XYZterms and azUzterms we have to average over all orientations of the molecule to obtain: -1 bzz)2 = -15 y2 -453 + 4p(axx)2 = 45 2T3v4 y2I, = 7(g)NEo2 NEo2 where N is the number of molecules. Thus the depolarization ratio for Rayleigh scattering pp is given by pp=-= 3Y21, I,, 4562 + 4y2 where pp refers to the use of incident polarized light and y2, where we choose principal axes, contains no aij terms.Clearly for a molecule in the cubic groups aZZ= aYY= azz,so that pp = 0.The greatest value of pp would be obtained for a molecule in which aZxis finite but ayYand azztend to zero. In this case pP ---f 113. For Raman scattering identical equations apply except that we must substitute 01’ for 01 and (Y’)~for y2. For molecules belonging to the cubic groups, as for Rayleigh scattering, pp for the totally symmetric vibration is zero (a‘xZ= dYY = dZz; = 01‘~~01’~~ = a‘yz= 0). For a vibration which is antisymmetric with respect to any symmetry element of the molecule as we have seen earlier a’zZ+ a’yy+ a’zz= 0 so that a’ also is zero, but y’2 is finite and hence pp = 8. For totally symmetric bands (i.e.vibrations which maintain all the symmetry elements of the original molecule) the depolarization ratio for the experimental situation depicted in Figure 1will lie between 0 and $. The intensity of a Raman line at a displacement Av from vo (the exciting frequency) is given by : where K contains the constant terms. In this expression it is assumed that the scattering molecules are in a non-degenerate electronic ground state and that vo is well removed from any region of absorption. As with the i.r. effect, absolute intensities are difficult to measure in the Raman effect. It is customary to consider two factors, the geometrical optical effect based on spectrometer performance (including the refractive index of the sample) and the internal field effects intrinsic to the system under study.Where accurate 122 Beattie intensity and depolarization measurements are made it is formally possible to calculate derived mean bond polarizabilities from the observed isotropic part of the Raman scattering [that proportional to (z’)2]using elementary Wolken- stein30 theory. The use which has been made of derived bond polarizabilities is limited. Their interest would be principally in calculating intensities of fundamental modes of vibration in the Raman effect, In considering Raman intensities pictorially from a sketch of the appropriate normal mode it is very easy to make errors by mentally adding vectors rather than tensor components.3l Before leaving this section it is useful to summarize some of the more important features : (i) Group theory enables the prediction of the i.r.- and Raman-active fundamental modes of vibration (and combination or overtone bands) for isolated molecules32 or for crystals in the centrosymmetric space gr0ups.3~ (ii) Assuming approximate force fields to be available for the molecule under study, vibrational frequencies can be calculated from standard programmes requiring the input only of atomic masses, Cartesian co-ordinates, and force constants. The extension to unit cells of crystals is straightforward?* (iii) In the i.r.effect the criterion for activity is a change in dipole moment with normal co-ordinate. In the Raman effect the criterion is change of polarizability with normal co-ordinate and polarized bands (with pp in the range 0-$) only occur for totally symmetric modes.* Where a centre of symmetry is present i.r.bands are Raman inactive and Raman bands are i.r. inactive. Raman vibrations of centrosymmetric molecules (or crystals) retain the centre of symmetry. We shall now briefly consider one molecule, boron trichloride. There is only one totally symmetric mode: Clearly the polarizability ellipsoids are different at the extremes of the vibration; hence a’ # 0 and the vibration is Raman active. The directions of the axes of the polarizability ellipsoid and its symmetry remain unchanged and hence there are no off-diagonal terms. As x = y if z is chosen as the three-fold axis, the Raman tensor is * Note that pure rotational Raman bands are depolarized.30 M. Wolkenstein, Compt. rend. Acad. Sci. U.R.S.S.,1941,30,791; M. Wolkenstein,J. Phys. U.S.S.R., 1941, 5, 185; M. Eleashevich and M,Wolkenstein, J. Phys. U.S.S.R., 1945, 9, 101,326. 31 S. F. A. Kettle, I. Paul, and P. J. Stamper, Inorg. Chirn. Acta, 1973,7, 11. 32 See, for example, F. A. Cotton, ‘Chemical Applications of Group Theory’, Interscience, New York, 1963. 33 R. Loudon, Adv.Phys., 1964,13,423 (errata, 1965,14,621). 34 I. R. Beattie, N. Cheetham, M,Gardner, and D. E. Rogers, J. Chem. SOC.(A), 1971,2240. Vibrational Infrared and Raman Spectroscopy in Inorganic Chemistry There is no change of dipole during the vibration and the mode is i.r. inactive. The only other non-degenerate mode of this molecule is the out-of-plane deformation.Clearly there is a dipole moment change in this vibration. The band is i.r. active with the transition moment transforming as z. c1*I a: c1’ /”\,,* To examine the Raman activity, consider the undisplaced polarizability ellipsoid and the ellipsoids at either extreme (Figure 5).* (Note that in this vibration in the undisplaced position the dipoIe moment is zero. This is not true of the polarizability. The polarizability of an undisplaced molecule is always finite.) I I1 IT1 0 Q Figure 5 * In the Journal of Chemical Education for 1967, vibrations v, and v, of carbon dioxide are depicted as producing no change of the polarizability ellipsoid from that of the undisturbed molecule.Thiserror has recently been repeated in a monograph on Raman spectroscopy.Beattie Ellipsoids I and 111are identical but differ from the undisplaced ellipsoid II. Thus the polarizability change is of the form shown in Figure 5, so that (01’)o --+ 0 and the band is Raman inactive. (Ina non-centrosymmetric molecule, of which BC13 is an example, it is only vibrations of this type which are i.r. active and Raman inactive. All other i.r.-active vibrations are also Raman active.) This leaves four modes to find (3n -6, less the two already given). These are the (degenerate) in-plane deformation and antisymmetric stretch, both of e’ symmetry and most easily discussed using character tables.35 A. Solid State.-When radiation falls on an isotropic non-absorbing medium it is reflected and refracted, with changes in polarization characteristics. Ignoring fluorescence effects the refracted ray will be attenuated by elastic (Rayleigh) and inelastic (Brillouin and Raman) ~cattering.~~ Crystals are not normally isotropic; they are birefringent, implying that the refractive index is not the same in all directions.The variation of refractive index with direction in the crystal may be represented by a triaxial ellipsoid called the indicatrix. For crystals of orthorhombic or higher symmetry the indicatrix axes coincide with the crystallographic axes. For the general triaxial ellipsoid there are two circular sections which pass through the origin. The directions perpen- dicular to these sections are termed the optic axes, leading to the term ‘biaxial’ for such crystals.In the case of uniaxial crystals the indicatrix is an ellipsoid of revolution and the optic axis is parallel to the principal symmetry axis.37 When radiation falls on a birefringent material in general two rays of different polarization characteristics are produced, both or one of which will not obey the normal laws of refraction. To avoid this splitting of the exciting (and observed) radiation it is essential in single-crystal studies to use the indicatrix axes, or axes of the appropriate elliptic section of the indicatrix. However, for uniaxial crystals, as one of the indicatrix axes is also an optic axis it is important to avoid pro- pagation (or observation of Raman light) along the optic axis.38 Because the elliptic section perpendicular to an optic axis is circular, if one is slightly off the axis in propagating the incident light, two rays may be produced which will interfere, resulting in loss of the polarization characteristics of the incident light.Such behaviour is less likely to occur for biaxial crystals as the optic axes are usually well removed from the indicatrix axes which are frequently also the crystallographic axes and hence also frequently the alignment axes. If the crystal has intense absorption bands in the region under study the refractive index will undergo large changes in the region of these bands (Figure 6) leading to major changes in the reflectivity of the sample. Further, where the 38 For a general discussion of infrared and Raman spectroscopy see G.Herzberg, ‘Infrared and Raman Spectra’, Van Nostrand, New York, 1945; for the theory of the Raman effect see J. A. Koningstein, ‘Introduction to the Theory of the Raman effect’, Reidel, Dordrecht, 1972. a* M. Garbuny, ‘Optical Physics’, Academic Press, New York,1965. 37 N. H. Hartshorn and A. Stuart, ‘Crystals and the Polarizing Microscope’, Arnold, London, 1970; V. C. Varghese, J. Opt. SOC.Amer., 1967,57, 1351. 38 S. P. S. Porto, J. A. Giordmaine, and T. C. Damen, Phys. Rev., 1966,147,608. Vibrational Infrared and Raman Spectroscopy in Inorganic Chemistry h Figure 6 Change of refractive index (n)in the region of an absorption band (absorption index K) indicatrix axes are not symmetry-determined not only will their magnitude change, but also their direction will change on passage through an absorption band.In the case of absorption measurements there is the additional difficulty of pleochroism (anistropy in the absorption index). For crystals of lower than orthorhombic symmetry the directions of the axes of the absorption index and the refractive index (indicatrix) need not coincide except in the case of the unique axis (b)for monoclinic crystals. B. Raman Spectra of Single Crystals.-The problem here is to consider the interaction between the incident refracted light beam and elastic waves in the crystal. For centrosymmetric crystals the prediction of optically active modes can be made rigorously using group theory.33 A factor-group analysis is an essential first step in solid-state spectroscopy, providing that the full X-ray structure is known.39 The procedure for a factor-group analysis is essentially the same process as carrying out a conventional point-group analysis on a molecule.The differences 39 See, for example, S. S. Mitra, and P. J. Geilisse, ‘Progress in Infrared Spectroscopy’, Plenum Press, New York, 1964, Vol. 2; see also R. S. Halford, J. Chem. Phys., 1946,14,8; S. Mitra, Solid State Physics, 1962, 13, 1;C. H. Ting, Spectrochim. Acta (A), 1968, 24, 1177; R. A. Cowley, Proc. Phys. SOC.,1964,84,281; J. E. Bertie and J. W. Bell, J. Chem. Phys., 1971,54,160; S. H. Chen and V. Dvorak, J. Chem. Phys., 1968,48,4060; W.Vedder and D. F. Hornig, Adv. Spectroscopy, 1961, 2, 189; G. R. Wilkinson, W. C. Price, and E. M. Bradbury, Spectrochim. Acta, 1959,14,284. Beattie between molecules and crystals are: (i) the extended elements of crystals such as glide planes or screw axes become for molecules more restricted elements such as mirror planes or rotation axes; (ii) in the analysis a polyatomic grouping is regarded as invariant to a symmetry operation which causes only intramolecular rearrangements between molecules or ions which are related to one another solely by a primitive translation; (iii) aprimitive unit cell (which is not necessarily that used by the crystallographer) is the one that is important for vibrational spectroscopy and may contain more than one complex ion or molecule leading to splitting of internal modes of vibration via correlation coupling; and (iv) the point group isomorphic with the corresponding factor group is useful in dis-cussions of activity of modes.In considering crystals it is useful to examine both the factor group approach and the molecule or ion site symmetry, plus coupling with other units in the primitive cell. Thus in Volume I of International Tables for X-ray crystallography under Space Group number 62 we find ~wxz-D%~. The isomorphic point group is therefore D2h and the complete notation given is P21/n21/m21/a.In addition the four-fold positions either have one mirror plane (m =c,)or a centre of symmetry (i= G). The cell is of course primitive (P). An example will indicate the elegance of single-crystal studies.For an orthorhombic crystal, if the plane-polarized laser beam is incident parallel to crystal z, with the electric vector in the x-direction and observation is made along y with the analyser set in the z-direction, then this is termed a z(xz)y observationso that cos ~T(V-vi)t Rzx Rzy orPzv= Rxz&!?zocos 2n(v -vt)t = EY,where Pzv refers to the particular band under observation and Rt3 (= a’ij) is the Raman tensor component of the crystal. In this observation, ideally only the Rzz tensor component is active as the intensity observed depends only on RZx2.For the crystal of symmetryPnrnu-D162~ this implies that only 62s modes will appear in the spectrum. By carrying out (xx),(xy), (xz),and (yz)observationson an orthorhombic crystal of this symmetry it is (ideally) possible to assign the observed bands unambiguously to symmetry classes40 (Figure 7).As chemists we are usually interested in molecules or ions in the crystal rather than in the whole primitive cell. We therefore may make an initial assumption that the molecule or ion carries over its derived polarizability tensor from the discrete species to the crystal unchanged. If the derived polarizability tensor of the molecule is a’tj and that of the crystal Rtj, then transformation from molecular to crystal axes is required. It is, however, necessary to take care that the character 40 I. R. Beattie, M. J. Gall, and G.A. S. Ozin, J. Chem. SOC.(A), 1969, 1001.127 Vibrational Infrared and Raman Spectroscopy in Inorganic Chemistry er 1222 Figure 7 Oriented single-crystal Raman spectra40of NapSa06,2Hz0 table ‘axes’ are lined up correctly with those used for crystal and molecular axes41 For molecular axes For crystal axes ThusPc” = TPm” = T2Em”where Tis the appropriate transformation matrix. SimilarlyEcv = TEVnv,so that Pc” Ta‘TtEc”=z or R = Tor’Tt where Pis the transpose of T. Because the matrices are orthogonal it is also the inverse of T. In this way it is ideally possible from single-crystal measurements to obtain the relative magnitudes of the tensor components for each fundamental mode of vibration of the molecule or ion in the crystal. From these values it is possible to predict relative intensities of Raman bands in solution and their depolarization ratios-within the framework of the approximation used (see Table 2).In the case of non-centrosymmetric crystals, predictions by group theory are not exact. Thus the case of zinc oxide which crystallizes in the system P6rnrn-c6, with four atoms in the primitive cell has been carefully examined by Damen, Porto, and Tell.42 There are 3 x n(= 4) = 12 phonon branches of which nine will be optical and three will be acoustic. (The quantum of energy 41I. R. Beattie and G. A. Ozin, J. Chem. SOC.(A), 1969, 542. 48 T. C. Damen, S. P. S. Porto, and B. Tell,Phys. Rev., 1966,142,570. Beattie Table 2 Relative intensities and depolarization ratios for NazSzO6,2HzO in the solidstate and in aqueous solution40 Free ion Powder Solution PP mode Ire1 Ire1 calc. obs.calc. obs. calc. obs. v1 40 52 112 122 0.004 0 < p < 0.02 alg v2 23 34 54 54 0.005 0 < p < 0.02 v3 100 100 100 100 0.426 0.35i-0.03 v4 17 18 26 16 0.75 0.74i-0.02 eg v5 16 22 14 13 0.75 0.73k0.02 v6 65 58 50 34 0.75 0.75f0.02 in an elastic wave is termed a phonon by analogy with the term photon for an electromagnetic wave.) Application of group theory leads to the predictions for the optical modes rcryst = a1 + el + 2ez + 2b of which the b branches are inactive and the e2 branch is Raman active only. The only modifications which have to be made to the simple theory concern the a1 and el branches which are affected by the macroscopic electric field associated with the i.r.-active longitudinal optical (LO) mode.Thus the transverse optical modes (TO) and the longitudinal optical mode (LO) may appear at different frequencies. (In essence for LO modes the change in dipole occurs along the propagation direction for the phonon wave whereas in TO modes it occurs per- pendicular to the propagation direction.) In i.r. absorption experiments it is the TO mode which interacts with the photons which, in wave terminology, are also transverse waves. For molecular crystals the change of dipole with vibra- tional co-ordinate may be small and hence the separation of LO and TO modes may be small. In the case of zinc oxide we will deal with the non-degenerate case first: The character table for c6v shows that for a1 modes the Raman tensor components are Rxx = Ryy = Rzz;for the i.r.vector we have pzonly. Consider an x(zz)y observation. When a photon interacts with a phonon both energy and momentum must be conserved (Figure 8). For conservation of energy vp = vi -VI The momentum is given by hk where k is the wave vector defined by k = 24A (A being the wavelength).43 Thus for conservation of momentum kp = ki -ks In Figure 8 the phonon propagates in the xu-plane (incident/observation plane) 43 C. Kittel, introduction to ‘Solid State Physics’, Wiley, New York, 1971 ;C. A. Wert and R. M. Thomson,‘Physics of Solids’, McGraw-Hill, New York,1964. Vibrational Infrared and Raman Spectroscopy in Inorganic Chemistry obscrved photon vs X-incident photon F, Figure 8 Interactionsof photon with phonon in a (zz)observation for zinc and the polarization is defined by the direction of the dipole moment change, noted above from character tables as pz.The double-headed arrows directing the polarization direction are thus all parallel to z. Because for the phonon (Aveffectively) the polarization direction (z) is perpecdicular to the propagation direction (xy-plane) the a1 mode is a pure TO. (Note also that because AV < v and(v -Av),9 is ca. W.) The case of the el mode is much more complex because of the degeneracy. The character tables merely provide ps,pLy;asz,aYz,but do not tell us what pairs go together.We must either appeal to first principles or to a tabulation where this is made clear. It must be emphasized that although for a degenerate mode there is an ,infinite number of possible linear combinations which act as a representation of the mode concerned, by making an observation we select one ‘half’ of the combination and thereby define the other half. From Gilson and Hendra’s we find, el : pxwith axz py with ayz We thus obtain Figures 9a and 9b for x(zx)z and x(zy)z observations, bearing in mind that it is the i.r. vector that defines the polarization direction. It is clear that for the (zy) observation a pure TO mode is obtained (at 407 cm-1). By contrast the (zx) observation shows the phonon polarization to be at ca.45” to the propagation direction, giving both TO and LO observations with bands at 395 cm-1 (TO) and 581 cm-1 (LO). Additional complications arise which will not be discussed here, due to interaction of the a1 and el nhonons. Because dipolar interactions reinforce one another for LO modes, LO modes are perturbed relative to TO modes arising from the same factor group fundamental. Hence LO modes occur at the higher 44 T. R. Gilson and P. J. Hendra, ‘Laser Raman Spectroscopy’, Wiley, London, 1970, Beattie Y Y 1 2 0 X X Figure 9 Conservation of energy and momentum diagrams for zinc oxide: (a) x(zx)z;(b) X(ZY)Z*~ frequency, the difference in frequency giving a measure of the strength of the i.r. transition. C. Powders.-The Raman spectra of powders generally yield less information than that obtained from single crystals of known structure.Further, because of scattering within the powder, depolarization ratios cannot normally be obtained. However, powders deriving from isotropic materials, if immersed in liquids of suitable refractive index, can yield useful depolarization ratios. D. Infrared Spectra of Single Crystals.