摘要:

DISCUSSIONS OF THE FARADAY SOCIETYN 0 . 4 7 1969Bonding inMetallo-organic CompoundsTHE FARADAY SOCIETYLONDONDistribution arrangements overleaTHE SOCIETY’S PUBLICATIONSTransactions of the Faraday SocietyDiscussions of the Faraday SocietySymposia of the Faraday SocietyPublished monthlyNormally published twice a yearPublished annuallyMEMBERSof the Faraday Society receive current issues of bothTransactions and Discussions free on publication, and ofSymposia at a reduced price on request. Enquiriesregarding membership of the Society should be addressed to:The Secretary, The Faraday Society,6 Gray’s Inn Square, London WC1 (Telephone: 01-242 8101)NON-MEMBERSmay obtain the Society’s publicationseither through their own bookselleror by making application as follows:Annual Subscriptions:Back Issues:Discussionsand Symposia:FORto current issues of EITHER Transactions, Discussiorzsand SymposiumOR Transactions onlyComplete Volumes (comprising Transactions, Discussions andSymposium)from Vol.41 (1945) onwardsThe Aberdeen University Press LtdFarmers Hall, Aberdeen, Scotland, AB9 2XTAPPLY TOAND FORAll current issues and back numbersBack Issues: Complete Volumes (comprising Transactions and Discussions)from Vol. 1 (1905) to Vol. 40 (1944)Butterworth dk Co. (Publishers) Ltd, 88 Kingsway, London WC2APPLY TOOVERSEAS ADDRESSESAustralia: Butterworth & Co. (Australia) LtdSydney: 20 Loftus StreetMelbourne: 343 Little Collins StreetBrisbane: 240 Queen StreetToronto: 14 Curity Avenue, 314New Zealand: Butterworth & Co.(New Zealand) Ltd.Wellington: 4915 1 Ballance StreetAuckland: 35 High StreetSouth Africa: Butterworth & Co. (South Africa) Ltd.Durbun: 33/35 Beach Grove Canada: Butterworth & Co. (Canada) LtdU.S.A.: Discussions and Symposia-current and back issues :Enquiries to 88 Kingsway, London. WC2.Complete Volumes 140:Johnson Reprint CorporationNew York: 1 1 1 Fifth AvenueA GENERAL DISCUSSIONONBonding inMetallo-orgunic Compounds25th-27th March, 1969A GENERAL DISCUSSION on Bonding in Metallo-organic Compounds was heldat the University of Cambridge on the 25th, 26th and 27th March 1969. ThePresident, Prof. G. Gee, C.B.E., F.R.S. was in the Chair and 110 members andothers were present.Among the overseas visitors were:Dr. and Mrs. J. H. AmmeterDr. H. Arzoumanian FranceDr. P. Baekelmans BelgiumProf. T. L. Brown U.S.A.Prof. F. A. Cotton U.S.A.Prof. L. F. Dahl U S A .Dr. W. K. Glass EireProf. J. A. Ibers U.S.A.Dr. J. M. Larnaudie FranceProf. P. M. Maitlis CanadaDr. A. R. Manning EireDr. A. May ItalyDr. J. Messier FranceDr. K. W. Muir U S A .Dr. J. F. Ogilvie CanadaProf. R. Pettit U.S.A.Dr. R. Prins NetherlandsSr. R. M. Rawlinson EireDr. G. L. D. Ritchie AustraliaMr. A. Van der Ent NetherZandsProf. L. L. Van Reijen NetherlandsDr. P. B. Venuto U.S.A.Dr. J. L. Verbeek NetherlandsProf. W. Zeil GermanySW'ifzerlan0 The Faraday Society and Contributors 1969Printed in Great Britain at the University Press, AberdeeCONTENTSPage 720273748535971798493107112121126136144General Introduction-From Ferrocene to Uranoceneby G.A. GamlenThe Geometries of and Bonding in Certain Transition Metal Complexesby R. McWeeny, R. Mason and A. D. C. Tow1Approximate Molecular Orb it a1 Calculations on Met allo- organ ic Complexesby 1. H. Hillier and Mrs. R. M. CanadineA SimpliFed Molecular Orbital Model *fop. Octahedral Metal CarbonylCompounds by Wayne P. Anderson and Theodore L. BrownCorrelation between Crystal Structure and Carbonyl-Bond StretchingVibrations of Methyl Benzene Transition Metal Tricarbonylsby H. J. Buttery, G. Keeling, S. F. A. Kettle, I. Paul and P. J. StamperInteractions of Carbonyl Groups in Compounds Containing Metal-MetalBonds by M.L. N. Reddy and D. S. UrchGENERAL DIscussIoN-Prof. J. N. Murrell, Prof. R. Mason, Mr. A. F.Orchard, Dr. 1. H. Hillier, Mrs. R. M. Canadine, Prof. J. N. Murrell,Dr. D. S. Urch, Dr. J. R. Miller, Dr. G. Bor, Prof. N. N. Greenwood,Dr. A. J. RestExtension of the Woodward-Hoflman Rules to Organometallic Systemsby R. Pettit, H. Sugahara, J. Wristers and W. MerkSome Bonding Questions Prompted by Studies of the Fluxional MoleculeTriscyclopentadienylnitrosylmolybdenum by F. A. CottonGeometries of Transition Metcll Complexes Containing Simple Alkenesby Ljubica ManojloviC-Muir, Kenneth W. Muir and James A. IbersOrganometallic Chalcogen Complexes. Part 18.-A Diluted Single CrystalE.S. R. Study of the Electronic Structure of Tricobalt EnneacarbonylSulphide: Antiaromaticity in a Transition Metal Carbonyl Cluster System.by Charles E.Strouse and Lawrence F. DahlGENERAL DIscussIoN-Mr. A. Hamnet, Dr. D. S. Urch, Dr. R. Pettit, Mr.C. H. Campbell, Prof. F. A. Cotton, Mr. K. F. Wagstaff, Dr. H. A. 0.Hill, Prof. J. A. Ibers, Dr. Michael GreenStudy of the Bonding in Pentacarbonylmariganese Deriuatiues by PhotoelectronSpectroscopy by S. Evans, J. C. Green, M. L. H. Green, A. F. Orchard andD. W. TurnerPhotoelectron and Ultra-violet Spectroscopy of Transition Metal CarbonylDerivatives by P. S. Braterman and A. P. WalkerMossbauer Spectra, Structure, and Bonding in Iron Carbonyl Derivativesby R. Greatrex and N. N. GreenwoodBonding and Structure in Fe(II) Low-spin Compounds using the MossbauerE’ect by G.M. Bancroft, M. J. Mays and B. E. PraterGENERAL DIscussIoN-Mr. A. F. Orchard, Dr. M. G. Clark, Dr. M. J.6 CONTENTS149157165172178183190199205Mays, Prof. N. N. Greenwood, Dr. G. M. Bancroft, Dr. A. J. Rest, Dr.H. A. 0. HillStructure of Molecules of Type (CH,),X-C=C-Y (X = Si, Ge, Y = H,Cl) determined by Electron Diffractionby Werner Zeil, Joachim Haase and Marwan DakkouriElectron Impact Studies on Organo-Beryllium and -Aluminium Compoundsby D. B. Chambers, G. E. Coates and F. GlocklingSome Properties of Metal Complexes Containing One Metal-Carbon Bondby H. A. 0. Hill, J. M. Pratt and R. J. P. WilliamsFormation of Cobalt-Carbon a-Bondsby Michael Green, J. Smith and P. A. TaskerEvidence for a n-Donor Eflect in Transition Metal-Carbon Bonds from H-DCoupling Constantsby J. D. Duncan, J. C. Green, M. L. H. Green and K. A. McLauchlanBonding in Hexamethyl-dialuminium and Related Compoundsby K. A. Levison and P. G. PerkinsComplexes of Carbon Suboxide with Silver: a new Chemistry of SiIuerby E. T. Blues and D. Bryce-SmithGENERAL DrscussIoN-Prof. N. N. Greenwood, Dr. I. H. Hillier, Prof.P. G. Perkins, Prof. Theodore L. Brown, Dr. H. A. 0. Hill, Dr. M. G.Clark, Dr. Michael Green, Dr. J. F. Ogilvie, Dr. M. IT. Newlands, Prof. D.Bryce-Smit hAUTHOR INDE