-In the i.r. effect the material under study, of necessity, has absorption bands in the wavelength region of interest. Thus there are major changes of reflectivity and refractive index as a function of wavelength. This contrasts sharply with Raman spectroscopy where the frequency of the exciting light is constant and the material under study has frequently no absorption bands in the region of interest if the laser line is carefully chosen.For i.r. studies of crystals normally some form of preparation is necessary in terms of cutting, grinding, and polishing. This severely limits the value of single- crystal studies to the chemist who is dealing with reactive compounds. Further, the sample size necessary is usually appreciably greater than that for Raman studies, where in addition the crystal can be examined in a glass-walled vacuum ampoule. In transmission spectroscopy of single crystals it is necessary to prepare thin sections in one or more crystallographically different directions. Similarly, for reflectance measurements it is necessary to prepare reasonably flat surfaces in one or more directions.Using commercially available instrumentation a sample 131 Vibrational Infrared and Rainan Spectroscopy in Inorganic Chemistry area of not less than 0.25 cm2 is desirable. (This size will be appreciably reduced if tunable i.r. lasers become available.) Because of the vector-vector-vector relationship for i.r. spectroscopy compared with the vector-tensor-vector relationship for Raman spectroscopy, trans- ference from molecular axes to crystal axes in the i.r. case merely requires the projection of the molecular axes on to the corresponding crystal axes to effect the required transformation (i.e.x on X, y on Y, and z on Z). The same problems arise in terms of orientation of the crystal with regard to the electric vector of the incident and collected radiation, except that for i.r.spectra of triclinic and monoclinic* crystals the indicatrix and absorption index axes (which need not coincide) may change in direction with frequency. For a non-absorbing medium the variation of the reflectivity (R) with angle of incidence is shown in Figure 10. At the Brewster angle the reflected ray is plane 1.o R 0.8 0.6 0" 30' 60' Angle of incidence Figure 10 Reflectivity as a function of angle of incidence for n = 2 (Reproduced by permission from M.Garbuny,'Optical Physics',AcademicPress,New York,1965) polarized in the plane of incidence. Normally, i.r. reflectance and transmission measurements are carried out on single crystals at approximately normal incidence.The reflectivity is then given by (n -1)2 + k2R= (n +--I-k2 * Except for the unique 6-axis. Beattie where k is the real absorption index and n is the real refractive index. On the low-frequency side of an absorption band the refractive index increases rapidly with the frequency leading to a high reflectivity (Figure 6). Thus errors occur in band intensity, position, and contour when carrying out measurements based solely on the amount of light transmitted. In certain cases erroneous bands have been found in transmi~sion.~~ In i.r. reflectance spectroscopy both n and k can in principle be obtained from measurements of R as a function of wavelength at a fixed angle of in~idence.~~ The most widely used approach is a Kramers-Kroenig analysis based on phase and amplitude changes after reflection.The complex reflectivity Y* = reie = (n* -I)/(n* + 1) with n* = n + ik; 8 is the phase (angle) change on reflection. Equating real and imaginary parts one obtains (1 -r2) n= (1 -2~0s8 + r2) and 2r sin 8k= (1 -2rcos 8+ r2) Thus to find n and k it is necessary to determine 8. If w (= 2nv) is the angular frequency and the chosen point on the frequency scale is wz, then an exact expression is known : e(wi) = -O3 lnR(w) -lnR(w) do:lo wi2 -02 where w takes all values from 0 to co. Thus if the fractional reflectance R(w) is known for all frequencies from zero to infinity the calculation is correct. Un- fortunately only a section of the spectrum is measured and it is customary to generate the ‘wings’ by computational techniques.47 In addition, the Kramers- Kroenig analysis is not sensitive to minor changes of reflectance in the restrahlen region and is very sensitive to the reflectance minima.48 Certainly where the extinction coefficient is < 0.1, dispersion analysis is to be preferred.It is interest- ing to note that using a Kramers-Kroenig analysis in 1964 Hunt, Perry, and Ferguson49 reported US of KMgF3 at 140cm-1 (lattice mode) whereas in 1967 Perry and Young,4s using more reliable spectroscopic data at longer wavelengths, found 165 cm-l, which compares favourably with the value of 168 cm-1 found 4s W. G. Spitzer, R. C. Miller, D. A. Kleinman, and L. E. Howarth, Phys. Rev., 1962, 126, 1710.48 See, for example, J. R. Sweet and W. B. White, Appl. Spectroscopy, 1969, 23, 230; D. E. McCarthy, ibid., 1969, 22, 460; see also A. E. Tshmel and V. I. Vettegren, Spectrochim.Am, 1973,29A, 1681. 47 G. Andermann, A. Caron and D. Dows,J. Opt. SOC.Amer., 1965,55, 1210; see also J. D. Neufeld and G. Andermann, ibid., 1972,62, 1156. 48 C.H. Perry and E. F. Young,J. Appl. Phys., 1967,38,4616. 49 G.R.Hunt, C. H. Perry, and J. Ferguson, Phys. Rev., 1964,134, A689. 133 Vibrational Infrared and Raman Spectroscopy in Inorganic Chemistry using classical dispersion analysis. Transmittance data50 on KMgF3 in Nujol mulls or polythene discs yielded: VI 478, v2 300, v3 156 cm-l. The reflectance data yielded45 for the transverse modes: VI 458, v2 299, v3 168 cm-l.In a classical dispersion analysis each oscillator j is defined by its strength (PI), width (yj), and frequency (vj). (up -v2)Then n2 -k2 == EO + 24nppj2 (v52 -v2) + ypv2vp i and The summation extends over all the lattice oscillators and EO is the high-frequency dielectric constant (essentially the polarizability of the electrons with the nuclei fixed). The fractional reflectivity at normal incidence, R, can be calculated if the dispersion parameters are found.51 In Figure 11 the reflectivity, refractive index, and absorption index of quartz 10 20 30 WPm Figure 11 Reflectivity, refractive index, and extinction coefficient of quartz for the ordinary ray: (a) theoretical curve adjusted to give best fit with experimental observations (points); (b) und (c) calculated values of n and K,respectively, from (a)(Reproduced by permission from M.Garbuny, ‘Optical Physics’, Academic Press, New York, 1965) 6o I. Nakagawa, A. Tsuchida, and T. Shimanouchi,J. Chem. Phys., 1967,47,983. 61 W. G. Spitzer and D. A. Kleinman, Phys. Rev., 1961,121,1324. Beattie for the ordinary ray (polarized perpendicular to the optic axis) are shown as functions of wavelength. This may be compared with the transmission spectrum of a single crystal for the ordinary ray of quartz, using a piece thinned to 0.0262 mm (or roughly one thousandth of an inch), shown in Figure 12. 10 20 Figure 12 The transmission of a quartz plate 0.00262cm thick for the ordinary ray. The theoretical curve is adjusted to give the best fit to the experimental points (Reproducedby permission from Phys.Rev., 1961,121,1324) E. Powders.-Nujol (and other) mulls are used routinely in the study of the i.r. spectra of solids. However, the problem of the transmission of light through an idealized assembly of isotropic spheres of the same diameter, separated by distances large compared with the wavelength of the light used and having absorption bands in the region of the wavelength under study is by no means clear. Bearing in mind the anisotropy of the crystalline material frequently studied, the random shape and size of the particles and the fact that they are frequently comparable in size to the wavelength of light indicate some of the difficuIties in powder i.r.spectroscopy. (Attenuated total reflectance studies on powders are also worth considering.52) The theory of diffraction developed by Mie for an isotropic sphere53 reduces to Rayleigh scattering (proportional to v4) where the radius of the sphere is much less than the wavelength of the incident radiati0n.~4 Where the radius of the sphere is much greater than the wavelength of the incident radiation the Mie formula leads to the Huygens-Kerchoff theory and for the radius tending to infinity it leads to geometrical optics. Conventional mulling leads to particles in the 1-50 pm range so that the Mie theory is important. There is little point in attempting to develop the theory of the Nujol mull here, 53 M. H. Brooker, J.Chem. Phys., 1970,53,4100. 53 See, for example, M. Born and E. Wolf, 'Principles of Optics', Pergamon, London, 1959. 64 A. H. Pfund, J. Opt. SOC.Amer., 1934,24, 143. 135 Vibrational Infrared and Raman Spectroscopy in Inorganic Chemistry except to emphasize that band contours, band maxima, and band intensities can all be affected by the nature of the experiment carried out (particle size, mulling agent, etc.). A paper on the Christiansen filter effect is well worth reading,55 but a few examples will indicate the difficulties. Figure 13 shows the effect of refractive index of the medium on the absorption Figure 13 A function of the frequency plotted against a function of the extinction coefficient for several values of the refractive index diference between compound and substrate: [f (v)contains the refractive index diference and band width; f(E) contains the radius of the particle. The originalpaper60 shouldbe consulted for details].Curve (a), which is a normal absorption curve, occurs for a refractive index diflerence of zero bands obtained: the curves are calculated.56 This reference also contains an extensive discussion of the transmission of light through powders in substrates. As an experimental example, in Figure 14 the spectrum of Co [Hg(CNS)4] in the CrN stretching region is shown in (a) a potassium iodide disc and (6) a potas- sium chloride disc.57 A difference of more than 50 cm-1 in the band maxima is apparent. It is, however, important to note the band distortion5* in case (b).66 R. B. Barnes and L. G. Bonner, Phys. Rev.,1936,49,732; see also R. L. Henry, J. Opt. SOC. Amer., 1948,38,775; J. M. Hunt, M. P. Wisherd, and L. C. Bonham, Analyt. Chem., 1950, 22,1478. 66 H. Primas and H. Gunthard, Helv. Chim. Acta, 1954, 37, 360; see also G. Duyckaerts, Spectrochim. Acta, 1955,7,25; R. N. Jones, J. Amer. Chem. Sor., 1952,74,2681. 67 G. Duyckaerts, Analyst, 1959, 84,201. 58 J. W. Otvos, H. Stone, and W. R. Harp, Spectrochim. Acta, 1957, 9, 148; see also N. T. McDevitt and W. L. Baun, Spectrochim. Acta, 1964,20,799. Beattie I I I I 2300 2200 2100 2000 vlcm-l Figure 14 Absorption band in the C=N stretching region for CO[H~(CNS)~]dispersed in (a) potassium iodide (b) potassium chloride6' Figure 15 shows the effect of particle size on the i.r.spectrum of quartz for the 12.5 pm band.S9 An article by Sherwood on bandshapes in solids for both i.r. and Raman spectra is of interest here.60 12 13 14 15 A Figure 15 Efect ofparticle size on the i.r. spectrum of quartz: (a) < 2 pm (b) 14-16 pmaO 6B W. M. Tuddenham and R. J. P. Lyon, Analyt. Chem., 1960,32,1630. 6o P. M. A. Sherwood, Spectrochim. Acta, 1971,27A,1019. Vibrational Infrared and Raman Spectroscopy in Inorganic Chemistry F. Fluids.-Most of the features of the spectroscopy of liquids and solutions are covered by a study of gases, except that for liquids and solutions rotatiois are not quantized.'jl Early Raman spectroscopy was carried out on colourless (non-absorbing to mercury arc excitation), pure (to obtain the maximum con- centration) liquids. By contrast there is a considerable literature on the gas-phase i.r.spectra of molecules. This difference in the literature reflects the experimental difficulty in studying the (weak) Raman effect for dilute gases (and solutions). In the gas phase at low pressure the possibility of high-resolution studies in the determination of molecular structure, e.g.from the vibration-rotation band contours, becomes attractive for relatively simple molecules. This becomes particularly important for molecules which do not have a dipole moment and hence are not suitable for microwave studies, although it should be pointed out that the small dipole moment of HC-CD is sufficient for a microwave spectrum to be obtained.It is also interesting to note that even for tetrahedral molecules the ground vibronic state possesses a small dipole moment produced by centri- fugal distortion effects. For methane = 5.38 k 0.10 x D, allowing the observation of the pure rotational spectrum in the i.r. effect.62 Finally we note that observation of torsional frequencies, for example, may allow the calculation of energy barriers for internal rotation. 3 Applications This section is intended to direct interest to areas where vibrational spectroscopy can be of value. For reasons of space it cannot be exhaustive. Solids are discussed first, followed by melts, solutions, and gases, with a final section on Raman spectroscopy under resonance or near-resonance conditions.Very readable accounts of the use of lasers in light-scattering studies in general have been given by port^^^ and by Smith.64 Vibrational spectroscopy is, in principle, applicable to all systems except dilute monatomic gases and can be of unique value as a purely analytical technique.65All the constituents in a mixture will ideally, if present in sufficient concentration, yield their own characteristic vibrational spectrum (as modified by the environment). Where single crystals suitable for X-ray studies are available the contribution of vibrational spectroscopy to structural chemistry may be small. However, anomalous vibrational spectra in terms of band position, intensity, or number of bands may direct attention to compounds useful for detailed X-ray diffraction See, however, G.Birnbaum, Mol. Phys., 1973,25,241. A. Rosenberg, I. Ozier, and A. K. Kudian, J. Chem. Phys., 1972,57, 568. 63 S. P. S. Porto, Spex Speaker, 1968,13 (2), 1 ;1969,14 (2), 1;see also T. C. Damen, R. C. C. Leite, and S. P. S. Porto, Phys. Rev. Letters, 1965, 14,9. O4 R. A. Smith, Endeavour, 1970, 29, 71. 13~This includes the group frequency approach: L. J. Bellamy, 'The Infrared Spectra of Complex Molecules', Methuen, London, 1964. Beattie techniques.66 The Raman67 and i.r.68 spectra of oriented single crystals rep- resent one of the most sophisticated uses of vibrational spectroscopy. However, from the point of view of the chemist the value of such studies is open to question.They are clearly of interest in studying intermolecular forces,69 while a know- ledge of lattice dynamics is frequently necessary in interpreting other phenom- ena in solids. X-Ray diffraction studies on single crystals may yield ambiguous results for a variety of reasons, including the swamping of light atoms by heavy atoms, leading to inaccurate light-atom positions. Close similarity of scattering factors (e.g., C and N) and disorder can similarly lead to difficulties in interpretation. In such cases vibrational spectroscopy of oriented single crystals70 or even powders71 may be valuable in assigning struc- tures. Order-disorder transitions,72 defects in crystals,73 and the dynamics of polynuclear ions doped into crystals74 may be conveniently studied by this technique. Isotopic substitution may define a particular point group, an example being provided by the recent studies of Nakamoto and co-workers75 on octahedrally co-ordinated metal atoms.Isotopic substitution at this atom can demonstrate the presence or absence of a centre of symmetry. This technique, as with other spectroscopic methods, is particularly valuable for materials not suitable for rigorous X-ray diffraction studies. Vibrational spectroscopy has assumed some significance in the study of surfaces76 and non-crystalline materials77 including biological ~ystems.7~ The application of matrix isolation vibrational spectroscopy to elucidate the nature of high-temperature and ‘short-lived‘ species has received much attention 6E J.Chatt, C. Eaborn, S. D. Ibekwe, and P. N. Kapoor, J. Chem. SOC.(A), 1970,1343. E7 I. R. Beattie and T. R. Gilson, Proc. Roy. SOC.,1968, A307,407; J. F. Scott and S. P. S. Porto, Phys. Rev., 1967, 161, 903; J. F. Scott, L. E. Cheesman, and S. P. S. Porto, Phys. Rev., 1967, 162, 834. G.R. Wilkinson, W. C. Price, and E. M.Bradbury, ‘Molecular Spectroscopy Conference Proceedings’, ed. E. Thornton and H. W. Thompson, London, Pergamon, 1959;T. S. Robinson and W. C. Price, ‘Molecular Spectroscopy’, ed. G. Sell, Institute of Petroleum, London, 1954;T. S. Robinson and W. C. Price, Proc. Phys. SOC.,1953, B66,969. (9 G. R. Wilkinson, ‘Molecular Spectroscopy’, ed. P. Hepple, Institute of Petroleum, London, 1968;J.R. Ferraro, ‘Spectroscopy in Inorganic Chhmistry’, 1971, Vol. 2, p. 57, Aca-demic Press, New York. ‘O I. R. Beattie, K. M. S. Livingston, D. J. Reynolds, and G. A. Ozin, J. Chem. SOC.(A),1970,1210. 71 I. R. Beattie and D. J. Reynolds, Chem. Comm., 1968,1531. 72 C.H.Wang and R. B. Wright,J. Chem. Phys., 1973,58,1411,2934. 79 See also R. C. Newman, ‘Infrared Studies of Crystal Defects’, Taylor and Francis, London,1973. 74 G. R. Field and W. F. Sherman,J. Chem. Phys., 1967,47,2378. 76 K.Nakamoto, Angew. Cliem. Internat. Edn., 1972, 11, 666. 76 R. Eischens, Accounts Chem. Res., 1971,5, 74; M. Fleischmann, P.J. Hendra, and A. J. McQuillan,J.C.S. Chem. Comm., 1973, 80. 77 D. J. Derouault, P. J. Hendra, M.E. A. Cudby, and H. A. Willis, J.C.S.Chem. Comm., 1972,1187. 78 S. C. Erfurth, J. K. Kiser, and W. L. Peticolas, Proc. Nat. Acad. Sci. U.S.A., 1972,69,938. 139 Vibrational Inflared and Rainan Spectroscopy in Inorganic Chemistry recently.79 An excellent example of this technique was provided by Claassen and Hustonso who, using combined i.r. and Raman data, were able to demon- strate the D3h molecular symmetry of the unstable species Xe03F2. The advantages of narrow linewidths, leading to well-resolved spectra and the possibility of accurate isotopic shift measurements,sl are offset by the loss of rotational fine structure, and, more seriously, constraints imposed by the matrix, including site effects. Errors can arise due to the extremely low concentrations of material isolated in the substrate, normally < 1 mole percent, and ideally < 0.1 mole percent.Thus spurious bands are frequently observed due to ubiquitous impurities such as water and nitrogen;s2 despite the use of double oven techniques polymer formation may still predominate in the matrix;83 impurities may react with highly reactive species produced in the matrix, leading to erroneous interpretation of experimental data;s4 and in the case of high-temperature species the compound under investigation may react with the Knudsen celLS5 In the absence of adequate isotopic studies it is frequently difficult to be sure of the formula of the fragment present in the matrix, let alone whether or not it carries a charge. Nonetheless, matrix isolation is an important area of study and one which is receiving extensive attention.86 In the Raman effect, if the sample is strongly absorbing, leading to heating by the incident laser radiation, rapid rotation of the sample can be of value. In the i.r.effect, intensely absorbing samples can also be difficult to examine. In such cases attentuated total reflectance techniques are of interest for both crystalline and powdered materials.87 Passing on to melts and solutions, there is an important paper by Wilinshurst on the nature and existence of complex ions and ion pairs in strongly associated systems.88 Thus in a melt of composition LizZnC14 the observation of a band at ca. 318 cm-l in the i.r. spectrum is not interpreted as due to v3 of ZnCh2- but to ‘lattice-like vibrations of the system’.However the Raman spectrum of aqueous solutions thought to contain ZnC1g2- species showss9 v3 at 306 cm-l. Examples of apparently clear-cut structural studies on melts occur for ‘gallium 7B L. Andrews, Ann. Rev. Phys. Chem., 1971,22,109;J. S. Anderson and J. S. Ogden, J. Chem. Phys., 1969,51,4189; J. W. Hastie, R. H. Hauge, and J. L. Margrave,J. Amer. Chem. SOC., 1969, 91, 2536. H. H. Claassen and J. L. Huston, J. Chem. Phys., 1971,55,1505. 81 S. D. Gabelnick, J. Phys. Chem., 1972, 76, 2483. 8a H. Huber, M. Moskovits, and G. A. S. Ozin, Nature Phys. Sci., 1972,236, 127; H. Dubost and L. Abouaf-Marguin, Chem. Phys. Letters, 1972, 17,269. 83 N. Acquista and S. Abramowitz, J. Chem. Phys., 1972, 56, 5221 ;1.R. Beattie, S. B. Brum- bach, D. Everett, R.Moss, and D. Nelson, Faraday Symposium, London, 1973. J. M. Kelly, H. Herman and E. Koerner von Gustorf, J.C.S. Chem. Comm., 1973, 105; C. P. Marino and D. White, J. Phys. Chem., 1973,77,2929. 86 R. D. Wesley and C. W. De Kock, J. Phys. Chem., 1973,77,466; see, however, J. W. Hastie, R. H. Hauge, and J. L. Margrave, in the press. 86 J. W. Nibler and D. A. Coe, J. Chem. Phys., 1971, 55, 5133. 87 K. Tsuji and H. Yamada, Bull. Chem.SOC.Japan, 1968,41,1975; I. Simon,J. Opt. SOC.Amer., 1951, 41, 336; J. Fahrenfort and W. M. Visser, Spectrochim. Acta, 1962, 18, 1103; W. N. Hansen, Spectrochim. Acta, 1965, 21, 209. 88 J. K. Wilmshurst, J. Chem. Ph-vs., 1963, 39, 1779; see also J. P. Devlin, D.W. James, and R. Frech, J. Chem. Phys., 1970, 53, 4394. 8p S. D. Ross, ‘Inorganic Infrared and Raman Spectra’, McGraw-Hill, New York, 1972. 140 Beat tie dichloride’ formulated as Ga [GaC14] 90 and for Re~07,~l which is apparently molecular asin the gas. In solution, depolarization and intensity measurements have been used to study the formation of complex ions in systems where the number of species formed is relatively restricted.92 However, the elegant results of Evans and Dean93 on SnF6-nXn2- (X = halogen) in solution using leFn.m.r. spectroscopy indicates the value of selecting the correct technique for the system under study. Intensity measurements on Group IV tetrahalides,g4 oxy-anion~,~~ and complex oxy-acids96 have been used to discuss the nature of the bonding in these species.To some extent studies on liquids and gases merge in the sense that most of the earlier Raman work was on liquids, while the same compounds were frequently studied in the i.r. as gases, illustrating the relative sensitivity of the two tech- niques. A structural determination in terms of the distinction between two possible point groups may occasionally be made using vibrational spectroscopy, although assignment on the basis of non-observation of bands is open to question and has frequently led to results which have subsequently been shown to be erroneous. The series of compounds (MeaSn)sX, where X = N, P, As, or Sb, were shown to be pyramidal by a combination of selection rules and band positions.97 Rather less familiar uses of vibrational spectroscopy include a study of the kinetics of fast reactions using line-broadening in the Raman effect,98 studies of intermolecular interaction from the observation of linewidths and bandshape~;~gthe use of Raman experiments in interpreting nuclear spin relaxation experiments in simple molecules such as nitrogen;100 and the cor- relation of n-electron density with the vibrational frequencies of linear molecules.101 In the gas phase, high-resolution studies of vibration-rotation and pure rotation spectra are of importance for determining molecular parameters,1O2 including internal rotation.103 Where larger molecules are under study so that fine-structure analysis becomes extremely difficult both experimentally and theoretically, a study of band contours may be of value in assigning a structure or in developing a force field.lo4 By use of forward scattering techniques,l05 L.A. Woodward, G. Garton, and H. L. Roberts, J. Chem. SOC.,1956, 3723. g1 I. R. Beattie and G. A. S. Ozin, J. Chem. SOC.(A), 1969, 2615. G. W. Chantry and R. A. Plane, J. Chem. Phys., 1960,33,736. 93 D. Evans and P. A. W. Dean, J. Chem. SOC.(A), 1968,1154. 94 L. A. Woodward and D. A. Long, Trans. Faraday SOC.,1949,45,1131. 95 G. W. Chantry and R. A. Plane, J. Chem.Ph.vs., 1961,34, 1268. G. W. Chantry and R. A. Plane, J. Chem. Phys., 1961,35, 1027. s7 R. E. Hester and K. Jones, Chem. Comm., 1966, 317; see also G. Engelhardt and P. Reich, Z. Naturforsch., 1967, 22b,352.98 M. M. Kreevoy and C. A. Mead, J. Amer. Chem. Soc., 1962,84,4596. 99 R. P. Young and R. N. Jones, Chem. Rev., 1971,71,219; A. K. Atakhodzhaev, Optics and Spectroscopy, 1964, 16, 553. looR. G. Gordon, J. Chem. Phys., 1965,42,3658. lol R. M. Gavin arid S. A. Rice, J. Chem. Phys., 1971,55,2675. lo2 J. Overend, Ann. Rev. Phys. Chem., 1970,21,265. lo3 H. L. Strauss, Ann. Rev. Phys. Chem., 1968, 19, 419. Io4 F. N. Masri and W. H. Fletcher, J. Chem. Phys., 1970, 52. 5759; W. F. Edge11 and R. E. Moynihan, J. Chem. Phys., 1966, 45, 1205. 141 Vibrational Infrared and Raman Spectroscopy in Inorganic Chemistry Doppler broadening may be eliminated (analogous to the two-photon technique discussed on p. 113). Further, using pulsed techniques, 'picosecond spectroscopy' may be carried out.lo6 Pulse techniques have also been used to reduce fluores- cence* background.107 Gas-phase Raman spectra may be obtained routinelylOS to lo00 "C (much of the intensity coming from hot bands) in sharp contrast to studies using i.r. spectroscopy where the upper temperature limit for isothermal cells is of the order of 300 "C(owing mainly to problems of radiation from the windows and lack of availability of suitable window materials and gaskets).For non-iso- thermal cells much higher temperatures may be usedlOg but there is a danger of formation of smokes, which can cause errors in interpretation. In the area of high-temperature chemistry, flames can be examined by Raman spectroscopy using lasers to obtain spatial resoIution.ll0 Raman spectroscopy has the disadvantage of being relatively insensitive, but the advantage of detecting homonuclear diatomics such as oxygen and nitrogen which are constituents of many flames.In this connection it is interesting that vibrationally excited nitrogen has been studied using Raman spectroscopy.111 Break down of selection rules in gases can occur via collisions or by the application of an electric field. Thusvibrational transitions in mixtures of carbon dioxide and hydrogen (under pressure) were observed in the i.r. effect at 6505 cm-1 and assigned as 2349(C02) +4160(Hz) cm-l.l12 Compressed nitrogen alone shows an i.r.-active band at 2331 ~m-l.1~~At fields in the region 5-30 kV and high pressures, the fundamental vibration-rotation spectrum of hydrogen can be studied in the region of 4160 cm-l.114 Finally we note that i.r.emission from gases has been studied using a radiofrequency discharge as the excitation source.115 4 The Use of Polarized Light Excitation of Raman (or Rayleigh) scattering with circularly polarized light is important for chiral or non-chiral molecules. We consider initially species that are not optically active. Circularly polarized light is conveniently visualized as two plane-polarized waves at right angles to one another differing in phase by 42 but otherwise iden- tical. The y-component may lead the x-component (by ~/2)or lag behind (by d2). *Fluorescence lifetimes are usually > lo-# s. lo5 B. P.Stoicheff, J. Mol. Spectroscopy, 1970, 33, 183. looR. R. Alfano and S. L. Shapiro, Phys. Rev. Letters, 1971, 26, 1247. lo' P. P. Yaney, J. Opt. SOC. Amer., 1972, 62, 1297. Io8 1. R. Beattie and J. Horder, J. Chem. SOC. (A), 1969, 2655; I. R. Beattie, 'Molecular Spectroscopy', Institute of Petroleum, London, 1971. lo@ W. Klemperer and L. Lindeman, J. Chem. Phys., 1956,25,397; W. Klemperer, ibid.,p. 1066. 110 C. J. Vear, P. J. Hendra, and J. J. Macfarlane, J.C.S. Chem. Comm., 1972, 381. ll1 L.Y.Nelson, A. W. Saunders, A. B. Harvey, and G. Neeley, J. Chem. Phys., 1971,55,5127. 112 J. A. A. Ketelaar, Rec. Trav. chim., 1956,75,857. 113 M. F. Crawford, H. L. Welsh, and J. L. Locke, Phys. Rev., 1949, 75, 1607. 114 M. F. Crawford and R. E. Macdonald, Canad.J. Phys., 1958,36,1022.116 H. M. Mould, W. C. Price, and G. R. Wilkinson, Spectrochim. Acta, 1960,16,479. Beattie These are represented as k or left/right. Consider a forward scattering experiment. For the totally symmetric mode of a molecule such as SFe or cc14the induced dipole will be parallel to and in-phase with the incident field. Thus the observed Raman scattering will be either circularly polarized left or circularly polarized right, identically with the incident light. Now consider an oriented molecule AX4 as shown in Figure 16. This azy Figure 16 Raman scattering of circularly polarized light for an oriented square-planar AX4species (a) alg mode (b) blg mode term for the vibration shown turns y polarized light into x polarized and vice versa.Thus the vibration converts left circularly polarized light into right circularly polarized light and vice versa. For randomly oriented molecules in a liquid it is (as discussed on p. 121) necessary to average over all orientations. This was originally discussed by Placzek,l16 who defined a ‘reversal coefficient’, I+ 2pp=-=-I-l-p where p is the depolarization ratio. Figure 17 shows117 a rather spectacular demonstration of this technique applied to antimony pentachloride,* where conventional depolarization measurements are hindered by the proximity of a strong depolarized band close to the weak polarized band. The study of optically active materials in the i.r. and the Raman effect has also recently undergone a major change.The rotation of the plane of polarization of linearly polarized light by optically active materials is well known. In the region of an absorption band the dispersion (change with frequency) of the rotation becomes anomalous (optical rotatory dispersion) in a way similar to that shown in Figure 6 for the refractive index. Because of anomalous optical * I am indebted to Dr. A. J. McCaffery for permission to include this spectrum. G. Placzek, ‘The Rayleigh and Raman Scattering’, U.C.R.L. Translation 526(L), Cali- fornia, 1962; see also W. M. McCIain, J. Chern. Phys., 1971,55,2789. 11’ R. Clark, S. R. Jeyes, A. J. McCaffery, and R. A. Shatwell, to be published. Vibrational Infrared and Raman Spectroscopy in Inorganic Chemistry I00 500 vlcm--1 Figure 17(a) Circular polarization Raman spectrum of antimony pentachloride without polarization analysis (I+ + I-); (b) circular polarization Raman spectrum of antimonypentachloride with polarization analysis (I+ -I-) rotation, coupled with the change in refractive index, there is also a differential absorption of left and right circularly polarized components (circular dichroism).These effects are known collectively under the term ‘Cotton effect’. Optical rotation in the i.r. spectrum has been known and studied for a long time. It is only recently (1972) that vibrational Cotton effects have been detected.ll8 It has been known for some time that the intensity of Rayleigh and Raman scattered light from optically active molecules should show a slight difference for 118 R.J. Dudley, S. F. Mason. and R. D. Peacock. J.C.S. Chern. Conzm., 1972, 1084. Beattie right and left circularly polarized light.119 The principaI Raman scattering from chiral molecules derives from the 01‘2 term. However, there are minor contribu- tions from the optical activity tensors G’ and A. Whereas d2terms may be considered to arise through dipole-dipole contributions and hence are symmetric, the a’G’ and a’Aterms are products of a vector dipole and a (symmetrical) axial vector for G’ (the magnetic dipole) or a (symmetrical) field gradient tensor for A. The a‘G and a’A scatterings are ca. 103 times weaker than the a’2 scatterings but they are different for left and right circularly polarized light in chiral molecules.The difference may be detected using modulation of the incident circular polarization and amplifying only the modulated component of the scattered light. It appears that Raman c.i.d. (circular intensity differential) spectroscopy may become an important technique for probing asymmetric centres.120 5 Resonance Processes The discussion so far has relied heavily upon the Placzek theory of Rayleigh and Raman scattering.116 However, when the frequency of the exciting radiation lies close to an electronic absorption band for the molecule under investigation the simple Placzek theory is not applicable and the approach of Albrecht and other workers must be adopted.121 This approach has the great merit of showing the relationship between scattering processes and electronic transitions- respectively second-order and first-order interactions between light and matter.Classically the picture of an oscillating dipole induced in the molecule by the incident electromagnetic radiation, with the sinusoidal movement of the nuclei superimposed to lead to Raman scattering, is attractive. Placzek showed that provided the ground electronic state of the molecule was lz,that the Born- Oppenheimer approximation was applicable, and that the exciting frequency was much lower than any electronic absorption frequency of the molecule, then a quantum mechanical analogue of the classical picture could be applied. This introduced the Boltzman distribution leading to an explanation of the low intensity of anti-Stokes relative to Stokes lines.However, it still leaves open the origin of the intensity of the Raman effect. A better quantum mechanical picture for the normal Raman effect is that, although the exciting radiation is much lower in frequency than the frequency of any electronic transition for the molecule, nonetheless on application of a perturbation the admixture of electronic excited states into the ground state will ll9 P. W. Atkins and L. D. Barron, Mol. Phys., 1969, 16, 453; L. D. Barron and A. D. Buckingham, Mol. Phys,, 1971, 20, 11 11. lZo L. D. Barron, M. P. Bogaard, and A. D. Buckingham,J. Amer. Chem. SOC.,1973,95,603; L. D. Barron and A. D. Buckingham,J.C.S. Chem. Comm., 1973,152.lZ1 A. C. Albrecht, J. Chem. Phys., 1961,34,1476; A. C. Albrecht, and M. C. Hutley, J. Chem. Phys., 1971,55,4438; A. H. Kalantar, E. S. Franosa, and K. K. Innes, Chem. Phys. Letters, 1972, 17, 335; J. Behringer and J. 2. Brandmuller, 2. Elektrochem., 1956, 60,643; D. G. Rea, J. Mol. Spectroscopy, 1960, 4, 499; 0. S. Mortensen, Mol. Phys., 1971, 22, 179; W. L. Peticolas, L. Nafie, P. Stein, and B. Fanconi, J. Chem. Phys., 1970, 52, 1576; J. Behringer, ‘Raman Spectroscopy’, Plenum Press, New York, 1967; L. A. Nafie, P. Stein, and W. L. Peticolas, Chem. Phys. Letters, 1971, 12 131. Vibrational Infrared and Raman Spectroscopy in Inorganic Chemistry occur. Further, those normal modes which are vibronically active in an electronic transition lying close to the frequency of the exciting light should exhibit major enhancement of their Raman intensity.122 In this way those vibrations which contribute (via vibronic mixing) to the ‘forbidden’ intensity in an electronic transition close to the frequency of the exciting line will be enhanced. Following on from the discussion it is apparent that if the exciting frequency (VO) lies close to an absorption band of the material under study, then those vibrations which mix the (excited) electronic level close to vo with the nearest level corresponding to a strongly allowed transition will become greatly enhanced.This is the resonance Raman effect and Raman bands greater in intensity by several orders of magnitude may be obtained for appropriate fundamentals.By this method low concentrations of material may be examined using Raman spectroscopy; the interpretation of such data may be complicated because of the selective nature of the enhancement. Under resonance conditions (as with the electronic123 Raman effect) the scattering tensor need not be sym- metrical. Modes which are predicted to be inactive in the normal Raman effect may become active under resonance conditions. For example, the azgvibration of molecules belonging to point group D4h is inactive in the normal Raman effect. However, the scattering tensor under resonance conditions is given by (-“.rv i) The pp values are also anomalous and for partially asymmetric tensors lie in the range Q < pp < co. For a completely asymmetric tensor such as the one above pp becomes infinity.Although Placzek predicted this behaviour forty years ago, it has apparently been experimentally recognized for the first time only very recently. In an elegant paper on resonance Raman spectra of haemoglobin and cytochrome c, it was found that several prominent bands exhibited the inverse polarization.124 In particular this led to rigorous assignment of the azg vibrations of the tetragonal haem chromophores. An indication of the value of resonance Raman spectroscopy is found in a study of the Iz+ ion in fluorosulphuric acid at molar. The absorption maximum in the electronic spectrum is at 6400 A and using 6328 a (He/Ne laser) excitation an excellent resonance Raman spectrum is obtained showing the fundamental and a series of 0~ertones.l~~ This result may be compared with an experiment on iodine dissolved in carbon tetrachloride at 10-3 molar using i.r.excitation to avoid absorption.126 A pulsed gallium arsenide laser as source led to stimulated emission (see p. 149) as shown by the nearly equal intensities of the Stokes and anti-Stokes lines (ca. 1% of the incident power) and the angular la%A. H. Kalantar, E S Franzosa, and K. K. Innes, Chem.Phys. Letters, 1972, 17, 335. J. A. Koningstein, ‘Introduction to the Theory of the Raman Effect’, Reidel, Dordrecht, 1972. T. G. Spiro and T. C. Strekas, Proc. Nar. Acad. Sci. U.S.A., 1972,69,2622. R. J. Gillespie and M. J Mort0n.J. Mol. Spectroscopy, 1969,30, 178. la(G.W. Chantry, H. A. Gebbie, and C. Hilsum, Nature, 1964, 203, 1052. Beattie correlationof the emitted light with the incident beam. Another example of the use of resonance Raman spectroscopy is the recent detection of the p-phenylene- diamine radical in methanol at a concentration of the order of m01ar.l~~ However, even under resonance conditions Raman spectroscopy is insensitive compared with many other physical techniques (Table 1). When carrying out experiments to observe Raman spectra, fluorescence phenomena may be ob- served. The relationship between resonance fluorescence, resonance Raman, and Raman spectroscopy is interesting. The wavelength of the incident light is normally of the order of 5000 A leading to an oscillating dipole of period ca.10-15 s, that of the molecular vibration being ca. 10-13 s. CIassicaZly these time- scales are much faster than the frequency of rotation of the molecule (ca. s), accounting for the observation of anisotropic information from an apparently isotropic medium. The Raman effect is tensorial because two sets of axes are fixed in space: the laboratory frame of reference and the molecular axes for each randomly oriented molecule which is fixed in space for the lifetime of the observation.* Since this is a classical view the analogy must not be applied too literally. The other extreme, that of resonance fluorescence, is well defined. The molecule may be regarded ascapturing a photon, hv, thereby being raised from a particular rotational/vibrational level of the ground electronic state to a particular rota- tional/vibrational level of an excited electronic state 128 The selection rules are rigorous: dv may take any value; AJ is dependent on the symmetries of the ground and excited states but for a 2-2transition is & 1.The mean lifetime of the excited state is ca. 10-7 s so that, in a classical sense, the molecular axes now relax with time and there is only a vector relationship deriving from the laboratory axis, leading to a depolarization ratio of unity for the example cited. This is an area which is important to chemists because of the possibility of examining species at very low concentrations. It is also an extremely interesting region of study, theoretically and practically, for understanding vibronic effects in molecular spectra and the basis of the Raman effect.? A fascinating observa- tion is that the LO and TO modes of ZnSe, both of which arise from the same mechanical motion in the crystal, show completely different behaviour under resonance conditions.Thus the Raman excitation mechanism for these LO and *Rotational Raman bands are depolarized. 7 In planning experiments in this area it must be appreciated that there is a (Fourier transform) relationship between the (Gaussian or Lorentzian) half-width in time and the corresponding half-width in frequency. Thusfor a one picosecond pulse the half-width will be of the order of 10 cm-l. The further one moves from the long-lived resonance fluorescence process the shorter the necessary time-scale becomes and hence the greater the uncertainty in the frequency of the exciting radiation (see also P.F. Williams, D. L. Rousseau, and S. H. Dwortsky,Phys. Rev. Letters, 1974,32, 196). lP7E. Mayer, R. B. Girling, and R. E. Hester, J.C.S. Chem. Comm., 1973, 192. IPS See, for example, R. F. Barrow, I. R. Beattie, W. G. Burton, and T. R. Gilson, Trans. Farada.v Soc., 1971, 67, 583; W. Holzer, W. F. Murphy, and H. J. Bernstein, J. Chem. Phys., 1970,52,399; W. J. Tango and R. N. Zare, J. Chem. Phys., 1970,53,3094; M. Kroll and D. Swanson, Chem.Phys. Letters, 1971,9,115; I. R. Beattic and R. 0.Perry, J. Chem. SOC.