ISSN:0366-9033    

DOI:10.1039/DF9694700001

出版商:RSC    

年代:1969

数据来源: RSC

摘要:

GENERAL INTRODUCTIONFrom Ferrocene to UranoceneBY G. A. GAMLENImperial Chemical Industries Ltd., Petrochemical and Polymer Laboratory,P.O. Box 11, The Heath, Runcorn, CheshireReceived 28 th April, 1969It is characteristic of Faraday Society Discussions that they are held at opportunemoments in the development of the subject. This one is no exception becausemetallo-organic chemistry has reached that particular stage where considerablesimplification is possible through the replacement of facts by concepts. Thus wemay contemplate a return to the position in which chemistry can be regarded asa unified whole with metallo-organic chemistry occupying its place as the bridgebetween organic and inorganic chemistry. This will only be realized, however, ifour theory of bonding is sufficiently good to account for the situations found at theextremes of organic and inorganic chemistry and cast in a sufficiently simple formfor it to be readily comprehensible and profitable to use.There is no doubt that molecular orbital theory can be refined to include allinteractions between electrons and nuclei but unfortunately such calculations requirethe services of a large‘ computer and cannot be considered as handy for the averagechemist.The necessity of this second requirement for success was recognized someyears ago by a number of theoretical chemists and especially by 0rgel.l For thisreason, ligand field theory was devised in which the considerable simplification thatwas made depended upon some severe approximations regarding the contributionof the ligand to the bond.This theory has been of the greatest help in increasing our understanding of thebonding in complex compounds, particularly of those properties which are associatedmainly with the metal ion.With increased understanding, however, there came alsoa fuller realization of the limitations imposed by the approximations used especiallyas regards the state of the ligand once the bond to the metal had been formed. Avaluable task for the theoretical chemists is therefore to look for closer approximationsto molecular orbital theory without sacrificing the utility of ligand field theory. Asexamples of this approach we have the simplified model for octahedral metal carbonylcompounds proposed by Anderson and Brown2 which nevertheless gives a goodrelationship between force constants and ligand characteristics, and Hillier and Mrs.Canadine’s paper using the SCCEI method.The results in this case can now betested by new experimental techniques such as photo-electron and n.m.r. spectro-scopy, which will considerably increase their value.The present-day preoccupation with the role of the ligand springs in part fromthe unusual organic reactions which can be catalyzed by metallo-organic compoundsand which are of increasing industrial interest. To deduce the mechanism of thesecatalytic reactions requires a much deeper understanding of the nature of the metal-ligand bond than we have at present and is hampered by the inadequate and out-mode8 FROM FERROCENE TO URANOCENEformalism with which we try to convey our ideas.For example, the arrow used torepresent the donation of n-electrons from an olefin to a metal,R R\ /CCI1 -,MR/ \Ralthough describing how the bond is formed gives no indication of the nature of thelinks (especially in the olefin) once it has formed.It is clear that there is room for improvement of various terminological aspects ;a number have already been suggested and of these Cotton's hapto nomenclatureparticularly deserves mention as it would remove, inter alia, the confusion surroundingthe symbol 71. This is used indiscriminately to describe complexes in which thebonding is at least partly by non-linear orbital overlap as well as complexes in whichthe ligand donates n-electrons to the metal in a c-bond.Perhaps it is time for aresponsible body such as this Society to suggest to the International Union of Pureand Applied Chemistry that a commission should be set up for a general reform ofmetallo-organic nomenclature-r should it be organo-metallic ?In addition to the many other reasons why this meeting is opportune there is,for example, the realization that this is a huge area of chemistry which potentiallynumbers as many compounds in its fold as does organic chemistry. It also comes ata time of rapid expansion in the number of physical techniques available to study thenature of the bond and when the existing techniques are being so improved that theyallow much more stringent tests to be applied to theoretical explanations.Some of these techniques unlock physical properties which were hitherto virtuallyinaccessible ; for example, the electron density in different orbitals around the nucleusof certain elements can be deduced from Mossbauer effect spectra. These have beenused by Greatrex and Greenwood to make structural deductions about a largenumber of substituted iron carbonyls and by Bancroft, Mays and Baxter tocorrelate, among other things, the partial centre shift values of various ligands totheir bonding characteristics.This is a development which is sure to be followedclosely by all chemists interested in bonding.Another powerful technique of great potential which is quite new to the organo-metallic field is photo-electron spectroscopy.' This reproduces directly the electroniclevel structure from the innermost shells to the atomic surface.It is a characteristicfeature of this branch of spectroscopy that chemical structure effects cause shifts ofthe inner levels and thus provide information on chemical bonding. The spectrashown below (fig. 1) illustrate this for platinum and potassium chloroplatinate whileEvans, Green, Green, Orchard and Turner and Braterman and Walker havediscussed the bonding in pentacarbonyl manganese derivatives.It is not possible in a short introduction to discuss all these exciting new techniquesindividually but perhaps one typical example may be quoted : the existence of a cis-effect has been suspected for many years in square planar palladium and platinumcomplexes.Fryer and Smith lo have recently demonstrated its reality in trans-phosphine palladium complexes by estimating the charge on the chlorine atoms fromthe nuclear quadrupole resonance spectra. It is clearly significant that the order ofincreasing cis-effect of the ligands is the reverse of their trans-effectG . A . GAMLEN 9These advances in physical techniques are particularly welcome at the presenttime because they provide an insight into quite complex electronic interactions ofligands which may arise either through the central metal atom or by a field effect.A theoretical treatment, to be satisfactory, therefore has to consider the molecule asa whole ; yet metallo-organic molecules have becomesteadily more and more complex as time goes byand provided bonding problems of increasing com-is one of many possible examples to illustrate thiscomplexity and has been chosen because in additionto well-recognized forms of bonding it also has ametal atom [Fe(B)] which is acting as both a donorand accept0r.lThe polynuclear " cluster " compounds, whichmay show multiple bonding of a high order,12 alsoexemplify this requirement to treat the molecule asa whole and raise the question of whether metalatoms joined together in a cyclic system bear asimilar relationship to their individual parts as, forexample, benzene does to six individual carbonatoms.The compound Rh,(C0)16 which wasoriginally discovered in 1943 and whose true naturewas not revealed for twenty years when its structurewas determined by Corey, Dahl, and Beck l3 illus-trates a number of such points, (fig.3). As wellas being the first example of a hexanuclear metalcarbonyl it was also the first polynuclear metalcarbonyl recognized to contain a carbonyl groupbridging three metal atoms. Significantly, it is alsoan exception to the inert gas rule in that the entiremolecule contains two electrons in excess of the'' xenon '' configuration for each rhodium atomplexity. The compound Fe2(C0)6C6H~02, (fig. 2)KINETIC ENERGY - I I 1CV 80 75 70RINnlNC. FNFRGYFIG. 