(A), 1970, 2071. Vibrational Injirared and Raman Spectroscopy in Inorganic Chemistry TO photons is different although the vibration leading to their occurrence is the ~ame.~29 6 Phenomena at Very High Fields In discussing Rayleigh scattering we made use of the formula P = aE which assumes a linear relationship between the induced electric dipole moment P and the electric field strength E.The Raman effect was then introduced as a dependence of a (and hence of P)on the normal co-ordinate Q. At very high fields, found for example in a pulsed laser beam, this approach is inadequate. Some idea of the fields involved is given by considering a 1011 W pulse focused to a 0.1 mm spotsize: fields of the order of lo9 V cm-l and lo7 G (with radi- ation pressures of the order of 106 atm). A more general statement of the relationship between induced polarization and electric field is: P = aE + @E2+ QyE3... . .. plus time and spatial dependent derivative terms.l30 The introduction of factors such as 4 and Q varies with author; here the practice of Buckingham and Orrlsl has been followed. It should be noted that second harmonic generation arises fram E2terms, which contain a cos2 function, leading to frequency components at 2v0. Similarly third harmonic generation arises from the E3 term. The above equation may be used to discuss a whole variety of non-linear phenomena including the Kerr effect (electric birefringence) and the intensity dependence of the refractive index. Prior to the invention of the laser the only non-linear optical effect involving high-frequency components in the field was the Raman effect.The Raman effect is a two-photon process-the simultaneous annihilation of one photon with the production of a new photon together with a molecular transition. The intensity of the scattered light for the spontaneous Raman process is directly proportional to that of the incident light. The phenomena to be discussed below all involve high laser powers. An essen-tial feature of such radiation fields is the occurrence of stimulated effects. The power radiated due to a transition from a state Ei to a state Ejis given by p(vd = hvij[niAij + (nt -nj)Bij ~(vtj)] where Atj is the Einstein coefficient for spontaneous emission, Bij is the Einstein coefficient for stimulated emission, and ni and nj are the number of atoms in 129 R.C. C. Leite, T. C. Damen, and J. P. Scott, in ‘Light Scattering in Solids’, ed. G. B. Wright, Springer-Verlag, New York, 1969; see also T. C. Damen and Jagdeep Shah, Phys. Rev. Letters, 1971, 27, 1506. R. W. Terhune and P. D. Maker, ‘Non-Linear Optics’, Chapter 4 of ‘Lasers’, ed. A. K. Levine, Arnold, London, 1968; P. D. Maker and R. W. Terhune, Phys. Rev. (A), 1965, 137, 801 ;G. C. Baldwin, ‘An Introduction to Non-Linear Optics’, Plenum, New York, 1969. ls1 A. D. Buckingham and B. J. Orr, Quart. Rev., 1967,21, 195. Beat tie states i and j, respectively. Note the occurrence of U(VZ~),the energy density, as a multiplier in the term for stimulated emission. This equation, given in 1917, is also fundamental to laser action.Note that a photon produced by stimulated emission is identical in phase and direction to the incident photon. Clearly such a process as that described by the second term of the equation above can lead to amplification of an electromagnetic wave . A. The Hyper-Raman Effect.132-The term /3 is often called the first hyper- polarizability. Its components transform as the products (ijk) of the Cartesian vectors x, y, z. For centrosymmetric materials and those in the point groups D6, D4, and 0,p is zero.133 In the same way that the Raman effect is conveniently thought of as arising from acll/aQ terms, the hyper-Raman effect may be con- sidered as arising from aP/aQ terms, the hyper-Raman tensor in p‘tjk is a third-rank tensor, Scattering occurs at 2v0 -t VR (where VR is a molecular transition frequency).The selection rules for the hyper-Raman effect are different from those of the Raman effect; all i.r.-active bands are hyper-Raman-active and polarized; it is in principle also possible to observe modes forbidden in the i.r. and Raman effect. The intensity is typically ca. that of the Raman effect. However, under resonance conditions, this may be greatly enhanced by many orders of magnitude giving scattering comparable to that of the normal Raman effect. Little use has been made of this technique. B. The Stimulated Raman Effect.-As with the above section on non-linear phenomena, the discussion of the stimulated Raman effect will be extremely elementary. The Raman effect can be considered as a wave-mixing experiment : vo incident, vs scattered, and a phonon wave representing a vibrational mode.The conventional Stokes Raman process is illustrated in Figure 18a. Note that the dotted line represents a so-called virtual state. A convenient way of con- sidering this is to remember that there is a probability distribution associated with all energy levels in this molecule. A virtual state can be thought of as a region of low probability associated with several energy levels. As the virtual state approaches an eigenvalue of the system, so the more ‘real’ the state becomes. Conventionally, we think of a molecular vibration as causing a change in polarizability and hence producing scattered light at a frequency shifted by +du = VR from the exciting Tine.The molecular (thermal) vibrations are random and hence the resultant scattered radiation is of low coherence. By contrast, at very high flux densities the electromagnetic radiation incident on the sample may be considered to cause the molecular oscillators to vibrate in a synchronous fashion. They may be termed to be ‘phased coherently’. The non- linear driving force is second order in the field strength. The scattered photons will also create a high flux density, leading to stimulated phenomena. The situation is complicated by self-focusing of the radiation at high fluxes 132 R. W. Terhune, P. D. Maker, and C. M. Savage, Phys. Rev. Letters, 1965, 14, 681 ; S. J. Cyvin, J. E. Rauch, and J. C. Decius, J. Chem. Phys., 1965,43,4083.133 S. C. Abrahams, J.Appl. Cryst., 1972, 5, 143. 149 Vibrational Infrared and Raman Spectroscopy in Inorganic Chemistry (a) Thcrnial Stimulated A Figure 18 (a) Stokes Raman scattering, thermal and stimulated; (b) inverse (Stokes) Raman scattering; (c)four-wave mixing experiment due to the intensity dependence of therefractive index, the refractive index being greater towards the centre of the beam, so that the beam gets narrower and narrower until diffraction spread equals the focusing effect. Further, although the incident laser beam at frequency vo generates a Stokes-shifted beam vs and, in addition, an anti-Stokes shifted beam VA, these interact to produce Raman lines of higher order.lS4 In the case of benzene, for example, the observed effect is a series of Stokes and anti-Stokes lines spaced at the ‘breathing frequency’ of 1:11 W.J. Woodbury and W. K. Ng,Proc. Inst. Radio Engrs., 1962, 50, 2367; B. P. Stoicheff, Phys. Lelters, 1963, 7, 186; see also A. J. Glass, I.E.E.E. J. Quantum Electronics, 1967, 3,516. Beattie the carbon ring (992 cm-1). These are not overtones as the frequency shifts are in the ratio 1:2:3 . . . with no anharmonicity difference. Further, the lines have intensities comparable with that of the source. The anti-Stokes radiation may form a series of cones (VO + VR, vo + 2VR. . . . .) progressively out from the central beam which contains the unshifted frequency (UO) and the Stokes frequencies (VO -UR, YO -2VR. . . . .).The appearance of these rings is associated with the conservation of wave vector. The disadvantage of the stimulated Raman effect for chemists is that usually only one Raman-active frequency of the system is (selectively) excited. Further, stimulated Brillouin scattering may occur before the possible onset of stimulated Raman scattering. Experimentally the difference between stimulated Raman and stimulated Brillouin scattering is that the Raman process produces light while the Brillouin process produces sound as well (the frequency shifts for Brillouin scattering being much smaller than for the Raman effect). C. The Inverse Raman Effect.-An alternative description of stimulated Raman scattering is that, in the presence of a photon vs, it is possible for a photon vo to be annihilated with the creation of a further identical photon vs (together with a transition between molecular energy levels, YR).The two photons vs can clearly go on to create further vs photons leading (in a high vo flux) to amplifi- cationat VS. However, irradiation of the system with light at frequency VA = vo + VR leads to absorption at that frequency. This is the basis of the inverse Raman effect.135 Light of frequency VA is absorbed because it can be converted into molecular vibration quanta plus photons at the laser frequency vo (Figure 18b). Experimentally this is achieved by the simultaneous irradiation of a sample with two intense, collinear beams of light, one of frequency YO and one a continuum. Under these conltions absorptions are seen in the continuum at frequencies VA = vo + YR.A number of papers have been published on the use of pulsed lasers to enable inverse Raman spectroscopy to be carried out on the picosecond timescale.136 One clear advantage of this technique is the elimination of fluorescence effects. D. Four-wave Mixing Experiments.-To complete this review it is worth men- tioning a technique which is not a conventional study of a vibrational spectrum. The results of such experiments, however, yield vibrational data. Consider the simultaneous irradiation of a sample with two intense laser beams, one at YO and one at vt, where vt refers to a tunable laser. Under these conditions four-wave mixing can occur, leading to generation of radiation at v = 2v0 -vt.When YO -vt = VR (a molecular transition frequency) the output at 2v0 -vt = UA(see Figure 18c) can become large owing to resonant enhance- 13S W. J. Jones and B. P. Stoicheff, Phys. Rev. Letters, 1964, 13, 657. lS6 R. R. Alfano and S. L. Shapiro, Chem. Phys. Letters, 1971, 8, 631 ;R. A. McLaren and B. P. Stoicheff, Appl. Phys. Letters, 1970, 16, 140. Vibrational Infrared and Raman Spectroscopy in Inorganic Chemistry ment.137*138The processes illustrated in Figure 18c are synchronous and phased coherently. As radiation is emitted at 2v0 -vt = vo + VR = VA (the anti-Stokes frequency) the phenomenon has been termed Coherent Anti-Stokes Raman Scattering. (This is not the same as stimulated anti-Stokes scattering which requires inversion between initial and final states of the molecule.) A gain of many orders of magnitude over conventional Raman spectroscopy may be obtainable.The beam is highly collimated and fluorescence effects are almost eliminated, a feature of great importance in biological materials for example. A spectacular demonstration of this effect is the observation of the Raman spectrum of benzene to which fluoroscein had been added.lS9 It is interesting to compare this new technique with conventional Raman spectroscopy. Two laser beams are focused and crossed in the cell. The crossing angle is usually ca. 1O, is not critical, and is determined by wave-vector matching. Because the output beam (VA) is highly collimated (ideally diffraction limited) spatial filtering should be easily carried out.For benzene, using laser peak powers of the order of 1 kW, the four-wave mixing experiment shows an im- provement over conventional Raman spectroscopy of the order of five orders of magnitude. Further, the Raman radiation is spread uniformly over 1 steradian. That for the four-wave mixing experiment is collimated to the order of lo-* steradian. Thus the discrimination against fluorescence is of the order of nine orders of magnitude. Finally the CW equivalent power of the lasers need only be of the order of a few milliwatts and a monochromator may be unnecessary. It has been suggested that for peak powers of the order of 1 MW detection levels of ca. 10-6 Torr are feasible in gases. The observed intensity is proportional to the square of (a)the number density of molecules, (b) the (normal) Raman scattering cross-section, and (c) the reciprocal of the (normal) Raman half-width.However, under some conditions the intensity may be directly proportional to (a), (b), and (c), although the signal may be masked by nearby strong Raman modes or electronic effects. A point of some importance for gases is that, although the signal increases as the pressure squared, it also increases as the square of the coherence length. Thus if the length of the laser path through the gas is greater than the coherence length the signal will be constant with pressure. For solids and liquids the coherence length is only of the order of 1 mm. For gases it is tens of centimetres. It appears likely that this technique (Coherent Anti-Stokes Raman Spectro- scopy or CARS) will profoundly affect some branches of Raman spectroscopy.Further, two incident light sources illuminate the sample, the one resultant ray being used for CARS observation. Clearly (as with the hyper-Raman effect) more types of polarization measurement are possible than with the conventional Raman effect. In this way it may be possible to differentiate not only totally symmetric modes, but also possibly non-totally symmetric modes of different symmetries. It should also be noted that all Raman modes are CARS active, 137 J. Lukasik and J. Ducuing, Phys. Rev. Letters, 1972, 28, 1155. 138 p. R. RCgnier, F. Moya, and J. P. E. Taran, Appl. Phys. Letters, 1973, 23, 240. 139 R.F. Begley, A.B. Harvey, R. L. Byer, and B. S. Hudson, J. Gem. Phys., 1974,61, 2466. Beattie and in addition some Raman-inactive modes occur in the CARS or four-wave mixing experiment .I40 I am indebted to many former and present colleagues at Southampton for helpful comments, to workers at the King’s College, London, at the National Physical Laboratory, and at the Universities of Reading and East Anglia. I am particularly indebted to Dr. A. L. Harvey of the Office of Naval Research, Washington, for helpful discussions and for allowing me to see material before publication. I am also grateful to Dr. A. J. McCaffery for allowing me to use his work on circularly polarized light prior to publication. 140 B. S. Hudson, in the press.

ISSN:0306-0012    

DOI:10.1039/CS9750400107

出版商:RSC    

年代:1975

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Olefin Metathesis and its Catalysis By R. J. Haines DEPARTMENT OF CHEMISTRY, UNIVERSITY OF CAPE TOWN, CAPE TOWN, REPUBLIC OF SOUTH AFRICA and G. J. Leigh SCHOOL OF MOLECULAR SCIENCES, UNIVERSITY OF SUS SEX, BRIOHTON, BN1 9QJ 1 Introduction The olefin metathesis reaction (1) is one of the most remarkable catalytic reactions discovered in recent years1* R’CH R’CH RICH =CHR* ” -k R*CH RICH=CHRS” + RICH A non-catalytic counterpart of this reaction was observed in 1931 by Schneider and Frohlich,2 who established that at 725°C propylene could be converted into ethylene and butene. However, it was not until the discovery of hetero- geneous and homogeneous catalysts, which can promote the reaction at much lower temperatures and minimize side-reactions, that the potential of the metathesis reaction could be realized. The reaction is essentially thermoneutral, involving just the making and breaking of carbon-carbon double bonds.Equilibrium can be reached from either side of reaction (l), and the distribution of products is then statistical. The thermal activation of this entropy-controlled reaction is symmetry forbidden according to the Woodward-Hoffman rules,a which is consistent with the high temperatures necessary for the reaction dis- covered by Schneider and Frohlich. The catalysis of the reaction was first reported by Banks and Bailey,l their discovery stemming from a study of the catalytic activity of activated molyb- denum hexacarbonyl supported on alumina. They found that over this system linear olefins of three to eight carbon atoms were readily converted into mixtures of homologues of lower and higher molecular weight, and in particular that propylene was converted into ethylene and but-2-ene according to equation (2).*This reaction has been variously called the ‘olefin disproportionation reactiony1 and the ’olefin dismutation reaction’* but because it involves the interchange of alkylidene radicals (transalkylidenation) we prefer the term ‘olefin metathesis’ originally proposed by Calder~n.~ R. L. Banks and G. C. Bailey, Znd. and Eng. Chem. (Product Res. and Development), 1964, 3, 170. a V. Schneider and P. K. Frohlich, Znd. and Eng. Chem., 1931,23, 1405. See, for example, R. Hoffmann and R. B. Woodward, Accounts Chem. Res., 1968,1, 17.C. P. C. Bradshaw, A. J. Howman, and L. Turner, J. Caralpis, 1967, 7, 269. 6 N. Calderon, H. Y, Chen. and K. W. Scott, Tetrahedron Letters, 1967, 3327. 155 Olefn Metathesis and its Catalysis 2 propylene + ethylene + but-2ene (2) They also observed the formation of cyclopropane and methylcyclopropane from ethylene, and this has still not been adequately explained. Temperatures of ca. 150 “Cand pressures of ca. 30 atmospheres were employed. Prior to Banks and Bailey’s discovery, the polymerization of cyclic olefins by both heterogeneous (e.g. molybdenum oxide supported on alumina)6 and homogeneous catalysts [e.g. tungsten(v1) chloride-triethylaluminium]7-9 had been reported, but it was not recognized at the time that the ring-opening implicit in the polymerization involved olefin metathesis.The proposed mechanism involved cleavage of a bond adjacent to the olefinic bond. The first homogeneous catalysis of olefin metathesis was reported by Calderon and co-workers in 1967.5J0 They used [WCh]-EtOH-EtAICl2 as a catalyst mixture, and were the first to recognize that olefin metathesis involves trans- alkylidenation, and that ring-opening polymerization employing molybdenum or tungsten systems as catalysts also involves this step.11 The object of this review is to describe the scope of the reaction, to enumerate the theories which have been advanced to explain its catalysis (with particular emphasis on the nature of the catalysts), to evaluate critically the available literature (there is undoubtedly much industrial research which has not been revealed), and to attempt to present a model consistent with all observations.We shall discuss homogeneous systems in more detail than heterogeneous systems. Several reviews and catalogues on, and related to, this topic have appeared in the literat~rel~-~~ but none adequately covers all the aspects dis- cussed in this review, and a number are unavailable to the average reader. H. S. Eleuterio, Ger. Offen., 1960, 1 072 811 (Chem. Abs., 1961, 55, 16 005 h).G. Natta, G. Dall’Asta, G. Mazzanti, and G. Motroni, Makromol. Chem., 1963, 69, 163.* G. Natta, G. Dall’Asta, and G. Mazzanti, Angew. Chem. Internat. Edn., 1964, 3, 723. O G. Natta, G. Dall’Asta, I.W. Bassi, and G. Carella, Makromol. Chem., 1966, 91, 87. lo N. Calderon, E. A. Ofstead, J. P. Ward, W. A. Judy, and K. W. Scott, J. Amer. Chem. SOC.,1968, 90, 4133. l1 K. W. Scott, N. Calderon, E. A. Ofstead, W. A. Judy, and J. P. Ward, Abstracts 155th NationalMeeting of the American Chemical Society, San Francisco, April 1968, paperL54; Advances in Chemistry Series, 1969, 91, 399. la G. C. Bailey, Catalysis Rev., 1969, 3, 37. la F. D. Mango, Adv. Catalysis, 1969, 20, 307. S. Yoshitomi, Sekiyu Gakkai Shi, 1970, 13, 92 (Chem. Abs., 1970, 73, 3386~); Sekiyu to Sekiyu Kagaku, 1971, 15, 89 (Chem. Abs., 1971, 75, 119 670g); Nenryo Kyokaishi, 1967, 46, 514 (Chem. Abs., 1968,68,29 168a). l6 J. Tsuji, Kagaku No Ryoiki, Zokan, 1970, 89, 169 (Chem.Abs., 1970,73, 35 770k). l6 C. Inoue and K. Hirota, Yuki Gosei Kagaku Kyokai Shi, 1970,28,744 (Chem. Abs., 1970, 73,98 312h). l7 M. L. Khidekel, A. D. Shebaldova, and I. V. Kalechits, Russ. Chem. Rev., 1971, 40, 669. l* F. D. Mango and J. H. Schachtschneider, ‘Transition Metals in Homogeneous Catalysis’, cd. G. N. Schrauzer, Marcel Deuer, New York, 1971, p. 223. loN. Calderon, J. Macromol. Sciencp, 1972, C7, 105. N. Calderon, Accounts Chem. Res,, 1972, 5, 127. a1 L. Marko, J. Hung. Chem. SOC.,1972,27,213. Ia D. J. Cardin, M. J. Doyle, and M. F. Lappert, Chem. SOC,Rev., 1973,2,99. O3 R. L. Banks, Fortschr. Chem. Forsch., 1972, 25, 39. a4 W. B. Hughes, Organometallic Chem. Synth., 1972, 1, 341, Haines and Leigh 2 Scope of the Reaction A.Acyclic Olefins.-(i) Mono-oZe$ns. Whereas acyclic unsaturated hydrocarbons containing internal olefinic bonds are generally inert towards homopolymeriza- tion involving addition across the double bond, such as Ziegler-Natta poly- merization,25 they participate almost as readily as alkenes containing terminal olefinic bonds in the olefin metathesis reaction. This reaction thus provides a valuable route to the synthesis of new alkenes from those which may be more readily available. Its potential for redistribution of olehic bonds in systems containing func- tional groups is considerable, although thus far little exploited. Isolated examples are the metathesis of methyl oleate employing [WCl6]-[Me4Sn] as catalyst to give octadec-9-ene and the dimethyl ester of octadec-9-enedioic acid,26 and the metathesis of acrylonitrile and propylene over a heterogeneous catalyst derived from ammonium tungstate, to give ethylene and l-~yanopropylene.~~ Some problems may arise using olefins of this type due to catalyst poisoning, for example, where halogenocarbons are used, but they should be surmountable by suitable choice of catalyst.Thus p-chloroviuylbenzene and pent-2-ene, and 5-bromopent-l-ene and pent-Zene, undergo metathesis over [WCh]-EtNC12 in benzene.28 The metathesis of simple alkenes, particularly employing hetero- geneous catalytic systems, has, in contrast, been extensively studied. For a comprehensive catalogue of these reactions the reader is referred to the review by Bailey,lZ as well as to those by Banks23 and Hughes.24 The rate of reaction of olefins in the metathesis reaction is sterically con- trolled, as demonstrated by the decrease along the series CH2= >RCH2CH= > R2CHCH= >RzC=.~O Substitution of vinylic hydrogens by chlorine has been shown to deactivate the double bond towards metathesis.20 The industrial potential of the olefh metathesis reaction is considerable and obvious.The first commercial plant to utilize it, using in particular the Phillips Triolefin process to convert propylene into polymerization-grade ethylene and high-purity butene, was in operation within three years of the first report of the catalysed olefin metathesis reaction.29 Numerous industrial processes have been patented in which olefin metathesis is an integral step,12J7 for instance, a new route to high-octane alkylate.The process involves the alkylation of ethylene and but-2-ene, obtained by disproportionation of propylene, to di-isopropyl and but-Zene alkylate.30 It has also been shown that the yield of ethylene from a naphtha cracker can be economically increased through the addition of a propylene met at hesis st age.31 There are numerous reviews on this topic: see, for example, C. E. H. Bawn and A. Ledwith, Quart. Rev., 1962,16, 361 ;J. Boor, Ind. and Eng. Chem. (Product Res. and Development), 1970, 9,437, and references therein. z6 P. B. van Dam, M. C. Mittelmeijer, and C. Boelhouwer,J.C.S. Chem. Comm., 1972, 1221. 87 G. Foster, Ger. Offen., 1971,2 063 150 (Chem.Abs., 1971,75,63 172b). a8 J. I. O'Hara and C. P. C. Bradshaw, B.P., 1972,l 283 348 (Chem. Abs., 1972,77,113 786u). as Anon., Chemical Week, July 23, 1966, p. 70. *O R. S. Logan and R. L. Banks, Oil Gas J., 1968, 66, 131; see also Hydrocarbon Process, 1968, 47, 135. *l R. E. Dixon, J. F. Hutto, R. T. Wilson, and R. L. Banks, Chem. Age, 1967, 2,49. Olefn Metathesis and its Catalysis The metathesis reaction has provided a valuable method for characterizing polymer structures. For example, the monomer sequence distribution in a number of styrene-butadiene copolymers was determined by treating the polymer with a high proportion of but-2-ene in the presence of a suitable catalyst. This degraded the polymer to low-molecular-weight species, which were analysed by g.l.~.~~ This method has also been used to determine the extent of double-bond migration during free-radical cross-linking of butadiene.33 (ii) Di-and tri-olefns.Acyclic unsaturated hydrocarbons containing more than one olefinic bond also undergo the olefin metathesis reaction, although yields may be somewhat low over heterogeneous catalysts owing to coke formation on the surface. They can react either intermolecularly or intramolecularly. Intermolecular metathesis is effected on treatment of buta-1,3-diene with a tungsten oxide-silica catalyst that has previously been treated with sodium carbonate, to give ethylene and cyclohexadiene.34 For simple metathesis, hexa- triene should be obtained as product, and this must have cyclized to cyclohexa- diene under the reaction conditions. Deca-195,9-triene is formed on metathesis of hexa-lY5-diene by [MO(NO)~(PP~~)~CI~]-[M~~N~CI~].~~However, octa-1,7- diene reacts intramolecularly on treatment with [Mo(NO)~(PP~)~C~~I-[M~~A~~-C13] to give cyclohexene as a major product.35 Other reported reactions involving dienes include metathesis of buta-1,3-diene with pr~pylene,~~ of buta-1,3-diene with isobutene,a4 and of ~enta-l+diene.~~ B.Cyclic Ole&.-Metathesis is not restricted to acyclic olefins. As mentioned above, early reports showed that polyakenamers of the general formula [-CH=CH(CH2)n-] (n = 2,3,5,6, or more) are produced by treatment of the appropriate cyclic mono-olefin with heterogeneous catalysts such as molyb-denum oxide supported on alumina or homogeneous catalytic systems such as [WCls]-AIEt3.6-g Although it was not appreciated at the time that the reaction involves olefin metathesis, it was realized that the polymerization involves ring opening and not an addition polymerization of the Ziegler-Natta type because the latter would have yielded polymers containing cyclic repeat units.Further evidence against an addition mechanism was that cyclic olefins in which the rings are not highly strained were found not to be polymerized by normal Ziegler- Natta catalysts. The polymers formed were assumed to be essentially linear, and a mechanism in which a carbon-rbon bond adjacent to the olefinic bond is cleaved was proposed.8 Calderon and co-workers subsequently made a detailed study of the poly- merization of cyclo-octene and cyclo-octa-1 ,S-diene using [WCleI-EtAICh as 32 L.Michajlov and H. J. Harwood, Amer. Chem. Soc., Div. Polym. Chem., Preprints, 1970, 11, 1197. 33 W. Ast and K. HLmmel, Kautschuk Gummi Kunststofe, 1971, 24, 220. 34 L. F. Heckelsberg, R. L. Banks, and G. C. Bailey, J. Catalysis, 1969, 13, 99. 36 E. A. Zuech, W. B. Hughes, D. H. Kubicek, and E. T. Kittleman, J. Amer. Chem. Soc., 1970, 92, 528. G. Doyle, Ger. Offen., 1971,2047 270 (Chem. Abs., 1971, 75, 5202a); J. Catalysis, 1973, 30,118. Haines and Leigh catalyst, and they established that the polymerization mixtures contained low- as well as high-molecular-weight species.11~37 By means of selective and fractional extraction they isolated the former, and by gas chromatography, mass spectro- metry, and n.m.r.spectroscopy showed that they were macrocycles and multi- plets of the parent compound or, in the case of the cyclo-c?cta-1,5-diene reaction, oligomers of formula (C~HI~)~C~H~.~~ They also showed that further treat- ment of the high-molecular-weight species with [WC16EEtAlC12 led to the formation of low-molecular-weight macrocycles. These results led to the con- clusion that the ring-opening polymerization of cyclic olefins by catalysts of the type [WC16]-Rz~Cb-z is a special case of the olefk metathesis reaction essen- tially involving macrocyclization of the parent compound (1) by consecutive metathesis of larger and larger ring systems as shown. Intramolecular metathesis leading to ring reduction will obviously occur concurrently, and this explains why low- as well as high-molecular-weight species are found at equilibrium.On this basis, cyclic olefins that can be considered as multiples of the same mono-olefb should afford the same equilibrium mixture, and this has been observed.37 Strong evidence for the involvement of olefin metathesis in ring-opening poly- merization has come from studies of the ozonolysis products of the random copolymer produced from cyclo-octene and [1 -14C]cyclopentene with a [WO-C14]-EtAlClz-benzoyl peroxide catalyst.38 The high-molecular-weight species formed in the product mixtures obtained by polymerization of cyclic olefins are suspected to be primarily linear and not macrocy~lic,~~ but their formation is readily explained in terms of trace amounts of acyclic olefins being present in the reaction mixture and participating in the polymerization.The polyalkenamers obtained by ring-opening polymerization of cyclic olefins range from amorphous elastomers to crystalline materials, depending on the structure of the repeat units and the configuration about the olefinic bonds.40 The crystallizability of these polymers in general is associated with their stereo- regularity. Significantly, polymerization of substituted unsaturated cyclic olefins of medium ring size by this method is a conventional route to perfectly alternating copolymers, provided that side-reactions do not occur during the polymerization process. For instance, ring-opening of 5-methylcyclo-octene by s7 N.Calderon, E. A. Ofstead, and W. A. Judy, J. Polymer Sci.(Polymer Chem.), 1967,5A, 2209. 38 G. Dall’Asta and G. Motroni, European Polymer J., 1971,7,707. 3s K.W. Scott, N. Calderon, E. A. Ofstead, W. A. Judy, and J. P. Ward,Rubber Chem. Technol., 1971,44, 1341. 40 N. Calderon and M. C. Morris, J. Polymer Sci.(Polymer Phys.), 1967, 5A, 1283. 159 6 Olefn Metathesis and its Catalysis [WCls)-EtAlC12 gives a polymer with the repeat unit (2), which is equivalent to the alternating copolymer of butadiene, ethylene, and propylene.37 Similarly, a polyalkenamer containing the repeat unit (3), and equivalent to the alternating [-(CH~-CH=CH--CH+(CHaCH=C--CHa)-]I de (3) copolymer of butadiene and isoprene, is obtained20 by ring opening of (4).It has been established that the yield of low-molecular-weight macrocycles in the ring-opening polymerization of olefins is considerably increased under certain reaction conditions, e.g. high dil~tions.~~s~~-~3 For instance, metathesis of cyclo-octene in dilute benzene solution by [WC16J-EtAlC12-EtOH gives a mixture which contains the three low-molecular-weight species (C8H14)n (n = 1, 2, or 3) in high yield.41 Ring opening of cyclo-olefins by metathesis thus provides a convenient method for the synthesis of macrocycles, which are useful intermediates in the preparation of perfume bases. Intramolecular metathesis of cyclic diolefins can lead to products not obtain- able by more conventional routes.For example, catenanes have been identified in mixtures obtained by catalytic metathesis of cyclododecene by [WClsI-EtAlCl~-EtOH.4,45 It has been proposed that these are formed by a 360" twist of the cyclic olefin prior to the metathesis reaction, as shown in Scheme 1. Although most alicyclic olefins possessing more than one double bond readily undergo ring-opening polymerization, they do so only if these bonds are not c0njugated.1~ Thus the metathesis of cyclopentadiene, cyclo-octa-l,3-diene, or cyclohepta-l,3,5-triene has not been effected. It is believed that this inertness results from catalyst deactivation. For instance, addition of small amounts of cyclo-octa-l,3-diene to a tungsten-aluminium-catalysed polymerization of 41 N.Calderon, U.S.P., 1969, 3 439 056 (Chem. Ah., 1969,71, 38 438c). 49 N. Calderon, US.P., 1969, 3 439 057 (Chem. Abs., 1969,71, 80 807~). 45 E. Wasserman, D. A. Ben-Efraim, and R. Wolovsky,J. Amer. Chem. Soc., 1968,90,3286. 44 R.Wolovsky, J. Amer. Chem. SOC.,1970, 92,2132. 46 D. A. Ben-Efraim, C. Batich, and E. Wasserman,J. Amer. Chem. SOC.,1970,92,2133. Haines and Leigh m Scheme 1 cyclo-octene drastically reduces the activity of the catalyst.19 For a comprehensive review on the polymerization of cyclic olefins, see the excellent article by Calder0n.1~ C. Metathesis of Cyclic Olefins with Acyclic Olefins.--Cross-metathesis of cyclic and acyclic olefins is a convenient method for the synthesis of polyene species.For instance, it has been shown that the interaction of ethylene with cyclopentene, cyclo-octene, or cyclohexene over [Mo(CO)6]-Al203 or cobalt molybdate on alumina gives hepta-1 ,6-dieneY deca-1 ,Pdiene, or octa-1 ,7-dieneY re~pectively.~~ Similarly, treatment of cyclo-octa-1,5-diene with ethylene in the presence of [Mo(NO)~(PP~~)~C~~I-[A~~M~~CI~]gives deca-lY5,9-triene.35 The acyclic triene C14H24, the tetraene ClQH32, and the pentaene C24H40, as well as the diene CSH16, have been isolated from the reaction of cyclopentene with ~ent-2-ene.~' D. Alkynes.-Metathesis of alkynes has also been observed, although the reaction has received far less attention than reactions involving alkenes. Pent- 2-yne has been converted into but-2-yne and hex-3-yne, employing tungsten oxide-silica as catalyst, and of the 44% of the initial pentyne undergoing reaction, 53% was converted into hexyne and butyne.48 Terminal alkynes, e.g.propyne, also participate in this type of reaction, but in all cases studied the selectivity is low.49 This is because these alkynes preferentially cyclotrimerize to yield derivatives of benzene under the conditions required for metathesis. I* G. C. Ray and D. L. Crain, Fr. P., 1968, 1 511 381 (Chem. Abs., 1969,70, 114 580s).J.-L. Herisson and Y.Chauvin, Mukromol. Chem., 1970, 141, 161. 4* F. Pennella, R. L. Banks, and G. C. Bailey, Chem. Comm., 1968, 1548. 49 J. A. Moulijn, H. J. Reitsma, and C. Boelhouwer, J. CuruZysis, 1972, 25,434.161 Olefin Metathesis and its Catalysis 3 Catalyst Systems and the Nature of the Catalytic Site Both heterogeneous and homogeneous systems have been employed as catalysts in the olefin metathesis rea~tion.l~e~~1~4 The former are normally composed of a ‘promoter’, such as molybdenum or tungsten oxide, and a refractory ‘support’ having a high surface area, such as alumina or silica. However, the functions of the components cannot be rigidly defined, and the terms ‘promoter’ and ‘support’ should be used with caution. The homogeneous catalysts are generally derived from a transition-metal complex, e.g. wCl6] and an organometak derivative or a Lewis acid such as EtAlCl2 or LiBu. A. Heterogeneous Catalysts.4f the heterogeneous systems thus far studied, those derived from oxides or carbonyls of molybdenum, tungsten, or rhenium exhibit the highest catalytic activity.l~~~-~~ The sulphides of molybdenum and tungsten and the oxides of iridium, lanthanum, niobium, osmium, ruthenium, rhodium, tantalum, tellurium, and tin have also been found to be effective promoters, but the catalytic activity of systems containing these species is con- siderably less.54-56 Various refractory materials have been employed as supports.These include magnesium silicate, magnesia-titania, alumina-titania, the oxides of metals such as zirconium and titanium, and the phosphates of aluminium and magnesium.54 Silica and, in particular, alumina have been found to be the most effective and are the most widely used.In fact, alumina has been observed to effect the metathesis of propene in the absence of any promoter, although its activity for such is low.57 Generally, heterogeneous catalyst systems require activation before use. Many of the metal oxide systems need to be heated for up to five hours in a stream of an inert gas, optionally containing dioxygen, to temperatures of 600 “C. It is believed that the high temperatures effect some chemical interaction between the promoter and the support, and also desorb polar material which may poison the catalyst surface. The activity of a number of catalysts based on metal oxides can be increased by modifying their preparation. For example, the alumina used as support in some systems has been pretreated with a strong inorganic or organic acid (hydrochloric or acetic acids) prior to impregnation with the oxide.58 Rhenium oxide has been sublimed directly on to the alumina support.5Q.60 Hydrochloric acid and chlorinated hydrocarbons which yield R. L.Banks, U.S.P., 1966, 3 261 879 (Chem. Abs., 1966, 65, 12 lose). 61 L. F. Heckelsberg, R. L. Banks, and G. C.Bailey, Ind. and Eng. Chem. (Product Res. and Development), 1968, 7, 29. British Petroleum Co. Ltd., Dutch P., 1966, 6 511 659 (Chem. Ah., 1966, 64, 19 408c). sa K. V. Williams and L. Turner, B. P., 1968, 1 116 243 (Chem. Abs., 1968, 69’29 085s).L. F. Heckelsberg, R. L. Banks, and G. C. Bailey, Ind. and Eng. Chem. (Product Res. and Development), 1969, 8, 259. 66 L. Turner and K.V. Williams, B. P., 1967, 1 096 200 (Chem. Abs., 1968, 68, 77 69111). 66 R. B. Regier, U.S.P., 1972, 3 652 703 (Chem. Abs., 1972, 76, 139 91Oj). G. V. Isagulyants and L. F. Rar, Zzvest. Akad. Nauk S.S.S.R., Ser. khim., 1969, 1362. 6* British Petroleum Co. Ltd., Dutch P., 1966, 6 610 196 (Chem. Abs., 1967, 67, 53 607t). 6s British Petroleum Co. Ltd., Dutch P., 1966, 6 605 329 (Chem. Abs., 1967, 66, 48 081s). 60 L. Turner and C. P. C. Bradshaw, B. P., 1968, I 103 976 (Chem. Abs., 1968,68,86 834~): 162 Haines and Leigh hydrogen chloride at the activation temperature also increase the catalytic activity of WOpSiO2 systems.61 The activity and selectivity of oxide-based catalysts can also be increased by controlled treatment after activation, with gases such as CO and H2,50151~62163 although prolonged treatment deactivates them.Certain reducing agents have also been observed to have a beneficial effect on the activity of heterogeneous catalytic systems. For instance, the conversion of pent-Zene by WO3-SiO2 is increased by the addition of tributylphosphine to the feed,64 and the activity of supported oxides of molybdenum, tungsten, rhodium, vanadium, niobium, or tantalum is increased by treating with tri- ethylaluminium.65 The catalysts derived from metal carbonyls normally require much lower activation temperatures (120-140 "C),and are deactivated by air.66 Thus they must be activated under a high vacuum or in an inert atmosphere. Kemball and co-workers have established, by i.r.spectroscopy, that during the activation of [MO(CO)6]-&03, 2 or 3 moles of carbon monoxide are evolved.66 However, it has been shown subsequently that Mo(C0)4 is not an active species, and probably the active sites are completely free of C06' and possibly oxidi~ed.~~.~~ The activity of [Mo(CO)6]-&03 is enhanced by treatment with halogeno- olefms before admission of reactanksg Allyls of molybdenum, tungsten, and rhenium are apparent exceptions in not requiring activation, since, when adsorbed on alumina, they are immedi- ately active catalysts. However, possibly they activate rapidly as metathesis com- mences.7O Heterogeneous catalysts can function over a wide range of temperatures, the optimum temperature depending both on the nature of the support and on the promoter employed.Catalysts containing molybdenum or tungsten oxide generally require temperatures that are higher by about 100 "C to effect metathe- sis than the corresponding catalysts derived from molybdenum or tungsten hexacarbonyl. A higher operating temperature by about 200 "C is also required for systems containing silica as support compared with those using alumina. Silica-supported catalysts have the advantage that by operating at high tempera- tures (ca. 400 "C51)they are considerably more resistant to poisoning by polar molecules. Catalysts derived from rhenium oxide and alumina are noted for F. Pennella, Belg. P., 1968, 713 187. Oa L. F. Heckelsberg, U.S.P., 1968, 3 365 513 (Chem. Abs., 1968, 68,61424s).63 E. A. Zuech Ger. Offen., 1971, 2 017 841 (Chem. Abs., 1971,74, 5300). O4 L. F. Heckelsberg, Belg. P., 1968, 713 185. 66 Shell Internationale Research Maatschaapij N.V., Dutch P., 1969, 6 814 835 (Chem.Abs., 1969, 71, 49 21 lz). 66 E. S. Davie, D. A. Whan, and C. Kemball, Chem. Comm., 1969, 1430;J. Catalysis, 1972, 24,272. R. F. Howe, D. E. Davidson, and D. A. Whan, J.C.S. Faraday I, 1972, 68, 2266. 