1 .-Electron spectra from metal-lic Pt and KzPtCls showing shifts ofthe platinum NVI and NVII levels.and it has been suggested that this electron pair is strongly delocalized with low-lyingexcited states.I OH c9FIG.2.-Structure of Fe2(C0)6(C6H802)10 FROM FERROCENE TO URANOCENESimilar bonding problems arise in the unusual compounds containing a singlecarbon atom as ligand such as Fe,(CO),,C, in which the carbon atom is equidistantfrom the five iron atoms and therefore penta~o-ordinated,~~ (fig. 4), and the rutheniumarene complexes such as R U ~ C ( C O ) ~ , ( C H ~ ) ~ C ~ H ~ in which the carbon atom occupiesa6FIG. 3.-Structure of Rh6(CO)16.the centre of the octahedron of the metal atoms, (fig. 5). This compound, which wasdescribed by Johnson, Johnston and Lewis l5 and whose structure was determinedby Mason and Robinson,16 illustrates again the unexpected novelty of many of themetallo-organic compounds discovered in the past few years.Organo-metallic compounds are fascinating not only in their structural " many-sidedness " and complexity but also in the instability arising therefrom.Just astheory over the past ten years has had to be refined to provide a more dynamicelectronic description of the metal-carbon bond, so it now has to cope with a muchmore dynamic situation at the atomic level if we are to understand the behaviourof fluxional molecules l7 and n-ally1 complexes. This then is the background againstwhich we can trace the development of theory.(who also conceived theuseful rule-of-thumb " inert gas rule '') and then note that valence bond theory l9We may start from the lone pair concept of SidgwicG. A . GAMLEN 11represented a very real advance because it drew attention to the importance of d-electrons in the bonding of transition metal complexes ; by concentrating on bondingelectrons it satisfactorily explained the stereochemical problems then facing chemists.Its shortcomings were demonstrated, however, when improved methods of magneticThe Coordinate Link (Sidgwick 1927)Crystal F i e l d Theory(Bethe 1929, Van VleckValence Bond Theory(Pauling, 1935)Bonding e l e c t r o n s ,stereochemistry. Non-bonding e l e c t r o n s ,s p e c t r a , magnetism.Molecular Orbital Theorya-complexes of 2 /\ block n- complexes(Chatt and Shaw, \ 1959) (Dswar, Chatt, 1951)\/’\ ,\ , I\ /Metal Flho<oalkyls(Cotton, 1964)\FIG.612 FROM FERROCENE TO URANOCENEsusceptibility determination became available.Further, it was quite unable tocontribute an explanation for electronic spectra, although both the magnetism andelectronic spectra could be interpreted in terms of crystal field theory as developedby Van Vleck,20 Schlapp and Penny 21 and Jordahl,22 and in which the emphasiswas placed on the non-bonding electrons. These essentially electrostatic approaches,however, did not deal with the electron delocalization known to occur, as was shownby the work of Owen and Stevens 23 on the e.s.r. spectrum of ammonium hexachloro-iridate(1V). However, as early as 1935, Van Vleck 24 had pointed out that, if partlycovalent bonding was assumed, both m.0. theory and crystal field theory predictedthe same result for the splitting of metal d-levels into two subshells.It was thereforepossible to reconcile these approaches and various quantitative treatments havebeen proposed starting with the first general treatment of octahedral complexes byTanabe and Sugano 2 5 in 1954. Since then the molecular orbital theory contributionsby Cotton,26 Gray, Ballhausen 27 and Jarrgensen 28 have been particularly important.Some of the main steps in the historical development of the theoretical side areillustrated in fig. 6.While theoretical organo-metallic thinking has evolved in this way, an immensegrowth of synthetic techniques and materials has occurred. A major feature on theexperimental side has been the increase in the number of different kinds of moleculeable to act as ligand, including such unrelated entities as carbenes, carbollides andeven nitrogen itself.To reduce this richness and variety of ligand to manageable proportions it hasbeen necessary to set up a system for classifying ligands according to type.Onesuch scheme is shown in the following table.29TYPES OF LIGANDLII I(1) with one'or morefree electron pairs(2) no free electron pairs butwith n-bonding electrons,e.g., ethylene, cyclo-pentadienyl ion.I(c) with additional 71-I I(a) no vacant orbitals toreceive electrons from orbitals that can receivethe metal, e.g., HzO, n-electrons from the metalNH3 e.g., PR3, CN- orbitals, e.g.,OH-, C1-.The hydrocarbyl ligards found in Class 2 may then be sub-classified again accord-(b) vacant or vacatableelectrons that can bedonated to vacant metaling to the number of electrons they donate to the metal.TABLE 1.-A CLASSIFICATION OF ORGANIC GROUPS WHICH ACT AS LIGANDS TO TRANSITIONMETALS 30no.of electrons name of class examples of organic group1 Yl akyl or aryl groups2 alkene ethylene3 enyl n-all y 14 diene cyclobutadiene, butadiene5 dienyl n-cyclopentadienyl6 triene benzene7 trienyl n-cycloheptatrienyG . A . GAMLEN 13This table runs up as far as seven electrons donated but could now be extended toten for the di-anion of cyclo-octatetraene which is the ligand in the newly discovered~ r a n o c e n e , ~ ~ a sandwich-type compound like ferrocene.r 1Of the various ligand categories in this classification I propose to dwell briefly onlyon the first and second because they form a rough but useful basis for understandingthe multi-electron systems involving delocalization over a number of carbon atoms.Thus in very simple terms and using valence bond language, one can consider then-ally1 group as a resonance hybrid formed from a combination of a one- and two-electron donor system.Similarly, the cyclopentadienyl ligand is, in these terms acombination of an ally1 with an olefin donor.ONE ELECTRON DONORSStable alkyls of the p-block elements have been known for over 100 years and wereinvestigated so extensively by Frankland in the latter part of the 19th century thathe came to be known as the “ father ” of organometallic chemistry. They aretheoretically derived from the hydrocarbon by replacing an atom of hydrogen by itsequivalent of metal.For the most part, the bonding in these compounds fitted inwith current organic theory and presented no problems ; there is, however, the notableexception from the theoretical standpoint of the “ electron deficient compounds ”which can be exemplified by the dimer of aluminium t r i m e t h ~ l . ~ ~ This has beenshown by an X-ray study to have a bridged structure like aluminium(fig. 7). Each bridging methyl is considered to be held symmetrically to the twow,[CH4FIG. 7.4tructure of A12(CH&. Methyl distances are C-C distances (H atoms have been omittedfor clarity).aluminium atoms by only two electrons which are accommodated in three-centreorbitals formed by the overlap of the A1 sp3 and C sp3 orbitals.It is thereforeappropriate that among the papers to be presented at this meeting is one on bondin14 FROM FERROCENE TO URANOCENEin this and related compounds by Levison and perk in^,^^ which suggests the presenceof a rather stronger metal-metal bond than has hitherto been postulated.It was thought for a long time that the d-block elements were unable to formstable alkyls or aryls ; the discovery that ligands of high ligand-field strength such asFro. 8.the cyclopentadienyl anion, carbon monoxide and tertiary phosphines, stabilize bothtransition metal hydrides and alkyls marked a major step out of this confined thinkingand opened up new possibilities.A notable example of an aryl complex is the ruthenium phosphine complex,Ru(naphthalene)(P-P)2, where P-P = (CH3)2PCH2CH2P(CH3)2 discovered byChatt and D a ~ i d s o n .~ ~ Examination by infra-red and n.m.r. has shown that thecompound in solution is an equilibrium mixture of the arene complex and the 0-arylhydrido complex. An X-ray structure determination which has just been finishedby Kilbourn and co-w~rkers,~~ confirms that in the crystal the compound exists asa ruthenium aryl, (fig. 3).It is obviously important to explain how this stability comes about : Chatt andShaw 37 have advanced the following qualitative suggestion. Any fission of the metal-carbon bond is likely to be irreversible because all organo-metallic compounds arethermodynamically unstable. Such a homolytic fission will occur either by thepromotion of an electron into the anti-bonding orbital of the M-C bond or thepromotion of one electron from the M-C bonding orbital.It then follows that theenergy barrier to dissociation can be related to the energy difference between eitherthe M-C bonding orbital and the lower unfilled orbital or the M-C anti-bondingorbital and the highest filIed orbital, whichever is less. A similar explanation hasbeen put forward to explain in part the exceptionally stable ortho-substituted arylcomplexes.Among other stabilizing systems for these one-electron donor complexes arechelating nitrogen donors, such as dimethylglyoxime and especially the corrin ring ;the stable cobalt-carbon c-bond has the distinction of being the only one known tooccur in Nature, and Green, Smith and Tasker 30 discuss the requirements for stabilityof such a bond in terms of the energies of the 4 2 and d , , orbitals.The propertiesof this type of bond are also discussed by Hill, Pratt and Williams 39 who find thatsuch strong trans and cis-influences are present that the co-ordination number ofthe complex changes with change in donor strength of the ligand; more especiallyG . A . GAMLEN 15they find the alkyl ligand to be a very strong donor. Finally in this section thecuriously high thermal stability of the fluoro alkyl complexes deserves mention.TABLE~.-THE THERMAL STABILITY OF SOME FLUOROCARBON COMPLEXES COMPARED W HTHEIR HYDROCARBON ANALOGUESCZFSMn(CO)S stable up to 150" C2HSMn(C0)4 dec. 25".(C6F5)2Zr(n-CSH5)2 dec.218 (c 6H 5 ) 2Zr(n'C5H5)2 not knowntran~-(C~F~)~Ni[P(c~'H~)~]~ not dec. at 21 3" ~ ~ ~ ~ S - ( C ~ H ~ ) ~ N ~ [ P ( C ~ & ) ~ ] ~ dec. 125"There has been little detailed structural work on these classes of compoundsso far. However, initial measurements on bond length imply that the metal-carbonbonds are very similar in the hydrogen and fluorine complexes. Cotton has notedthat the carbon-fluorine stretching frequency appears to be lower than expected inthe metal complexes and has interpreted this in terms of a n-bonding mechanism.It is suggested the fluorine atoms would lower the energy of the a*-orbital of themetal-alkyl system, enough to permit effective back donation of d-electrons from themetal to carbon.40TWO-ELECTRON DONORSIn turning to the olefin complexes which are formed in such profusion by thed-block elements, we remember that Zeise's so indispensable to all reviewersof organo-metallic chemistry, was discovered only a few years after the platinumammines but fell outside existing theories and so remained anomalous and ignored.The description of the metal-alkene bond as a n-complex involving p-bondingwas established by Dewar 42 for silver-olefin complexes and Chatt and Duncanson 43for platinum-olefin complexes leading to what may now be termed the '' conventional "picture of this bond, the main features of which are shown in fig.9.The usefulness of this explanation is apparent from the fact that it has givensatisfactory service over the past 15 years and is only now being seriously re-considered.As was mentioned earlier, the problem today concerns the nature of the olefin onceit has complexed to the metal.In particular, the formation of the metal-olefin bondFIG. 9.4rbitals used in the combination of ethylene with platinum(II).destroys the n-symmetry of the olefin orbitals and removes the 0-71 inequivalence;mixing of the O- and n-components originally present in the olefin then allows somechange of hybridization from sp2 and sp3 so that the carbon atoms take on somealkyl-type character which is reflected in the bond length and angles of the boun16 FROM FERROCENE TO URANOCENEolefin. However, these changes are not great and it has required the recent improve-ments in accuracy and speed which automatic diffractometry brings to structuredetermination by X-rays before some of these rather small changes in a series ofrelated complexes could be measured with sufficient precision.An investigation ofC'3FIG. 10.-Structure of Zeise's salt.this calibre is described by Ibers44 and co-workers for simple alkene complexes.As they have pointed out, it is remarkable that so few results have been recorded forthis important class of compounds, and for this reason I make no apology in showingyou some unpublished results of Owston and co-workers 45 on a new structuraldetermination of Zeise's salt, (fig. lo).*B BFIG. I1 .-Crystal structure of Zeise's salt, K[C2H4PtCI3] . H20. 0, Pt ; 0, C1; 0, 0; 0, C ; 0, K.The two features of this particular determination are the carbon-carbon distancewhich at 1-37 A is barely significantly different from the distance in the free ligand.The second feature is that although one would expect the platinum-chlorine distancetrans- to the ethylene to be longer than either of the two cis-platinum-chlorine distances*A more recent and closer analysis has led to some changes in the values of the bond lengthsshown in the original diagram.The C-C bond distance is now 1.37 A; Pt-Cl (cis) 1.30 and 1.31while Pt-Cl (trans) is 1.33 A. These changes do not affect the general conclusions of the lectureG. A . GAMLEN 17it is not in fact very greatly different. The reason for this is clear when one examinesthe crystal structure of the complex, (fig. 11) : it is seen that the interaction betweenadjacent atoms in this remarkable array is such as to seriously affect any changes dueto the trans-effect.This emphasizes how careful one has to be in interpreting datafrom the crystal, because of the magnitude of the crystal forces which can distort theeffect one is attempting to observe.One of the interesting features of the bonding model proposed by Chatt andDuncanson was that the olefin would be locked perpendicular to the plane of thecomplex by virtue of the orbital hybridization used to accomplish back donation intothe anti-bonding orbitals of the olefin. The recent application of n.m.r. by Cramer 46to rhodium complexes and by Lewis 47 et al. to platinum complexes has establishedthat in some cases the olefin rotates about the olefin-metal bond (fig. 12) even thoughthe " frozen out " configuration has the olefin perpendicular