8o D. A. Whan, M. Barber, and P. Swift, J.C.S. Chem. Comm., 1972, 198. E. S. Davie, D. A. man, and C. Kemball, Chem. Comm., 1971, 1202. 'O A. Morris, H. Thomas, and C. J. Attridge, Ger. Offen., 1972, 2 213 948 (Chem. Abs., 1973, 79, 65 769h); J. P. Candlin, A. H. Mawby, and H. Thomas, Ger. Offen., 1972, 2 213 947 (Chem. Abs., 1973, 79, 97 101e); I.C.I.Ltd., Fr. P., 1972, 2 120 509 (Chem. Ah., 1973,79, 115 104h). 163 OlefinMetathesis and its Catalysis their activity at reIatively low temperatures, as demonstrated by a 38 % conver-sion of a but-1-ene feed into ethylene and hexenes with a 95.5% selectivity at 25 "C, at atmospheric pressure and at a 1600 vol./vol. gas hourly space velocity.52 B.Homogeneous Catalysts.-The first homogeneous system reported to catalyse the ole& metathesis reaction was tungsten hexachloride-ethylaluminium dichlorideethan~l.~Equilibrium is attained within minutes for a W :A1 ratio of 1:4 and a W:olefin ratio of 1:lO OO0.lo Neither constituent of this catalytic mixture catalyses metathesis alone. Subsequent studies have revealed that a wide range of organometallic or hydridic derivatives of Main Groups I, 11, In, or IV may be employed as co-catalyst in conjunction with [WclS].These include LiR (R = Bun or RMgX (X = C1, R = Bun or n-C~H11;X = Br, R = Prn, Bun, or n-GH11; X = I, R = B~~),~~-75R2AICl,&AI (R = Et or Bui), EtAIC12,7s-78 SnR4 (R = Me or Bun),79 LalH4, and NaBK.80 There is, however, no firmdividing line between homogeneous and hetero- geneous catalysis. Thus [wCls]-EtAIc12 (a homogeneous catalyst) acts hetero- geneously when adsorbed on alumina.81 It was initially believed that the function of the organometallic species was to reduce [WclS] to tungsten tetrachloride since the [WCls]-LiBun system shows a maximumof activity for a Li:W ratio of 2:l. Complexation of the olefin would then give [WCl4(ole&)2], the actual catalyst, in which the olefins are bonded in cis-positions.This is supported by the fact that [WCla]-Et& exhibits maximum activity over only a narrow range of W :Almolar ratios, the optimum being ca. 0.54.6.With an excess of Et3Al the activity is decreased, possibly due to over- reduction, but it may be restored by treatment of the mixture with dioxygen.77 However, wc14 obtained by reduction of [WclS] with H2 at high temperatures or prepared in situ by treatment of [WclS] with reducing agents such as zinc, magnesium, or sodium amalgam is not a metathesis catalyst,20s80 although the addition of aluminium trichloride to wc14 so prepared gives a highly active catalyst.82 Significantly, [WclS] and [WBrs] also form active metathesis cata- 71 J.-L.Wang and H. R. Menapace, J. Org. Chem., 1968,33, 3794. 7a M.L.Khidekel, V. I. Marin, A. D. Shebaldova, T. A. Bolshinskova, and I. V. Kalechits, Izvest. Akad. Nauk S.S.S.R., Ser. khim., 1971,663. P. A. Raven and E. J. Wharton, Chem. andInd., 1972, 292. 74 T. Takagi, T. Hamaguchi, K. Fukuzumi, and M. Aoyamo, J.C.S. Chem. Comm., 1972,838. 76 V. I. Marin, A. Shebaldova, T. A. Bolshinskova, M. L. Khidekel, and I. V. Kalechits, Kinetika i Kataliz, 1973, 14, 613. 70 A. Uchida, Y. Mukai, Y.Hamano, and S. Matsudu, Znd. and Eng. Chem. (Product Res. and Development), 1971, 10, 369. Y. Uchida, M. Hidai, and T. Tatsumi, Bull. Chem. SOC.Japan, 1972,45, 1158. 7* N. Calderon and H.Y.Chen, B. P., 1968,1 125 529 (Chem. Abs., 1968,69, 105 851f). 7@ C.P.C. Bradshaw, B. P., 1970,1 208 068 (Chem. Ah., 1970,72, 12 047v). 80 J. Chatt, R. J. Haines, and G. J. Leigh, J.C.S. Chem. Comm., 1972, 1202; see also S. A. Matlin and R. G. Sarnmes, ibid., 1973, 174. C. P.C.Bradshaw and J. I. Blake, B. P., 1972,1266 340 (Chem.Abs., 1972,76,126 352d). D.M.Singleton, US,P., 1970, 3 530 196 (Gem, Abs., 1970,72, 89 7342). Haines and Leigh lysts with the non-reducing AlCh or AIBr320983984 but it is conceivable that olefins can insert into aluminium-halogen bonds similar to the well-known olefin-insertion into aluminium-hydrogen bonds,*5 forming aluminium chloro- alkyls, and it is unlikely that the final systems contain WV or WVI. The organometallic or hydridic species which interacts with the tungsten compound to form the catalyst may bond through chloride, hydride, or alkyl bridges and/or may function as a Lewis acid, giving rise to acid-base equilibria of the type: [WClz] + [AlCl3]+ [WCIz-lIf + [AlC14]-in the case of AlCl3, the removal of a chloride ion from the tungsten providing an additional vacant co-ordination site for the incoming olefin.20 The addition of MC13 or AlBr3 to [WC16]-2LiBun increases the catalytic activity of this system considerably, and this may also be explained by the aluminium halide acting as a Lewis acid and effecting rapid isomerization of trans-[WC14(olelin)~]to cis-[WCL(olefin)~] by the mechanism outlined in Scheme 2.86 AlClt A.ctrans-[ WCla(olefin)z] 4-AlC& cis-[ WC14(olefh~] Scheme 2 The ability of lithium alkyls, tin tetra-alkyls, lithium aluminium hydride, or sodium borohydride to produce active metathesis catalysts with [WClS] would imply that although acid-base equilibria of the type described above can lead to an increase in the rate of metathesis, it is not an essential part of the catalyst const it ution.The homogeneous catalyst mixture [WCle]--LiBu* appears to yield substantially the same product from the metathesis of but-2-ene throughout the range 60-130 "C, although the rate increases with temperature. Above 130 "C,the catalyst is deactivated or destroyed.87 Controlled treatment of [WClS] with protic solvents such as ethanol, phenol, water, or acetic acid prior to the addition of the organoaluminium compound increases the catalytic activity of systems [Wc16]-RzA1c~3-z.5~10~78~88The reaction of [WClS] with ethanol produces a stoicheiometric amount of HCl together with species such as [WC15(OEt)].20 It is therefore not suprising that compounds such as [WOc14], WOa, [ {o-(MeOzC)C6&0 }Web], w(oPh)6], 83 P.R. Marshall and B. J. Ridgewell, European PoZymer J., 1969, 5,29. 8p J.-L. Herisson, Y. Chauvin, N. H. Phung, and G. Lefebvre, Compt. rend., 1969, 269, C, 661. See, for example, R. Koster and P. Binger, Adv. Znorg.Radiochem., 1965, 7, 263. 86 J.-L. Wang, H. R. Menapace, and M. Brown, J. Catalysis, 1972, 26,455. J.-L. Wang and H. R. Menapace, J. Catalysis, 1973,28,300. D. Medema, W.Brunmayer-Schilt, and R. vaa Helden, B. P., 1970, 1 193 943 (Chem.Abs., 1970, 73, 76 637r). Olefin Metathesis and its Catalysis and [WO(OPh)4] are active metathesis catalysts in the presence of a suitable cocatalyst. 47~84~890 90 Group V donors reduce the activity of catalysts derived from [WCle]. Treat- ment of [WCh]-EtAICIz with pyridine inhibits metathesis, as also does tri- phenylphosphine (but less Pyridine reduces [WCle] to [WC14(pyridine)2],92 and the ligand may block possible co-ordination sites for the olefin. It may also be that the aluminium compound removes PPh3 more readily from [WC14- (PPh3)2] than it does pyridine from [WC14(pyridine)2].91 Direct comparison of the effects of the two ligands is not really justified, however, because although the reactions for both ligands employed a W:ligand ratio of 1:1, they used different W: A1 ratios.Furthermore, for W: A1 ratios greater than those employed above, [WCle(pyridine)z)-EtAIClz, as well as [WCh (CzH4(PPh2)2 )2)-EtAIC12, are effective met at hesis cat a1 yst s.93 Systems derived from high-valent molybdenum compounds are far less effective catalysts than those obtained from the tungsten analogues. For instance, [MoCI~I-E~~A~,~~[Mo(a~ac)3l-EtAl,~7 [M0Cl5]-PrnMgBr,~~ and [MoC15]-LiAlH4*O show very little or no activity, although the [MoC14]-AlBr382 system has been claimed to be effective. Catalytic activity for a number of molybdenum systems can be induced by treatment with nitric oxide prior to addition of an organoaluminium compound.This has been established in particular for [MOCI~L~] = PPh3, C5H5N, or (L PrnCN), [GH5Mo(C0)31], [MoC13(PhC02)2], [MoO2(acac)], [MoOCI~], and [MoClb] in the presence of ligands such as C5H5N, Ph3P0, Bu%PO, and OCtn3- PO.35 The enhancement of activity is ably demonstrated by the [MoCh(PhCO&I- [Me3Al2C13] system. Treatment of the molybdenum compound with nitric oxide for 0.5 h before the addition of the aluminium sesquichloride produces a catalyst which gives a 60% yield of oct-4-ene from pent-l-ene after 17 min. Without the NO treatment this catalyst effects only a 0.8% conversion of the pent-l-ene after 1 h.24 Interaction with nitric oxide should yield molybdenum nitrosyl species, and, consistent with this, derivatives of the type [Mo(N0)2L2X2] (X = C1, Br, or I etc.; L = PPh3, AsPh3, Ph3P0, C5H5N, or EtC5H4NY etc.) are highly effective catalysts in the presence of an organoaluminium halide.35e94 These systems have been studied in some detail, and a number of significant features have been noted.35 For instance, a wide range of Mo:Al ratios can be employed, but 1 :4-5 gives highest activity.Further, the catalyst requires a pre-formation period to reach maximum activity, this being 1 h in the case of [Mo(NO)Z(C~H~N)~C~~]-E~AICIZ.During this period the two nitrosyl stretching vibrations associated with [MO(NO)Z(C~H~N)ZCIZ] shift to higher frequency and eventually disappear.24~95 This is explained as initial complexation of two J.-L. Wang, M.Brown, and H. R. Menapace, Ger. Offen.,1972, 2 158 990 (Chem. Ah., 1972,77, 125 933c). 90 H. Knoche, Ger. Offen., 1970, 2 024 835 (Chem. Abs., 1971,74,44 118b). 91 M. Kothari and J. J. Tazuma, J. Org. Chem., 1971, 36, 2951. O3 R. E. McCarley and T. M. Brown, Znorg. Chem., 1964, 3, 1232. O3 L. Bencze and L. Marko, J. Organometallic Chem., 1971, 28, 271. O4 E. A. Zuech, Fr. P., 1969, 1 575 778 (Chem. Abs., 1970,72,999 882). Ob W. B. Hughes, J. Amer. Chem. SOC.,1970, 92, 532. Haines and Leigh equivalents of the organoaluminium reagent with the chlorides and a conse- quential increase in the NO stretching frequencies, followed by attack of two further equivalents of the organoaluminium compound on the oxygens of the nitrosyl groups, leading to the removal of these groups from the rn0lybdenum,~4 as outlined in Scheme 3.In this scheme it is apparent that a function of the aluminium compound is to produce two adjacent vacant co-ordination sites. PY PY DY NO EtC1,AlCI I Ci 1JY PY PY + 2NOAlEtCIa Scheme 3 The catalytic activity of systems derived from compounds of the type [Mo-(NO)abX2] depends on the nature of L and of the anion X, an increase in activity being observed along the series AsPhs < 4-EtC5H4N PhsPO and along the series I < Br < Cl.24 It was therefore suggested that the neutral and anionic ligands are not removed from the metal in the formation of the actual catalytic intermediate. Tungsten derivatives [W(NO)2L2X2] also form active metathesis catalysts but their activity is not as high as that of the corresponding molyb- denum systems.35 Carbon monoxide is also capable of increasing the activity of metathesis catalysts derived from Group VI metal halide complexes and organoaluminium compounds.93 It has been found that [WC~~(C~H~N)Z]-E~AICIZ under an atmos- phere of carbon monoxide exhibits a very different catalytic activity from the same system under argon.The interaction of CO with [WC14(C5H5N)2]-EtAlC12 gives tungsten carbonyl species, and this led to experiments demonstrating that [W(C0)3(PPh3)2C12] in the presence of EtAlCl2 is highly active in converting pent-Zene into but-Zene and he~-3-ene.~~ [WCh {CzH4(PPhz)z }2]-EtAlC12 is also a metathesis catalyst, but excessive exposure to CO can destroy its a~tivity,~3 owing to its conversion into ~(co)6].96 Tungsten carbonyl derivatives D.y(CO)5L] or [W(CO)4L2] in the presence of AlC13 or EtAlCl2 promote the metathesis reaction, provided that trace quantities of dioxygen are present.97 It has been suggested that, in the presence of the tungsten complex, oxygen inserts into a carbon-aluminium bond in [(EtAlC12)2], and the resultant dimer then rearranges to give the species (5).This is believed to bond to the tungsten at a site originally occupied by a carbonyl group. A ten-fold excess of EtAlClz was employed in these studies, however, and it is L.Bencze, J. Organometallic Chem., 1972, 37, C37. B’ L. Ramain and Y. Trambouze, Compt. rend., 1971, 273, C, 1409. 167 Olefin Metathesis and its Catalysis Et\Al 0c’\Al/ CA \lo’ La unlikely that all of the aluminium compound is associated with the tungsten, as must have been assumed in proposing this mechanism.The catalytic activity of [W(PMezPh)4CN2)2]-EtAlCl2 is also increased in the presence of dioxygen9* but this can be readily explained in terms of the creation of vacant or potentially vacant sites through the removal of dirnethylphenylphosphine as its oxide. Dioxygen also affects the kind of product obtained from olehs and wCl~]- EtAI in chlorobenzene.99 Dioxygen does not appear to be necessary to activate metathesis catalysts derived from zero-valent halogenocarbonyl anions of molybdenum or tungsten, [M(CO)5X]-(M = Mo or W; X = C1 or Br), and MeAlC1236 or Grignard reagents73 and from the related metal acyl species [M(C0)5(COR)]- (M = Mo or W; R = Me or Ph) and alkylaluminium dichlorides or sesquichlorides.lOOJO1 The activity of these systems is far less than that of most of the catalytic systems described above.The order of reactivity found for the disproportionation of pent-l-ene by MeAlClz-[M(C0)5COR]- was Mo > W and Ph > Me.100 The dinuclear carbonyl anions [(CO)5M-M’(C0)5In- (M = Mo or W; M’ = Mo, W, Mn, or Re; n = 1 or 2) and the metal carbene complexes [W(C0)5C(OMe)Et] and [W(CO)&(NMez)Me] also form active metathesis catalysts with alkyl- aluminium chlorides, but only in the presence of an excess of chloride ions.lo0J02 It is assumed that the function of the chloride is to produce [M(C0)5Cl]-.Homogeneous catalysts derived from rhenium salts are very much less active than the molybdenum and tungsten species. For instance, 48 h are required for [ReCls]-SnBu4 to convert pent-2-ene into its equilibrium mixture with but-2- ene and hex-3-ene.103 Reactions employing [ReCl4(PPh3)2]-EtAlCh and [ReOX3(PPh3)2l-EtAIClz (X = C1 01Br) also require long reaction times.lo4 At least one transition metal from each Periodic Group has been shown to exhibit homogeneous catalytic activity in the presence of an appropriate co- catalyst. Systems reported include ~iC14(C5H5N)2]-EtAlC12,105J06[(C5H5)2-TiCl]-[Me3A12Cl3],106 [Zr(aca~)4]-[Me3AlzCl3],~~~J~~[NbC15]-[MeA12C13] in the R. J. Haines, unpublished results. OaA.Uchida, K. Kobayashi, and S. Matsuda, Ind. and Eng. Chem. (Product Res. and Development), 1972, 11, 389. looW. R. Kroll and G. Doyle, Chem. Comm., 1971, 839. Iol W. R. Kroll and G. Doyle, U.S.P., 1972, 3 689 433 (Chem. Abs., 1972, 77, 151 431g). lo4 W. R. Kroll and G. Doyle, J. Catalysis, 1972, 24, 356. lo3J. A. Moulijn and C. Boelhouwer, Chem. Comm., 1971, 1170. Io4 E. T. Kittleman and E. A. Zuech, Fr. P., 1969, 1 561 025 (Chem. Ah., 1970,72, 31 193f). lo6D. H. Kubicek and E. A, Zuech, U.S.P., 1972,3 670 043 (Chern.Abs., 1972,77,100 716w). W. B. Hughes, E A. Zuech, E. T. Kittleman, and D. H. Kubicek, 23rd I.U.P.A.C. Con-gress, Boston, 1971, Abstract 566. Haines and Leigh presence of benzoic acid and nitric oxide,lo7 [(Fe(NO)2Cl>2]-[Me3Al~Cl~],106~108 [OSOC~(PP~~)~~[M~~~C~~],~~~~~~~[CoC12(4-vinylpyridine)ne)zl-[Me3AlzC13],106 [(Rh(NO)2Cl }z]-[M~sA~sC~~],~~~,~~~ [{(C3H&RhCl )2]-[Me3A12C13],106~10S [Rh-(NO)(PPh3)2C12]-[Me3A12C13],106Jo* [Ir(NO)(PPh3)2C12]-[Me3A12C13],106~107 PdBrzEtAlClz in the presence of PPh3,109 [C~(PPh3)3Cl]-[Me3Al2Cl3],~~~~~~~ [Cu2Clz(PPh3)2)-EtAlC12,111 [Ag(PPh3)Br]-EtAlC12,106J10[Au(PPh3)Cl]-[MeaAl~Cl3],~~69SmC13-[Me3A12C13],106,l1 ThC14-[ and UClr l10 [Me~A12Cl3].~06The activity of these systems is not as high as those derived from Group VI metal derivatives.For instance, the conversion of heptene was only 2.5 % after 90 h at room temperature employing SmC13-[MeA2C13] as catalyst and 0.2 % after 2 h at 80 "C using [Cu(PPh3)sClI-[Me3Al2Cl3]as the catalyst.The rhodium systems described above are fairly active as metathesis catalysts but, not surprisingly, they also effect rapid isomerization of the olefin.lO8 All of the systems described thus far require two components for catalytic activity. Lewandos and Pettit113 have reported a transition-metal species which does not require the presence of a co-catalyst to promote olefk metathesis. They have shown that for a W :olefin ratio of ca. 1:4 and at a temperature of 98 "C, toluene tungsten tricarbonyl effects a 28%conversion of non-4-ene into oct-4- ene and dec-Sene, in heptane as solvent, during 24 h. Metathesis was only observed when the reaction was carried out in a system where any liberated carbon monoxide could readily escape.No metathesis occurred in a closed system, or with large o1efin:W ratios, or when an excess of toluene was added. On the basis of these results and electron-counting considerations,l14 they suggested that the actual catalyst is [W(C0)2]. In view of the low activity of their catalyst under the experimental conditions employed, other possible mechanisms cannot be immediately discarded. Rhodium(1) complexes in the absence of any co-catalyst effect the metathesis of electron-rich 01efins.l~~ Thus [Rh(PPh&LCl] (L = CO or PPh) catalyses the reaction of olefin (6) with olefin (7) to give (8), the yield of (8) approaching Ph N Ph N C,H,MeN C,H,Me Ph CeH4Me N N N N N Ph Ph CeH4Me C,H4Me Ph C,H4Me (6) (7) (8) lo' W.B.Hughes and E.A. Zuech, U.S. P., 1971,3 562 178 (Chem. Abs., 1971,75,10 868b); US. P., 1972, 3 691 253 (Chem. Abs., 1973,78, 3657r). lo* W.B. Hughes and E. A. Zuech,U.S. P., 1971,3 558 517 (Chem. Abs., 1970,72,99 985w). loS P. H. Phung and G. Lefebvre, Fr. P., 1970, 1 594 582 (Chem. Abs., 1971,74, 87 316g). noD. H. Kubicek and E. A. Zuech, U.S. P., 1971,3 558 520 (Chem. Abs., 1971,74,99 441g). D. H. Kubicek and E. A. Zuech, U.S. P., 1972, 3 703 561 (Chem. Abs., 1973,78,71 378a). 11* E. T. Kittleman and E. A. Zuech, U.S. P., 1971, 3 554 924 (Chem. Abs., 1971,74,99 440 f); U.S. P., 1973, 3 708 551 (Chem. Abs., 1973, 78, 71 382x). llS G. S. Lewandos and R. Pettit, J. Amer. Chem. Soc., 1971, 93, 7087. 114 G. S. Lewandos and R. Pettit, Tetrahedron Letters, 1971, 789.ll6 D. J. Cardin, M. J. Doyle, and M. F. Lappert, J.C.S. Chem. Comm., 1972, 927. 169 Olefin Metathesis and its Catalysis the statistical value after 2 h in xylene at 140 "C.A mechanism involving a four-membered metallocycle as transition state has been proposed, but whether this can be applied generally to reactions involving normal olefins remains an open question. One other system, viz. [WOCl4], has been reported to catalyse olefin metathe- sis in the absence of a co-catalyst, but its activity is exceptionally low.84 The determination of the nature of the catalytic site for olefin metathesis is difficult. In most cases the catalyst is generated by interaction of a transition- metal compound with an organometallic species, and a wide range of products are possible.The catalyst may only be present in solution in low concentration. However, molybdenum and tungsten species form the most effective catalysts, a co-catalyst capable of bonding to the molybdenum or tungsten through halogen, alkyl, or hydride bridges being required. Thus, for [WCl6]-EtAICI&tOH, the catalyst is claimed to be a complex of the transition metal with an aluminium compound, with two tungsten-carbon bonds,ll6 and transition metal-aluminium complexes are formed by Ziegler-Natta catalysts. It is possible that more than one oxidation state is capable of promoting metathesis, but +II is most likely for Mo and W. In any case it is possible to over-reduce the catalyst, so that oxidation states greater than zero seem likely.C.Catalytic Selectivity.