to the plane of the system.However, the ethylene ligands do not readily exchange with free ethylene and thisrotation can be accommodated in terms of a Chatt-Dewar model.Clearly, the geometry of unsaturated ligands when co-ordinated to transitionmetals is a major point of discussion for this meeting as is the proposition ofMcQueeney, Mason and Tow1 48 that the ligand charge distribution when complexedis virtually identical with that possessed by an isolated ligand molecule in its firstexcited state.The archetypal molecule for this idea is the remarkable compoundPt(CS2)(PPh3)2 described previously by Baird, Mason, Ray and Wilkinson 49 butanother example is furnished by the recent structure determination by Hewitt et aLS0of bis-allenerhodium acetylacetonate which shows that the central C=C=C systemis no longer linear, the angle being reduced to about 148".There is also a significantdifference in the central C=C=C bond lengths, the longer bond being the one thatis formally co-ordinated.It has already been noted that our understanding of the nature of the metal ligandbond has been greatly helped by the development of a whole armoury of physicaltechniques and the papers at this meeting illustrate very comprehensively how thewide range of physical determinations yield accurate data which the " non-specialists "can apply to a whole series of related compounds and purposes.Amongst thesepurposes some mention must be made of the increasing use by the practical chemistof symmetry arguments in, for example, vibrational spectroscopy as well as in simpleU H 1 1C Me A Me C,\A C' \FIG. 12.4lefin orientations in the pIatinum (II) olefin complex, Pt (acac) C1 (cis-but-2-ene).molecular orbital calculations. The power of symmetry arguments is demonstratedby the correctness of the prediction that cyclobutadiene would be stabilized throughcomplex formation, and more recently their use to predict the existence of uranocene.No review of this kind is complete without a glance into the future. It is appro-priate to consider first the central metal atom, and particularly the consequences o18 FROM FERROCENE TO URANOCENEthe very recently discovered uranocene with its unique feature that the n-molecularorbitals of the ring share electrons with thef-atomic orbitals of the uranium.Itspreparation could pave the way for a whole new area of chemistry involving theactinide, lanthanide and rare earths-much as the discovery of ferrocene 18 yearsago set in motion the search for organo-metallic sandwich compounds based onthe d-block elements.As regards the ligand, knowing the state of the ligand once it has bonded isfundamental to predictive metal-complex catalysis in organic processes. Clearly,a great deal of further thinking and experimentation is required here, to take us outof the age of Enlightened Empiricism.Some mention must also be made of the rationalization that becomes possiblenow that the nature of the products of a concerted reaction involving a metal ioncan be predicted by the application of the extended Woodward-Hoffman rules asdescribed by Pettit 51 and his co-workers. This understanding should result in organicchemistry of a new elegance and sophistication-but it too will be only achievedif the nature of the bonding in metallo-organic compounds is better understood.My best thanks are due to Prof. J.Lewis and my colleagues at the Petrochemical& Polymer Laboratory for their help and advice in preparing this Introductory paper.L. E. OrgeI, Report 10th Soluay Council (Brussels, 1956), p. 311. ’ W. P. Anderson and T. L. Brown, this Discussion.I. H. Hillier and Mrs. R. M. Canadine, this Discussion.F. A.Cotton, J. Amer. Chem. Soc., 1968,90,6230.R. Greatrex and N. N. Greenwood, this Discussion.G. M. Bancroft, M. J. Mays and B. E. Baxter, this Discussion.ESCA: Atomic, Molecular and Solid State Structure Studied by Means of Electron Spectroscopyby K . Siegbahn, C. Nordlina, A. Jahlman, R. Nordberg, K. Haunrin, J. Hedman, G. Johansson,T. Bergman, S-E Karlsson, I. Lindgren, B. Lindberg. Nova Acta Regiae Societatis ScientiarumUpsaliensis, ser. IV. vol. 20, (Uppsala, 1967).S. Evans, J. C. Green, M. L. H. Green, A. F. Orchard and D. W. Turner, this Discussion.P. S. Braterman and A. P. Walker, this Discussion.lo C. W. Fryer and J. A. S. Smith, private communication.l1 A. A. Hock and 0. S. Mills, Proc. Chem. SOC., 1958, 233.J. V. Brencic and F. A.Cotton, Inorg. Chem. 1969, 8,7.l3 E. R. Corey, L. F. Dahl and W. Beck, J. Amer. Chem. Soc., 1963,85,1202.l4 E. H. Braye, L. F. Dahl, W. Hubel and D. L. Wampler, J. Amer. Chem. SOC., 1962,84,4633.l5 B. F. G. Johnson, R. D. Johnston and J. Lewis, Chem. Comm., 1967, 1057.l6 R. Mason and W. R. Robinson, Chem. Comm., 1968,468.l7 F. A. Cotton, this Discussion.l8 N. V. Sidgwick, The Electronic Theory of Valency, (Oxford University Press, London, 1927).l9 L. Pauling, The Nature ofthe Chemical Bond (3rd ed., Cornell University Press, New York,2o J. H. Van Vleck, Theory of Electric and Magnetic Susceptibilities (Oxford University Press,21 R. Schlapp and W. G. Penney, Physic. Reu., 1932,42,666.22 0. M. Jordahl, Physic. Rev., 1934, 45,87.23 J. Owen and K. W.H. Stevens, Nature, 1953,171,836.24 J. H. Van Vleck, J . Chem. Phys., 1935,3,805.25 Y . Tanabe and S. Sugano, J. Phys. SOC. Japan, 1954,9,753,766.26 F. A. Cotton, Chemical Applications of Group Theory (Interscience, New York, 1963).27 C. J. Ballhausen and H. B. Gray, Molecular Orbital Theory (Benjamin, New York, 1964).28 C. K. Jprrgensen, Absorption Spectra and Chemical Bonding in Complexes (Pergamon, Oxford,29 R. S. Nyholm, Report. 10th Soluay Council CBrussels, 1956), p. 230.30 M. L. H. Green, Organometallic Compounds, vol. U. The Transition Elements, ed. G. E. Coates,st A. Streitwieser Jr., V. MiiUer-Westerhoff, J. Amer. Chem. Soc., 1968,90,7364.1960).London, 1932).1962).M. L. H. Green, and K. Wade, (Methuen, London, 1968)G . A . GAMLEN 1932 P. H. Lewis and R. E. Rundle, J. Chem. Phys., 1953,21,986.33 R. G. Vranka and E. L. Amma, J. Amer. Chem. SOC., 1967,89,3121; see also T. Onishi andT. Shimanouchi, Spectrochim. Acta, 1964,20,325.34 K. A. Levison and P. G. Perkins, this Discussion.35 J. Chatt and J. M. Davidson, J. Chem. SOC., 1965, 843.36 S. D. Ibekwe, B. T. Kilbourn, Miss U. A. Raeburn and D. R. Russel, Chem. Comm., in press.37 J. Chatt and B. L. Shaw, J. Chem. SOC., 1959,705 ; 1960, 1718.38 M. Green, J. Smith and P. A. Tasker, this Discussion.39 H. A. 0. Hill, J. M. Pratt and R. J. P. Williams, this Discussion.40 F. G. A. Stone, Endeavour, 1966,25,33 ; M. I. Bruce and F. G. A. Stone, Preparative Inorganic41 W. C. Zeise, Pogg. Ann. 1827,9,632.42 M. J. S. Dewar, Bull. SOC. Chim. France, 1951, 18, C71.43 J. Chatt and L. A. Duncanson, J. Chem. SOC., 1953,2939.44 L. Manolovic-Muir, K. W. Muir and J. A. Ibers, this Discussion.45 Miss M. Black, R. H. B. Mais and P. G. Owston, private communication.46 R. Cramer, J. Amer. Chem. SOC., 1964,86,217 ; 1967,89,4621.47 C. E. Holloway, G. Hulley, B. F. G. Johnson and J. Lewis, J. Chem. SOC. A , 1969,53.48 R. McWeeny, R. Mason and A. D. C. Towl, this Discussion.49 M. C. Baird, G. Hartwell Jr., R. Mason, A. I. E. Rae and G. Wikinson, Chem. Comm., 1967,92, see also M. C. Baird and G. Wilkinson, J. Chem. SOC. A, 1967, 865.50 T. G. Hewitt, K. Anzenhofer and J. J. DeBoer, Chem. Comm., 1969,312, see also T. Kashiwagi,N. Yasuoka, N. Kasai and M. Kukudo, Chem. Comm., 1969, 317. '' R. Pettit, H. Sugahara, J. Wristers and W. Merk, this Discussion.Reactions, W. L. Jolly, ed., (Interscience, New York), 1968, 4, 177

ISSN:0366-9033    

DOI:10.1039/DF9694700007

出版商:RSC    

年代:1969

数据来源: RSC

摘要:

The Geometries of and Bonding in Certain TransitionMetal ComplexesBY R. MCWEENY, R. MASON AND A. D. C. TOWLDept. of Chemistry, University of SheffieldReceived 23rd January, 1969Two topics in structural transition metal chemistry are discussed. First the Chatt-Dewar theoryfor the bonding between metals and unsaturated ligands such as carbon disulphide, oxygen andacetylene is developed ; it is shown that the molecular orbital wave function contains wave functionsbelonging to the excited states of the individual fragments and the theory is then correlated withsome recent structural results. Secondly, the trans-influence of a number of ligands in severalplatinum(II) complexes is described and related to their electronegativities and ligand-metal p(a)overlaps.The Chatt-Dewar model for the bonding between transition metals andunsaturated ligands has been qualitatively successful for many discussions of theproperties of organometallic molecules ; during the last few years, semi-empiricaltheories of varying sophistication and reliability have developed this largely symmetry-based model to predictions of molecular energy levels and related matters.We nowexamine the implications of a simple molecular orbital theory on recent observationsof the geometries of unsaturated ligands co-ordinated to metals in low oxidation states.The model for the bonding between the metal and ligand is based, as in the Chatt-Dewar model, on the highest occupied orbitals of the metal M and ligand L beingrespectively 7r and a symmetry, the lowest empty orbitals being of opposite symmetry.The molecular orbitals are constructed, as usual, by a linear combination of orbitalsof the component systems.The a orbitals lie close together and interact quitestrongly to give the bonding and antibonding orbitals, aML and The 71 orbitals,initially filled on the metal and unfilled on the ligand, in general interact more weaklyto give the xML and orbitals. If we disregard mixture of all other orbitals, weconsider only the electron configurations ML[x2a2], M[&] and L[G~] which allcorrespond to spin singlet ground states (the complexes of interest are all diamagnetic).If we neglect overlap, the molecular orbitals ML have the normalized form,a = oM sin O+ oL cos 8,O* = aM cos 0-0, sin 8,n* = nM sin 4+n, cos 4,7~ = nM cos # + nL sin #,with the ground state of the complex being represented by the wave functionY = + I aanz!.(2)a and 5, for example, represent spin-orbitals with a and B spin factors respectively ;d and 4 are defined by the equations6, = 2 sin2 0 (a charge donated by the ligand),6, = 2 sin2 4 (n charge back-donated by metal).2R . MCWEENY, R . MASON AND A. D. C. TOWL 21Inserting (1) into (2) and expanding, we obtain the following terms in the wavefunction :sin2 8 cos2 4 I o M ~ M ~ M E M 1 + cos2 8 sin2 4 I o L o j n l ? t L I + cos 8 sin 6 cos 4 sin 4 x(- I OMZMZLEL I + [ OMEMOLE, I + i ~ M ~ M z L ~ L I - I 1 )+sin2 8 sin2 4 1 ~ M ~ M ~ L E L I + cos2 8 cos2 4 I QLcTLzM~M 1 +sin2 e cos 4 sin { I oMcM~MzL 1 - I o M ~ , i i L n M i +ax2 # sin 8 cos e( I o,nM?t:,aL 1 - I CMZMEMOL I ] +cos e2 cos 4 sin 4{ 1 O L ~ L ~ M E L I - I oLzLEL~M I 1 +sin2 4 sin e cos 6( 1 C T L ~ L Z L ~ M I - 1 ~ , ~ L z L ~ M 1 >.(3)The various terms are identifiable as antisymmetrised products of wave functionsrepresenting the states of the separate systems M and L. For example,4 [ o L ~ L ~ M E M i = A . 1/42 I o L ~ L 1 I/ J2 I ZMEM Iwith A antisymmetrizing the product of two normalized wave functions representingthe singlet ground states of [7&] and [o?], performing all possible electron inter-changes and summing with appropriate signs.The first two terms in eqn. (3) arise from the ionic wavefunctions M2-• L2+ andM2+ L2- while the last four terms describe singlet states belonging to the singlyionized functions, M- L+ and M+ L- respectively-they are, therefore, relativelyunimportant.That is to say, if the coefficients of the various wave functions in(3) were regarded as variational parameters, those of the " ionic states " woulddecrease in magnitude on optimizing the wave function. (A similar situation occursin the molecular orbital treatment of the hydrogen molecule where complete neglectof the " ionic states " yields the Heitler-London wave function). The remainingterms may now be written in the form,= aA(@Lg@Mg) + bA(@Le@Me) +C(AC3@L,03@M,0- 3@L,- 13@M,+ 1- 3@L,+ 13@M,- l]+A(l@L1@M)]-mLS and mMO are the ground states of L and M while QLemMe are excited singletstates produced by double excitations into the lowest empty orbitals.The secondterm must be expected to have lower weight than the first which represents the" no-bond " situation. We note that in the limit of very small overlap, our functionwill correspond to the well-known Mulliken charge transfer wave functions. Thesingly excited wave functions appear only when overlap is large and the molecularorbital method more appropriate. We infer from (3) that the coefficients a and care defined byc/a = tan 6 tan 4showing that the importance of the interaction terms, and hence the excited statecomponents of the wave function, increases with increasing charge transfer from themetal to ligand and vice versa.The appearance of triplet states in the term describing the interaction is of mostinterest to us. 3@L,m, for example, is the spin-triplet state of the ligand arising froma single excitation into the empty n orbital, with spins coupled to unity and with " z "component rn = 0, & 1.The functions in the square brackets arises by vectorcoupling the triplet states of the fragments to a resultant spin S = 0. The ter22 BONDING IN TRANSITION METAL COMPLEXESA(lmLIQM) arises from the same single excitations but with spins coupled to give asinglet state of each fragment. Since each term is individually normalized, thetriplet terms occur with three times the weight of the singlet term. A variationalcalculation would allow the ratio of these coefficients to vary but the general picturewill remain unchanged.In the complex the charge density is a weighted sum of densities associated withfragments in their various individual states. The terms in which the fragments arein their first excited triplet states appear with large weighting and all lead to a ligandcharge distribution virtually identical with that possessed by an isolated ligandmolecule in its first excited triplet state.The geometry of a ligand will thereforespontaneously change on co-ordination, the forces acting on the nuclei being morenearly those of the triplet state rather than those of the ground state.Recent X-ray structural analyses show that (i) the carbon disulphide ligand in(Ph,P),PtCS, has a geometry remarkably similar to that of its ,A2 excited state.,’(ii) The electron distribution in z-bonded oxygen complexes approximates that ofthe 3Z; excited state in a way which varies according to the electronic nature of theremaining ligands in the cornple~.~~ The state is, in valence bond terms,predominantly 0f-O- and the reactivity of (Ph,P),PtO, towards carbon dioxide,sulphur dioxide, aldehydes and ketones may be rationalized along these lines.’(iii) The geometries of acetylene, ethylene and butadiene co-ordinated to metals inlow valence states often approaches that of their excited states.6 For acetylene,Blizzard and Santry have recently discussed its co-ordinated geometry.8 Substitutedacetylenes are cis-bent on co-ordination in contrast to the excited state of the un-co-ordinated molecule which is trans-bent. It is obvious qualitatively that non-bonded interactions of the substituent groups with the metal will stabilize the cis-bentligand on co-ordination. However, Blizzard and Santry * point out that the cis-bentstructure can be explained solely in terms of the symmetry of the various orbitals ofthe metal and ligand-it is largely a question of the contribution of the carbon 2sorbitals to the various metal-ligand molecular orbitals.CNDO-MO calculationsindicate that the observed angles of bonding in co-ordinated acetylene can beaccounted for by the transfer of approximately 0-5 electron from the ligand nu orbitalsto its z* counterpart; this estimate is not inconsistent with other estimates of theextent of charge transfer in metal to ligand interaction^.^THE TRANS-INFLUENCE OF LIGANDSThe classification of ligands according to their “ trans-directing ” properties,i.e., their relative tendency to direct an incoming ligand into a trans-position tothemselves, for substitution reactions in planar platinum (11) complexes, has beenestablished for some time.1° The extension of this classification to a series basedon the effectiveness of a ligand to influence the rate of displacement of trans-ligandswas made by Basolo and Pearson l1 and forms the definition of the trans-effect ”.Two theories have some success in rationalizing the trans-effect in terms ofelectronic effects transmitted by the trans-directing ligand across the metal to theleaving group ; they are based respectively on the inductive and mesomeric effectsof the trans-directing ligand.2* A generalization of the relative importance ofinductive and mesomeric effects is difficult for little is still known of the detailedmechanisms of substitution in d6 and d8 planar complexes while the variations ofreaction rate with solvent, metal oxidation state, incoming group participation andsteric effects are not understood in any general way. These difficulties have led to anincreasing interest in observable ground state effects of a ligand L on a trans-liganR. MCWEENY, R. MASON AND A . D. C . TOWL 23X and infra-red and nuclear magnetic resonance spectroscopy has been used to studythe trans-bond weakening effect of a ligand 14* l5 (differentiating an equilibriumproperty (trans-influence) from the kinetic trans-effect).We prefer to examinebond lengths in this connection since again it is not clear that interpretation of infra-red and nuclear magnetic resonance data is at all straightforward-for example,few data are available for bond force constants in the complexes of interest.Structural data, from X-ray analyses, of some planar platinum(I1) complexes arecollected in table 1.TABLE 1 .