-The catalysts employed in olefin metathesis can also promote other types of reaction, including olefin isomerization, olefin oligo- merization/polymerization,and, in the case of homogeneous systems, alkylation of aromatic solvents. Double-bond migration is readily effected by supported metal oxide metathesis catalysts. This explains why Banks and Bailey observed the formation of a large number of unsaturated hydrocarbons in their original studies on but-1 -ene, pent-l-ene, and hex-l-ene.l It can be minimized by poisoning of the isomerization sites on the heterogeneous system with alkali-metal, alkaline-earth, or thallium(1) These presumably reduce surface acidity and inhibit isomerization occurring by a cationic mechanism.l17 The effectiveness increases along the series Na < K < Rb N TI1 < Cs, which is the order of the ionic radii.117 In some chemical processes, for instance the formation of detergent-range olefins from propylene, double-bond isomerization activity is essential.Magnesium oxide is a very selective catalyst for double-bond migration and has been em- ployed in conjunction with metathesis catalysts for obtaining mixtures containing a wide range of olefins.llg Isomerization has also been observed for most homogeneous catalytic systems. For instance, the metathesis of oct-l-ene by [Mo(NO)~(PP~~)~CI~]-E~AIC~~at ambient temperature affords, after 16 h, a mixture of all olefins from ethylene to pentadecene, and not just ethylene and tetradecene as expected.s5 Olefin isomeri- Il6 H.Hoecker and F. R. Jones, Makromol. Chem., 1972, 161, 251. 117 T. P. Kobylinski and H. E. Swift, J. Catalysis, 1972, 26, 416. 118 Shell Internationale Research Maatschaapij N.V., Dutch P., 1967, 6 607 427 (Chern. Abs., 1968,69, 43 389a). 11* R. L. Banks and J. R. Kenton, Belg. P., 1968, 713 190. Haines and Leigh zation is generally considerably slower than olefin metathesis, and can be mini- mized in metathesis reactions by performing the latter at low temperatures and for short periods. Thus little isomerization of pent-l-ene or oct-l-ene by [MO(NO)~(PP~~)~CI~]-E~AICI~is observed after I hour at 0-5 0C.35 Terminal olefins are isomerized more readily by metathesis catalysts than internal olefins.For example, whereas only bu t-2-eneY pent-Zene, hex-3-eneY and ethylene are found in the reaction mixture obtained by metathesis of pent-Zene by [ReC15F AIEt3-02, all olefins from ethylene to octene are formed in the corresponding metathesis of pent-l-ene.77 The extent of isomerization also depends on the non-transition-metal component of the catalyst. The catalytic system [Mo-(NO)2(PPh3)2Cl2]-EtAlC12 promotes isomerization more readily than [Mo-(N0)2(PPh3)2C12]-[Me3A12C13], suggesting that for these systems isomerization is effected by a cationic mechanism, as found for heterogeneous ~atalysts.~5 Oligomerization and/or polymerization of olefins has been reported for a number of homogeneous metathesis catalysts.Although definite attempts have not been made to identify the oligomers in the various reactions nor to establish the nature of the oligomerizations, it has been ascertained that the so-called oligomers formed on metathesis of pent-l-ene by [WCls]-Et&l or [MoC15]- EtsAl in chlorobenzene are not alkylated products of the solvent.77 Further, the small amounts of branched olefins formed in the disproportionation of pent-l- ene by [M~(NO)~(PP~~)~CI~]-[M~&~CIS]can only be explained by some dimeriza- tion process.35 Terminal olefins are oligomerized far more readily than internal olefins, as demonstrated by some reactions involving [WOC14]-EtAlC12 as catalyst.s4 Whereas oligomers in yields of 50 and 30%, respectively, were produced on metathesis of propylene and pent-l-ene employing this catalytic system, only disproportionation products were observed in the corresponding reactions involving pent-2-ene and hept-3-ene. These results are not particularly surprising when one considers that the metathesis catalysts are Ziegler-Natta in type.The oligomerization behaviour of [WCls]-Et&I, [MoC15]-Et3AlYor [ReC15]-Et3AI is destroyed by addition of an excess of EtsAl or PPh3.77 In fact, it has been found that the oligomerization behaviour of [WClsI-Etdl towards oct-l-ene in benzene increases rapidly as the W:AI ratio is varied from 0.25 to 1.O whereas the disproportionation activity decreases.99 These results suggest that the oligomerization is effected by a species of higher oxidation state than is active in the metathesis; a carbonium-ion process participating in oligomeriza- tion was eliminated because alkylbenzenes were not detected in the reaction of [WCls]-Et3Al with oct-l-ene in benzene.99 Heterogeneous catalytic systems for metathesis also readily promote the oligomerization of olefins, as demonstrated by the reaction of CoO-Mo03-Al203 with b~t-l-ene.~ Over SO% of the octenes formed in this metathesis reaction are branched, and this can only be attributed to dimerization of the butenes present in solution.A cationic polymerization mechanism120 has been proposed to account for this dimerization, and, consistent with this proposal, it is found that laoC.E. H. Bawn, Proc. Chem. SOC.,1962, 165.Olefin Metathesis and its Catalysis the oligomerization process can be eliminated by poisoning of the acid sites with sodium carb~nate.~ Addition of fluoride ions to oxide catalysts of molybdenum or rhenium also inhibits oligomerization.121 A sidereaction often encountered when aromatic solvents are employed in metathesis reactions utilizing homogeneous catalysts is the Friedel-Crafts alkylation of the solvent by the olefm. The mixture [WCI6]-EtAIC12 is a particu-larly effective alkylation catalyst under certain conditions. Thus, addition of pent-2-ene to a premixed solution of [WClS] and EtAlCl2 in toluene results in the almost exclusive alkylation of the solvent, and di- as well as mono-pentyl- toluenes are formed.g1 On the other hand, however, if the [WCls]-EtAIC12 catalyst is prepared in the presence of the pent-2-ene, rapid metathesis occurs.Q1 The tungsten species [WCls] promotes the Friedel-Crafts alkylation of benzene with propylene in the absence of any EtAIC12, although at temperatures up to 150 "C (much higher than those for [WC16]-EtAlC12).It has been suggested that the actual catalyst in the [WCls]-EtAlch system is a derivative containing the highly active Friedel-Crafts catalyst AICL, formed by the reaction of [wc16] with EtAIC12, co-ordinated to a reduced tungsten spe~ies.~lJ22 On this basis, it would be expected that [WCls]-Et3AI would be a less effective alkylation catalyst than [WCle]-EtA1C12 under comparable conditions, and this is found to be the case for both terminal and internal 0lefins.76,~~J~3 In fact, in the complete absence of water or dioxygen, [WCh]-Et3Al does not effect the alkylation of benzene with oct-1 -ene.99 The inability of [WCl6]-LiAlHg to promote alkylation of aromatic solvents is further indirect evidence for the suggested intermediate.80 Addition of pyridine and triphenylphosphine to [WCh]-EtAlC12 has been shown to inhibit the alkylation without completely curtailing the metathesis reaction.gf Like [WClS], [MoC15] and [ReC15] also promote the alkylation of aromatic solvents by but in the presence of SnBun4 the [ReC15] loses its alkylating ability while remaining an effective metathesis catalyst .lo3 4 The Mechanism of the Olefin Metathesis Reaction No single mechanism has been suggested to explain all olefm metathesis reactions and, indeed, it may proceed by more than one mechanism.Nor is it necessary that the reaction route should be the same for both homogeneous and hetero- geneous reactions. It is apparent that even different homogeneous catalysts use different mechanisms. A satisfactory mechanism should be extendable to hetero- geneous alkyne metathesis, which occurs in conditions very similar to those which promote olefin metathesi~.~~,~~ We first consider the general experimental evidence upon which the mechan- isms must be based. A. Factors to be Considered in Proposing a Mechanism.-(i) The Olefins. The lal R. Argankight, U.S.P., 1972, 3 697 613. la*J. R. Graham and L. H. Slaugh, Tetrahedron Letters, 1971, 787.lSs A. Uchida, Y. Hamano, Y. Mukai, and S. Matsuda, Ind. and Eng. Chem. (Product Res. and Development), 1971, 10, 372. lap J. Tsuji, T. Nogi, and M. Morikawa, Bull. Chem. SOC.Japan, 1966, 39, 714. Haines and Leigh metathesis of [2-14C]propene over a heterogeneous catalyst of rhenium oxide on alumina yields an ethylene sample free of 14C.125 2C3H6 + Ca + CHsCH=CHCH3 (cis-and trans-isomers) This excludes a linear mechanism, e.g. reaction (3). * * * * * 32c=c-c --t [C-c-c-c-c-C] c=c + c-c=c-c (3) On this same catalyst, methyl groups retain their integrity throughout the reaction,126J27 as shown by experiments with [1-14C]propene and [2-14C]propene. This precludes hydrogen transfer from olefin to metal and back again, and hence r-allylic intermediates.Similar studies over a cobalt molybdate-alumina catalyst yielded essentially the same results, but above 60 "Cisomerization of the propene occurs,127 and then an allylic intermediate cannot be excluded. An investigation of the homogeneous metathesis of deuteriated butenes also showed that hydrogen transfer to the metal does not occur, because no H-D exchange was ob~erved.~J~ This is also consistent with the results from the heterogeneous metathesis of ethylene and 2,3-dimethylbut-2-ene,128which show that if hydrogen does migrate to the metal it must always return to the carbon atom from which it came. This is unlikely. The observations cited are generally consistent with a transition state in which the four carbon atoms involved in the bond switching are equivalent.To account for this, Bradshaw et aL4proposed a 'quasi-cyclobu- tane' picture of the tiansition state, which has since been adopted by 0thers.1~~-~3~ An alternative model, which uses the same equivalence concept, pictures the four carbon atoms in the transition state as bound to a single metal ion essentially as met hylene fragment s.1139114 Kinetic studies of the classical type have done little to clarify the mechanism, because they are very difficult, and the results from different systems often con- flict. Various attempts have been made to rationalize the rates of metathesis of propylene over a Co0-Mo03-A.h03 catalyst in the temperature range 120-200 "C.The rates are consistent with a two-site mechanism, with an activation energy of about 8 kcal m0l+3~ The rates of olefin metathesis over the homogeneous catalysts derived from molybdenum(r1) dinitrosyls and alkylaluminium halides have been measured.95 At low o1efin:catalyst ratios the order of reaction with respect to olefin was found to be greater than unity, but it fell towards unity as the olefin concentration was increased.The order of reaction with respect to lS6 J. C. Mol, J. A. Moulijn, and C. Boelhouwer, Chem. Comm., 1968, 633; J. Catalysis, 1968, 11, 87. 18e F. L. Woody, M. J. Lewis, and G. B. Wills, J. Catalysis, 1969, 14, 389. la' A. Clark and C. Cook, J. Catalysis, 1969, 15, 420. D. L. Crain, J. Catalysis, 1969, 13, 110. 189 E. A. Zuech, Chem.Comm., 1968, 1182. 130 C. T. Adams and S. G. Brandenberger, J. Catalysis, 1969, 13, 360. lS1 R. Pettit, H. Sugahara, J. Wristers, and W. Merk, Discuss Faraday SOC.,1969, No. 47, p. 71. 13p M. J. Lewis and G. B. Wills, J. Catalysis, 1969, 15, 140; R. C. Luckner, G. E. McConocie, and G. B. Wills, ibid,, 1973, 28, 63; A. J. Moffat and A. Clark, ibid., 1970, 17, 264. 173 Olefn Metathesis and its Catalysis catalyst could not be determined. The results are consistent with rapid complexing and decomplexing of the olefin, the metathesis step being rate-determining. The activation energy for the process was found to be ca. 7 kcal mol-l, which is low, as expected for a reaction that is basically athermal and entropy-controlled.95 The stereochemistry of metathesis over the same homogeneous metal nitrosyl- aluminium alkyl catalysts has also been studied.133 At equilibrium of the metathesis of pent-Zene to but-2-ene and hex3-ene the cis and trans pairs of each olefin are in thermodynamic equilibrium, but in the early stages of the reaction the isomer ratios depend upon the starting pent-2-ene. Thus, cis-pent-2- ene yields, preferentially, cis-but-Zene and cis-hex-3-ene, and trans-pent-Zene yields, preferentially, the corresponding trans-isomers. The rates of isomerization and metathesis appear to be related, and the observations are explicable in terms of two olefin molecules bound cis to molybdenum, with their axes parallel, and undergoing metathesis via the ‘cyclobutane’ transition state.133 The isomer ratios from pent-2-ene in contact with [WC16]-EtAlC12 and ethanol (1 :4: 1) in a homogeneous system are in sharp contrast.10 Substantial amounts of cis-and trans-hex-3-ene are formed at the outset (i.e.after about 20 seconds at room temperature) regardless of which isomer of pent-Zene is used, but cis-isomers are present in slightly more than equilibrium proportions initially. The residual pent-Zene was also observed to approach slowly its equilibrium isomer content. These results are consistent with a random transalkylidenation coupled with a minimum of steric control.lO~71 It has also been inferred that the overall mechanism of the reaction over homogeneous tungsten catalysts is different from that over molybdenum nitrosyl derivatives.Thus, investigation of cyclo-octene polymerizations shows that high- molecular-weight polymers are formed during the early stages of the reaction, and this is unexpected if olefm exchange is more rapid than metathesis20 because otherwise the high-molecular-weight polymers would form more slowly. The explanation for these divergences may lie in the different structures of the catalysts, The generalizations above, complex as they are, may nevertheless be an oversimplification. Many of the reactions are accompanied by isomerization, even though this is generally a slower reaction. More interesting, ethylene over a heterogeneous molybdenum-alumina catalyst at 210 “C in a static reactor, rather than in the more usual flow systems, gives propene, butene, small amounts of hexenes and heptenes, and, as the major product, ~entenes.13~ The initial reaction would seem to be a reaction of ethylene to form propene, possibly via formation of methylene (carbene) and thence trimethylene (cyclopropane).However, a concerted reaction, such as depicted below, cannot be excluded. Reactions producing compounds with an odd number of carbon atoms have also been lSs W. B.Hughes, Chem. Comm., 1969, 431. lS4P.P.O’Neill and J. J. Rooney, J. Amer. Chem. SOC.,1972,94, 4383. Haines and Leigh observed in homogeneous systems,135 but these could arise by isomerization followed by metathesis. Molybdenum hexacarbonyl-alumina catalysts also catalyse the conversion of ethylene into cyclopropane and methylcyclopropane, although the reaction temperature (140 "C)is much lower.1 Cyclopropane itself rearranges to propene under very mild conditions, so that a common route for all these reactions seems possible.Further, olefins are converted quantitatively into the corresponding cyclopropanes by the action of carbene (derived from CH2N2) under the action of palladous salts.136 All this lends credence to the idea that carbenes and olefins may exist together on an appropriate metathesis catalyst. The metathesis reaction, whether homogeneous or heterogeneous, is rapid, isothermal, and entropy-controlled. The activation energy is less than 10 kcal mol-1. This makes the formation of free carbenes unlikely, but, at least where electron-rich olefins are involved, carbene-containing products have actually been isolated.Thus in the homogeneous metathesis of (9), catalysed by [RhCl-(Pph,)~],(10) has been identified as an i11termediate.~zJ15 It is not clear whether intermediates of this kind form in the general case, since the olefins involved are atypical. (ii) The Catalysts. There would seem to be no easy classification of the catalysts for either homogeneous or heterogeneous metathesis. The heterogeneous catalysts are generally compounds of molybdenum or tungsten, often supported on alumina,54 and they are not well-defined compounds. The homogeneous catalysts* are generally derived from high-oxidation-state halides and a reducing agent, generally a metal alkyl, such as ethylaluminium dichloride.5,20J~3 Although the proportion of the second metal to the primary metal can affect activity, and even the kind of products, neither the alkyl nor the second metal is necessary in every case; tungsten and [(n-CH3C6H5)W(C0)3] gives rise to a catalyst by loss of carbon monoxide and toluene.1'3 In the system [WC~~(P~~PCH~CH~PP~Z)~]-E~AICI~,carbon monoxide actually stimulates the catalyst.93 The alkyl-metal compound apparently reduces the tungsten halide. It has not yet been unequivocally demonstrated which oxidation state(s) of tungsten are responsible for the catalysis.It is believed that the catalytic species derived from [(PPh&Mo(N0)2C12] and organoaluminium halide contains molyb- * Note added in proof: it has been suggested that the [WClJ-metal alkyl system may give rise to catalyst systems that are wholly heterogeneous; unequivocal proof would seem difficult (E.L. Muetterties and M. A. Busch, J.C.S. Chem. Comm., 1974, 754). lS6 K. Hummel and W. Ast, Naturwiss., 1970, 57, 245. lS* R. Padisson, A. J. Hubert, and P. Teyssit, Tetrahedron Letters, 1972, 1465. Olefin Metathesis and its Catalysis denum(o).~5~QsIn one cases6'MO(C0)4' has been suggested as the active entity, although potentially oxidizing halogenated olefins stimulate reactivity.69 Later, the complete loss of CO from [Mo(CO)s] was postulated in catalyst formation.67 In another homogeneous system, a tungsten dicarbonyl fragment has Wn inferred as the catalyst.113 This suggests a d6 configuration for the transition metal, although substitution reactions at octahedral low-spin @-metal ions are usually slow.It is unlikely, however, that catalytic systems produced by re- ducing, say, Wch], contain Wo, and oxidation to above Wo is required for the preparation of some of the others. Thus [W(C0)5(PPhs)]-EtAlC1~~7requires dioxygen for catalytic activity, [WClsl--EtAIC12-EtOH presumably contains tungsten in an oxidation state of at least +x11,10 and there is also the range of various tungsten halide-aluminium halide mixtures, and the majority of the heterogeneous catalysts derived from metal oxides on supports such as alumina. The catalyst for the dismutation of electron-rich olefins contains rhodium(1) or rhodium(r~x).~~~ In summary, although active catalytic species may be derived from Moo or WO (d6)in some systems, the majority probably have fewer d-electrons and higher oxidation states.(iii) The Transition State. The synchronous making and breaking of double bonds as apparently required by the experimental evidence is thermally forbidden according to the Woodward-Hoffmann rules.3 The transition-metal catalyst makes the reaction possible, and in the absence of such a catalyst the reaction occurs only at ca. 725 "C.2 It is therefore of great theoretical interest to identify the transition state. Before considering the three current models it is worthwhile making two reservations. The first is that no mechanism yet proposed offers any role to the metal alkyl (or comparable reagent) other than that of a reducing agent or of a modifier of the base behaviour of the transition metal. Two of the models, which may apply only to specific systems, do not require a second metal compound at all.In the general case, a polynuclear metal catalysis has not been considered although the formation of compounds with halogen or hydrogen bridges between, say, tungsten and aluminium is very probable. The second reservation is that apparently synchronous transitions may be step-wise. Thus, the thermally forbidden conversion of quadricyclene (11)into norbornadiene (12) is catalysed by [Rhz(C0)4C12]. The conversion involves oxidative addition of Rhrto the quadricyclene, which is ratedetermining, and the thermally forbidden Haines and Leigh reaction is thus made possible.137J38 The olefin metathesis reaction could be facilitated similarly. However, if we assume synchronous bond-making and bond-breaking, a possible transition state is (1 3), the ‘quasi-cyclobutane’ transition state.4 R’z R~,c=cR’~ M+ C 3E R22 It should be emphasized that the fact that synchronous transformations of this kind are forbidden by the Woodward-Hoffmann rules simply means that they have excessively high activation energies for normal thermal activation.How a transition-metal ion might facilitate such forbidden reactions is discussed in the next section. The cyclobutane transition state cannot represent the whole picture, because under heterogeneous conditions even-chain olefins can undergo reactions that produce odd-chain pr0ducts.13~ These products could arise by isomerization, e.g.by but-l-ene isomerizing to but-Zene, which then undergoes metathesis with more but-l-ene to produce propene. However, the conversion of ethylene into propene must involve the complete fission of an ethylene molecule. Diazomethane decomposes, apparently at the sites on a heterogeneous catalyst that also catalyse the metathesis reaction, to yield dinitrogen and ethylene, and the latter presum- ably arises by dimerization of carbene.139 Thus, carbene is implicated to some degree in metathesis reactions involving ethylene, and carbene complexes can generate metathesis catalysts. 