-PLATINUM-CHLORINE BONDLENGTHS IN SOME PLATINUM(II) COMPLEXESmolecule trans-ligand Pt-Cl bond length (A)(Pr(acac), C1)- 0 2.28 f0.01 ''trans-(PEt,),PtCl, CI 2.30 50.01 l7(G2H 1 7)2PtzC12 C=C 2.31 &O-01cis-(PMe3),PtC12 P 2-37&0-01 l9trans-(PPh,Et),PtHCl H 2.42f0.01 2otrans-(PPhMe,),(SiPh,Me)PtCl Si 2.45f0.01 21The order of trans-influence is seen to be : Si>H>P>C=C, C b O .Although less data are available, a similar series may be provided for other d8(Ni(l1) and Pd(I1)) planar complexes and for octahedral d6 (Co(III), Rh(III), Ir(III),Grinberg's original discussion l 2 of the trans-effect was based on purely electro-static grounds, relating polarizability to the trans-directing ability of a ligand.Syrkin and Yashkin related polarizability of a ligand to the covalent character ofthe metal-ligand bond and, using valence bond methods, showed that in d8 planarcomplexes an increase in the degree of covdence in the M-L 0 bond decreasedthe covalence of the trans M-X a bond and hence increased the lability of X.23Fig.1 shows that for the data of table 1, the trans-influence of L increases smoothlywith decreasing electronegativity (the effective electronegativity of C = C is that oftrigonal carbon).24 Chatt et reached a similar conclusion from infra-reddata. This result is related to those of Syrkin and Yashkin; if we consider relativeenergies of L, M and X a-bonding orbitals, then if the ligand X is more electronegativethan L, the following qualitative scheme is obvious.P t (I V)) .22t1 EnY-MtAEi ?AE2 > AEI L- ' a'E,-XiThe a-molecular orbital electron density will be principally within the M-Lbond (high covalence) and on the ligand X (high ionic character of M-X).Gray and Langford 26 base a theory of the trans-effect on the magnitude of theM-L and M-X a-overlap integrals.The suggestion here is that if the a-donororbital of the ligand L has a greater overlap with the metal pa orbital than does theligand X a-orbital, then the M-L bond is strengthened at the expense of the M-Xbond. The overlap calculations of Gray and Langford have now been extendedcomprehensively to a number of ligands and metals and are given in table 224 BONDING I N TRANSITION METAL COMPLEXES01.5 I2.2 5 2 - 3 0 2 3 5 2.40 2.4 5Pt-CI distance, AFIG.1.-Variation of the Pt-Cl bond length in L-Pt-Cl as a function of the Pauling electro-negativity, XL, of the trans-ligand L.TABLE 2.-METAL (PO) - LIGAND HYBRID (0) OVERLAP INTEGRALSligand atom andhybridization Co(II1)0.450-470-5 10.400.430.360.380-3 10-340.370.560.510.480.5 10.450.470.500.520-480.500.5 10.54NI(I1)0.490.520.550-450.470.400-430.360-380-410.550.530.500.520.470.490.500-530.480-490-500.56Rh(1II) Pd(I1)0.490.520.550.450.470-400-420.370-3 80.380.510.500.470.470.440.440.440.480.430.440.430.510.480.520.550.450.480.410.420-370.3 80-390.500-490.470.470.430-440.440.480.430.430.430.51Ir(II1)0.480-5 10.540.440-470-400-410-360.370.3 80.490.490.460.460.420.430.430.470-420.420.410.50R(IU0.480.510.540.440.470.410.420.370.380.390.490.490.460.460.430-430.430.460-420.420-410.50Au(l1I)0.480.510.540-450.480-410.420.370-380.390.490.480.450.460-420.430.420.460.410.410.410.50These overlap integrals have been calculated as follows :(i) Atom radii (A) ; C(sp3) 0.77, C(sp2) 0.74, C(sp) 0.71 ; N(sp3) 0.70, N(sp2) 0.63 ;H, 0.27 ; 0, 0.66 ; F, 0.62 ; Si, 1.00 ; P, 1.00 ; S, 1.00; C1, 0.99 ; As, 1.12 ; Br, 1-14;Co(TII), 1.26 ; Ni(Il), 1.24 ; Rh(IV), 1.33 ; Pd(II), 1-31 ; Ir(III), 1-36 ; Pt(II), 1.33 ;Au(TII), 1-31.For the first row transitionmetals they are those of Richardson et aL2’; for the second and third row metalsthey are the functions of Basch and Gray.28(ii) S.C.F.wavefunctions were used for all atomsR. MCWEENY, R . MASON AND A . D . C. TOWL 25(iii) All metal wavefunctions are those of the + I oxidation state.Whilst thisassumption will undoubtedly introduce some error, it will probably be a systematicone and with little variation from metal to metal. Cotton and Harris conclude 29that failure to correct the metal wavefunction for varying metal charge in (PtC1,)2-leads to an error of less than 4 % in the M-L overlap integrals for charge variationsbetween 0 and + 1.(iv) All ligand wavefunctions are those of Clementi 30 for neutral atoms, doublezeta functions being used where available.(v) The calculated values are based on the method of Demuynck and Kaufmann 31and Mulliken's rules.For platinum(II), the a-overlap integrals have the order :Si-H, C, P>Cl>N>O>Fin general qualitative agreement with trends in the bondlengths noted in table 1and confirming, for the trans-influence of ligands, the suggestions of Gray andLangford relating to the trans-effect.We emphasize that the agreement is onlyqualitative-we cannot sensibly deal, for example, with the relative trans-influenceof tri-alkyl and triaryl phosphine ligands.Perturbation theory indicates that bond strength may be related to S2/AE (S theoverlap integral and AE, the energy separation between the orbitals being mixed).The above discussion now shows that both relative ligand a-orbital energies andrelative ligand-metal cr overlap integrals reproduces the trend of the trans-influenceability of a ligand.The bond length data additionally illustrate a mesomeric contribution to thetrans-influence which would, in principle, be exerted in one of two ways,(i) A n-acceptor ligand L is able, through the usual synergic effect, to effectivelyincrease its donor capacity which in turn increases the inductive contribution to thetrans-influence, and(ii) As has been pointed out previously, a n-acceptor ligand will directly competewith the ligand X for excess charge in the metal.The fact that ethylene and carbonmonoxide have little or no trans-influence and that, experimentally, the trans-influence of a a-bonded alkyl is greater than that of a formally a-bonded phosphineclearly indicate that the second process is dominant. In short, therefore, a strongtrans-influence follows from the ligands with large inductive, a-donor and weakn-acceptor properties. (In all of this discussion we assume that the ligand X hasnegligible n-bonding capacity.) By contrast, the large trans-effect of ethylene andcarbon monoxide can be accounted for, as Chatt et al.and Orgel 32 pointed out, interms of the stabilization of transition states and not from any initial ground statelabilization of the trans-ligand.We are grateful to the S.R.C. for support of this work.M. J. S . Dewar, Bull. SOC. Chim. Fr., 1951, 18, C71.J. Chatt, J. Chem. SOC., 1953, 2939.M. Baird, G. Hartwell, R. Mason, A. I. M. Rae and G. Wilkinson, Chem. Comm., 1967, 92.R. Mason and A. I. M. Rae, in preparation.J. A. McGinnety and J. A. Ibers, Chem. Comm., 1968, 235 and references therein.R. Mason, Nature, 1968, 217,543.'I R. Ugo, F. Conti, S . Cenini, R. Mason and G.B. Robertson, Chem. Comm., 1968, 1948.* A. C. Blizzard and D. P. Santry, J . Amer. Chem. SOC., 1968,90,5749.for example, J. W. Moore, Acta Chem. Scand., 1966, 20,1154.lo I. I. Chernyaev, Ann. Inst. Plafine U.S.S.R., 1926, 4,243, 26126 BONDING IN TRANSITION METAL COMPLEXESF. Bas010 and R. Pearson, Mechanisms of Inorganic Reactions, (Wiley, N.Y., 1967).l2 A. A. Grinberg, Actaphysiochim., 1935,3,573.l3 J. Chatt, L. Duncanson and L. Venanzi, J . Chem. SOC., 1955,4456.l4 D. M. Adams, J. Chatt, J. Gerratt and A. D. Westland, J. Chem. Sac., 1964,734 and referencesl5 A. Pidcock, R. E. Richards and L. M. Venanzi, J. Chem. Sac. A , 1966, 1707.l6 R. Mason, P. Pauling and G. B. Robertson, J. Chem. Sac. A , 1969,485.l8 R. Mason, G. B. Robertson and P. 0. Wimp in preparation ; see also Chem. Comm. 1968,therein.G. G. Messmer and E. L. Amma, Inorg. Chem., 1966, 5,1775.869.G. G. Messmer, E. L. Amma and J. A. Ibers, Inorg. Chem., 1967,6,725.'O R. Eisenberg and J. A. Ibers, Inorg. Chem., 1965,4,773.21 J. Chatt, C. Eaborn and S. Ibekive, Chem. Comm., 1966,700, with P. N. Kapoor, Chem. Comm.,22 A. D. C. Towl, Ph.D. Thesis (Univzrsity of Sheffield, 1968).23 Y. K. Syrkin, Bull. Acad. Sci. U.S.S.R. Classe. Sci. Chim., 1948,69; M. M. Yashkin, Compt.24 G. Pilcher and H. A. Skinner, J. Inorg. Nucl. Chem., 1962,24,937.2 5 J. Chatt, L. Duncanson and L. Venanzi, J. Chem. Soc., 1955,4461.26 H. B. Gray and C. Langford, Ligand Substitution Processes, (Benjamin, New York, 1966).27 J. W. Richardson, et al., J. Chem. Phys., 1952,36,1057 ; 1963,38,796.28 H. Basch and H. B. Gray, Theor. Chim. Acta., 1966, 4,367.29 F. A. Cotton and C. B. Harris, Inorg. Chem. 1967, 6,369.30 E. Clementi, I.B.M., J. Res. Dev. Supplement, 1965.31 J. Demuynck and G. Kaufmann, Bull. Soc. Chim. Fr., 1967, 1256.32 L . E. Orgel, J. Inorg. Nucl. Chem., 1956, 2,137.1967, 869 ; P. M. Harrison, Ph. D. Thesis (University of Sheffield, 1968).Rend. Acad. Sci. U.R.S.S., 1941, 32,555