100 The heat of formation of two molecules of ethylene is very similar to that of cyclobutane.One would therefore expect at least some cyclobutane to be produced from ethylene and a heterogeneous metathesis catalyst, but the con- version over a heterogeneous oxide-based catalyst is less than 0.1 %. Conversely, the yield of ethylene from cyclobutane over the same catalyst is only 3 %. These results suggest that cyclobutane is not involved in the transition state,114 though one can make the reservation that ‘quasi-cyclobutane’ is in a condition far removed from that of cyc10butane.l~~ For these reasons, an alternative mechanism involving the breaking of both cr-and n--components of the carbon-carbon double bond of the olefin has been proposed.l13J14 The transition state (14) is best described as four methylene fragments bonded to a single transition-metal ion, the carbon atoms remaining sp3 hybridized.Evidence from homogeneous systems supports this interpretation,l13 for example [(~T-CHQC~H~)W(C~)~] is a metathesis catalyst for non-4-ene in heptane H. Hogeveen and H. C. Volger, J. Amer. Chem. SOC.,1967,89,2486. L. Cassar and J. Halpern, Chem. Comm., 1970, 1082. 1s9 P. P. O’Neill and J. J. Rooney, J.C.S. Chem. Comm., 1972, 104. Olefin Metathesis and its Catalysis :'a g, II +M+ II C C Raa R4a at 98 "Cwhen the system is open, but not when carbon monoxide is kept in the system. In these latter circumstances isomerization occurs. A large excess of olefin also suppresses metathesis relative to isomerization. An excess of toluene inhibits both reactions of the olefin.These data are consistent with a species [(olefin)2W(CO)z] being involved. The conversion of (15) into (16) involves a change in the number of electrons in the valence shell of tungsten from 14 to 18, and (16) thus obeys the inert-gas rule. For a corresponding alkyne metathesis, the analogue of (15) should only possess 10 electrons, which seems rather unlikely, but compounds are known in which fCR groups are bonded to transition-metal ions, e.g. [M(CR)X(C0)4] (M = Cr, Mo, or W; R = Me or Ph; X = C1, Br, or I).l*O Although the evidence for the tetramethyl intermediate in these particular circumstances is convincing, the system may not be typical. Reaction tempera- tures are high (N 100 "C), whereas homogeneous metathesis catalysis usually occurs at ca.25 "C, and the reaction is inhibited by an excess of olefin. Gen- erally, very high o1efin:transition metal ratios are used (ca. 500:1), but in the tetramethylene system the proportions used are comparable. A third mechanism has been suggested, stimulated by the observations that [WCls]-alkyl-lithium is most active for metathesis with a tungsten :lithium ratio of 1:2 and that rhodium-catalysed carbocyclic ring rearrangements proceed via metal-carbon o-bonded intermediates.141 A conceivable pathway is that shown in Scheme 4. To establish the feasibility of this scheme, the reaction of [WclS] with 1,4-dilithiobutane in benzene was investigated, and it was found to yield ethylene quantitatively, presumably from the breakdown of an intermediate (19) [cf.(17) and (18) in Scheme 41.Experiments with deuteriated dilithiobutane, 1,4-dilithio-2,3-dideuteriobutane,produced ethylenes in the proportions ldoE. 0. Fischer, G. Kreis, C. G. Kreiter, J. Muller, G. Huttner, and H. Lorenz, Angew.Chem. Internat. Edn., 1973,12,564. 141 R. H. Grubbs and T. K. Brunck, J. Amer. Chem. SOC.,1972,94,2538. Haines and Leigh H,C-CHSA 7 1' HRC, M,CRHC CHR RH H*C CHR 11-M --I1 H*C CHR Scheme 4 CzHzDz:CzH3D:CzH4 = 6:88 :6. Hence rearrangements or methathesis definitely occur, and subsequent experiments proved that there is stereoselectivity. Cl, This work is only in its preliminary stages, and leaves several questions unanswered.There appears to be no reason why the transformation (17) -+(18) occurs, or why it should be rapid. A subsequent paper, which describes the synthesis and structure of the complex (20), proposes two possible mechanisms, but without any supporting evidence.142 The metallocycles of type (17) are not normally formed by ethylene,* but tetrafluoroethylene readily does so, and norbornadiene and [Ir(CO)Cl(PPh&] yield a stable complex containing a * Note added in proof: there is persuasive evidence that ethylene can form such a metallo-cycle in a titanium complex (J. X. McDermott and G. M. Whitesides, J. Amer. Chern. SOC.,1974,96, 947). 14*C. G. Biefeld, H. A. Eick, and R.H. Grubbs, Inorg. Chem., 1973, 12, 2166. 179 Olefin Metathesis and its Catalysis metallocycle, which, with acetylacetone, yields the compound (2l).I43 Thus the identity of the five-membered metallocycle with the metathesis transition-state (21) is open to doubt.Thirdly, it is not yet proven that metal-hydrogen bond forma- tion is not involved in the transformation (17) -+ (18), whereas experiments in other systems have excluded this route. Fourthly, the deuteriated ethylenes are not formed in the equilibrium proportions typical of the metathesis reaction. In addition, the conversion of ethylene into propylene and cyclopropane is not immediately explicable on this model. Finally, it does not explain why buta- diene should be a poison for metathesis, nor how alkyne metathesis might occur. However, the transition state does overcome the activation-energy ob- jections to synchronous mechanisms, as do the two other mechanisms so far discussed.The most recently proposed mechanism115 has already been mentioned, and is shown in Scheme 5. The carbene complexes (22) and (23) have been isolated, -CR~~=CR*,\(22) (u) Scheme 5 14a A. R. Fraser, P. H. Bird, S. A. Beman, J. R. Shapley, R. White, and J. A. Osborn, J. Amer. Chem. SOL, 1973, 95, 597. Haines and Leigh and the metathesis has been found to take place in boiling xylene, but only when :CR21 and :CR2a are derived from electron-rich olefins. Although the evidence for the mechanism proposed in this particular caseis compelling, it is unclear how far it can be extended to olefin metathesis reactions in general.B. Theories of Catalysis.-These will be discussed in order, from the simplest to the most complete. The basic problem can be grasped by considering the re- arrangement of molecular orbitals involved in the overall transition (Scheme 6). Scheme 6 If a cyclobutane intermediate is involved, then the transformation can be repre- sented as the sum of two individual steps, viz. Scheme 7 and its converse. Con- CR*~--CR~~ Scheme 7 sidering only the p-orbitals involved in double-bond formation in the two ethylenes, then the two diagrams for correlation of orbitals and states shown in Figure 1may be drawn. From either of these it is evident that the transformation I\ I-\ II + II 0 II + It 0 X Figure 1 of the ground state of two ethylene molecules into a ground-state cyclobutane in the absence of catalyst involves the transfer of electrons from one orbital to another, a process which requires a high activation energy so that, in Woodward Olefn Metathesis and its Catalysis and Hoffmann’s terms, the thermally activated process is symmetry-forbidden. The catalysis has been rationalized as follo~s.~44 The two olefin molecules form a complex with a transition metal, with symmetry properties as indicated in (26).The simple correlation diagrams of Figure 1 must then be redrawn to in- clude transformations of the metal orbitals, and particularly the d-orbitals. The orbi tal-correla t ion diagram is now as shown in Figure 2. A ground-state Complex Figure 2 reaction path from cyclobutane to the bis-(olelin) complex is now available, provided that the transition-metal ion has between 2 and 8 d-electrons, and this is consistent with experiment. In theory, too, a path exists for a non-trans-ition-metal catalyst with only 2 available electrons, in an orbital that is not a &orbital but of the appropriate symmetry, and such catalysts, e.g.potassium in graphite, are known.145 The tetramethylene model has received a similar treatment.ll4 In this case, the metal-tetramethylene complex has a set of molecular orbitals built up from sp3 carbon hybrid orbitals and the appropriate metal orbitals. The orbital-correlation lQ4The most recent discussion is by: F. D. Mango and J. H. Schachtschneider in ‘Transition Metals in Homogeneous Catalysis’, ed. G.N. Schrauzer, Marcel Dekker, New York, 1971, p. 223. See also F. D. Mango and J. H. Schachtschneider, J. Amer. Chern. SOC., 1967,89,2484. F. B. Carleton, personal communication. Haines and Leigh diagram provides a ground-state pathway, and the metal apparently contributes 6 electrons to the bonding scheme. It will be recalled that the experimental data from which the tetramethylene model was developed were derived from a system containing tungsten(o) (d9.114 The function of the metal in a quasi-cyclobutane intermediate has been described144 as follows, The metal injects an initially non-bonding pair of d-electrons into a carbon-carbon antibonding orbital combination, such that bonding between C-1 and C-2 (and C-3 and C-4) is weakened [see (27)], but, as a consequence of the nodal pattern of this combination, bonding between C-2 and C-3 (and C-4 and C-1) is strengthened.At the same time, the metal with- draws an electron pair from an orbital bonding between C-1 and C-2 (and C-3 and C-4). Considerations of this kind suggest that a d2 ion should be a better catalyst than ions with more d-electrons. The metallocycle model has also been discussed in these terms.144 Three paths for the rearrangement have been suggested, the asterisks being used in Figure 3 to denote the identities of the individual carbon atoms. * Y (b$ **= M *-C%* /\ M 6, ,cM Figure 3 Pathway (a) involves reversible insertion of the metal into a cyclobutane ring, maintaining the cyclobutane-metal bond at all times, since cyclobutane is never Olefin Metathesis and its Catalysis evolved.This is not considered likely.* Pathway (6) is similarly not likely because it implies metal-carbon interactions across the ring, and these cannot be strong if the ring is planar. However, in (28) the ring is far from ~1anar.l~~ Pathway (c) apparently suffers from further symmetry restrictions, which a d2 metal could not overcome because it would become do in the metallocycle. Since a tetra- methylene intermediate is considered improbable on activation-energy grounds, then all these theoretical discussions suggest the quasi-cyclobutane model as the only reasonable one? This kind of approach has been criticized on the grounds that orbital-correla- tion diagrams are not adequate, and that state-correlation diagrams should be ~sed.1~6It is claimed that the ‘allowed-forbidden’ problem is quite distinct from another, which is that the activation energy is normally too high in reactions such as the metathesis reaction, for steric reasons.The lowering of the activation energy by transition-metal catalysis for certain ‘forbidden’ reactions may be as great as 15 kcal mol-l for some d1Oand d8systems (e.g.AgI and RhI). A proper consideration of the state-correlation diagram for the olefin metathesis reaction shows that all the states of the bis(o1efin) complex and of the cyclobutane-metal intermediate are completely symmetrical with respect to reflection in the appro- priate planes, but that the ground states of the two species do not correlate.This is, however. only true if the metal ion formally has an even number of electrons, and this may not occur if odd-electron ligands such as NO are also present. The role of the metal ion is to provide excited states on the ion which are of lower energy than those excited states of the organic molecules which would have to be occupied if the non-catalysed transformation were to occur.146 However, in the example selected, all the states of the organic moiety-metal ion complex are of the same symmetry because of the presence of the metal ion, so that one might equally argue that catalysis occurs because forbidden tran- sitions become allowed. In any case, it is not clear what ‘forbidden’ means, unless it is that the activation energy is very high.The most sophisticated analysis of the cyclobutane system develops the argu- ments concerning state-correlation diagrams one stage further.147 The additional consideration introduced is that the relative energies of the metal ion d-orbitals * Note added in proof: an analogous step for alkyne metathesis has been suggested on the basis of reaction products obtained from [Fe(CO),] and diynes (H. B. Chin and R. Bau, J. Amer. Chem. SOC.,1973, 95, 5068). 146 W. T. A. M. van der Lugt, Tetrahedron Letters, 1970, 2281. G. L. Caldow and R. A. Macgregor, J. Chem. SOC.(A), 1971, 1654. Haines and Leigh will change during the course of the reaction. Consider, for example, the usual bis-olefin to cyclobutane conversion (Scheme S), with the transition metal Scheme 8 initially square-planar.It is evident that the act of removing electrons from the olefinic bonds and placing electrons between the initially unbound carbon atoms means for the metal an isomerization from square-planar to tetrahedral. When this is included in the correlation argument, it becomes apparent147 that a square-planar-tetrahedral isomerization is unlikely to be involved in the meta- thesis catalysis. For an axial-equatorial trigonal-bipyramidal exchange (Scheme 9), however, catalysis does seem more likely, with low-spin d6 metal ions, and possibly with dl and d2. Scheme 9 It must be emphasized that none of these theoretical discussions is valid if the mechanisms postulated are eventually found to be inapplicable.The recent demonstration148 that ethylene and butadiene (a poison for metathesis catalysts) can form considerable quantities of vinylcyclobutane under the influence of a catalyst such as tetrabenzyltitanium, which in the transition state may not possess any d-electrons at all, suggests that there may be catalysts without transition- metal ions. It is not clear whether this system also promotes metathesis (the products of such metatheses are the same as the starting materials), but possibly unsymmetrical olefins may break orbital degeneracies and promote new reaction paths. The scope for future research seems very large.C. A Proposed Mechanism.-It is not proven whether there is a mechanism common to all the metathesis reactions discussed. In fact, they fall into two natural divisions, those which operate at room temperature and below (most of the homogeneous systems) and those which operate at about 100 "C and above 148 L.G. Cannell, J. Amer. Chern. Soc., 1972, 94, 6867. Olefin Metathesis and its Catalysis (most of the heterogeneous systems, the tetramethylene system, and the ‘electron- rich-olefin’ system). All the reactions in the second group are those in which there is evidence for participation of CR2 fragments (whether termed ‘carbene’ or ‘methylene’). The fact that heterogeneous metathesis of propylene and the decomposition of diazomethane to yield ethylene and dinitrogen are very fast over the same catalyst may not be relevant.However, although there is good evidence that the catalysts can split the carbon-carbon double bond, the CR2 fragments retain their integrity, and there is no evidence for hydrogen transfer to metal. The production of cyclopropane from ethylene could occur as shown in Scheme 10. However, only one carbene (methylene) fragment is necessary, so that a or Scheme 10 process involving three carbon atoms (Scheme 11) is also feasible. Various versions of this can be sketched. Under the reaction conditions, cyclopropane will almost certainly rearrange to give propylene, and a further possibility is the (29) Scheme 11 insertion of a carbene fragment into cyclopropane to give methylcyclopropane, and thence various butenes.Such reactions are, however, generally of minor significance, but the intermediate (29) is adequate to explain the whole metathesis reaction, any stereoselectivity being ascribed to the steric characteristics of the catalysts, as shown in Scheme 12. Scheme 12 Haines and Leigh A variation of this has already been pr~posed,l~~J~~ in which a metaIIocycle (30) is envisaged as the transition state. It is not possible to make a definitive judgement. (30) recalls the four-carbon metallocycles (17) and (18), but in that system metathesis has not been detected with certainty, although hydrogen scrambling has been demonstrated.124 However, neither (29) nor (30) presents any conceptual difficulty for the metathetical reactions. We prefer not to commit ourselves, and shall refer to (29) or (30) as the trimethylene model.Whether this can be extended to homogeneous systems is open to question. Generally the activation energies for homogeneous systems are low, and it seems doubtful whether the activation energies for metathesis via metallocycle (30) with normal olefins or via a tetramethylene complex are small enough. It may be, however, that one function of the EtAlC12 is to generate a carbene during breakdown of an aluminium-tungsten alkyl or of a tungsten alkyl, and aluminium alkyls are known to react with some transition-metal complexes to form carbene complexes.149 Additionally, whereas oxidative addition to RhI (to give RhnI) is eminently reasonable for the route via (30), the corresponding transformation in the tungsten-catalysed homogeneous reaction (possible WI1-WN) seems less likely.We have assumed that alkyne metathesis follows a path analogous to that of olefin metathesis. If this is so, the cyclobutane model seems most appropriate. The formation of complexed cyclobutadienes from acetylenes is relatively common,l50 and these, upon thermal decomposition, would undoubtedly give the impression of alkyne metathesis. The analogue of the tetramethylene model (tetramethyne) does not seem feasible. If the effective atomic number rule is to be obeyed, then the critical transformation must be that between (31) and (32), so that M must be effectively free of any other ligands if it is to be a Group VI metal, and this is improbable.A similar objection holds to the trimethyne variant, and although metallocycles such as (33; R = H) are ~XIOW~,~~~their chemistry is virtually unexplored. RC M'CR=fR~R-CR W. Petz, J. Organometallic Chern., 1973, 55, C42. lKoSee, for example, P. M. Maitlis, Adv. Organometallic Chem., 1966, 4, 95. ls1 See, for example, R. P. Dodge and V. Schomaker, J. Organometallic Chem., 1965, 3, 274. 187 Olefin Metathesis and its Catalysis Internal acetylenes C2Rz react with MoCh or WBr5 to yield complexes (X =~(C~RZ)] C1, M = Mo; X = Br, M = W;R = alkyl or aryl), which react with organic nitriles to yield the well-characterized [M&(nitrile)2]. The acetylene complexes are believed to contain metallocycles (33), but their thermal decomposition does not lead to metathesis products.153 5 Conclusions Our understanding of the olefin metathesis reaction is in its infancy.There is obviously a close relationship between catalysts which promote this reaction and those which aid transalkylation of arornatic~~~~J53 and Ziegler-Natta p0lymerization.16~An understanding of how these catalysts function and why minor constitutional changes endow them with such different capabilities will go far towards an important goal of organometallic chemistry, namely the design of specific catalysts for specific reactions. 16’ A. Greco, F. Pirinoli, and G. Dall’Asta, J. Organometallic Chem., 1973, 60, 115. lSa L. Hocks. A. J. Hubert,and Y.Teyssie, Tetrahedron Letters, 1972, 3687. lS4 0.Henrici-Olive and S. Olive, Angew. Chem. Internat. Edn., 1967, 6, 790; see also ref. 25.

ISSN:0306-0012    

DOI:10.1039/CS9750400155

出版商:RSC    

年代:1975

数据来源: RSC