ISSN:0366-9033    

DOI:10.1039/DF9694700020

出版商:RSC    

年代:1969

数据来源: RSC

摘要:

Approximate Molecular Orbital Calculations onMetallo-organic ComplexesBY I. H. HILLIER* AND MRS. R. M. CANADINETReceived 7th January, 1969The results of approximate molecular orbital cdculations on the metallo-organic complexes,palladium bis x-allyl, ferrocene, and dibenzene chromium are presented. The calculations includeelectron interaction and all valence electrons are considered. The bonding involves mainly themetal orbitals and the x orbitals of the hydrocarbon ligand. The results are compared with thoseof other workers and with experimental data.The use of semi-empirical methods in the molecular orbital treatment of inorganiccomplexes has become widespread because the computation of accurate wavefunctionsfor these molecules by the Hartree-Fock self-consistent field method of Roothaanis limited by the large number of molecular integrals which are required.2 Thereis, however, a continuing search for simplsed methods of calculating reliable wave-functions, particularly for those molecules for which a crystal field model is un-satisfactory.One of the earliest such methods, introduced by Wolfsberg andHelmholz (WH),3 and subsequently modified,4 has been used to interpret the bondingin several inorganic molecules. This method, being essentially an extended Hiickelscheme, does not include electron repulsion terms explicitly, and uses empiricalparameters and equations for evaluation of the elements of the secular determinant.For these reasons, many authors have recognized that it may lead to unreliableresult^.^" In particular, the neglect of interatomic coulomb terms leads to too smalla metal-ligand charge separation.By the introduction of systematic and well-defined approximations in Roothaan’s method it is possible to develop other com-putational In this paper the self-consistent charge with electroninteraction (SCCEI) method, which has been developed in previous publications,6* lois used to describe the bonding in some organo-metallic complexes.6* *-loMETHODIn the LCAO-MO approximation, a molecular orbital +i is represented as a linearcombination of atomic orbitalswhere xku is atomic orbital k on centre u, and the coefficients Cik are determined bysolution of the secular equations, yielding the determinantwhere Siu,ju is the overlap intergral and Hi,,,v the matrix element of the FockHamiltonian.Since the approximations introduced to simplify evaluation of thelatter are discussed elsewhere,6* lo they will only be outlined here. Richardson’s* Chemistry Department, The University, Manchester 13.I- I.C.I. Limited, Petrochemical and Polymer Laboratory, The Heath, Runcorn, Cheshire.228 APPROXIMATE MOLECULAR ORBITAL CALCULATIONSseparated Hamiltonian procedure l1 is used to reduce the elements of the seculardeterminant into terms which involve atomic orbital energies and interatomic coulombterms. The former are approximated as the negative of the valency state ionizationenergies (VSIE) and the latter are simplified by using a point charge approximationfor the atomic potentials.The diagonal elements then becomewhere Eiu, the orbital energy associated with atomic orbital xiu, depends upon thecharge qu on atom u obtained via a Mulliken population ana1ysis.lelements similarly becomeThe off-diagonalT i u , j v , (4)where is the two-centre kinetic energy integral. The leading terms in boththese expressions (eqn. (3) and (4)) are similar to those used in the WH method, thedifference being the introduction of an empirical parameter F, usually having avalue near 2, in the off-diagonal terms, viz.,H i u , j v = ( F P P i u , jv(Hiu,iu + H j v , j v ) . ( 5 )The terms in eqn. (3) and (4), which are in addition to those occurring in the WHmethod may be considered as arising from interatomic coulomb or “ ligand field ”effects.The one-electron integrals in these equations are evaluated using the atomicorbitals of the basis set. The calculation is iterated until self-consistency is achieved,when the atomic charges and configurations used to evaluate the elements of thesecular determinant are virtually the same as those obtained by a Mulliken populationanalysis of the resulting molecular orbitals, the criterion being that the differencein the charges is less than 0.01.DESCRIPTION OF THE CALCULATIONSMolecular orbital calculations using this method have been performed on themetallo-organic compounds, Pd(allyl)2, ferrocene, and dibenzene chromium. Previousnon-empirical calculations 3-1 on compounds of the transition metals and un-saturated hydrocarbons have not included the interaction of the a- framework ofthe organic entity with the metal.In the isolated hydrocarbon ligands considered,benzene, cyclopentadiene, and the ally1 radical, the plane of the carbon skeleton isa symmetry plane of the molecule, so that no mixing of the 0- and n-carbon atomicorbitals can occur. However, once the ligand is co-ordinated to the metal, this isno longer the case, so that mixing of Q- and n-orbitals can occur on construction ofthe molecular orbitals. Even if the extent of such mixing is slight, there remainsthe further possibility of significant bonding arising from m.0. constructed of metalorbitals and the ligand orbitals of predominantly a-character with appropriatesymmetry. To assess the importance of such “a-bonding” in these compounds,our calculations have been performed with an a.0.basis set which includes all threecarbon 2p orbitals as well as the 2s and hydrogen 1s orbitals. The metal orbitalsused include the palladium 4d, 5s and 5p atomic orbitals, and the iron and chromiuI . H . HILLIER AND MRS. R . M. CANADINE 293 4 4s and 4p orbitals. The basis orbitals were taken as linear combinations ofSlater type orbitals for the neutral atom in all cases except palladium. Those forpalladium (for metal charge of + 1) were taken from Gray et aZ.,16 for iron andchromium from Richardson et aZ.,17 and for carbon and hydrogen from Clementi?VSIE for the carbon and palladium orbitals were taken from the data of Moore,19the latter from values smoothed over the second row of the transition metals, aftertaking into account the absence of several terms in the experimental data, and aregiven in table 1.VSIE for the other orbitals were taken from the literature.20TABLE 1 .-vSIE FUNCTIONS FOR PALLADIUMVSIE = (Aq2+ Bq+ C ) eV. (1 eV = 16021 x J)orbital4d5s5P2s2Pstartingconfiguration A B0-754.841.1 10.890.8 10.460.490-271.3011.094.2 110.917.327.838-776-556.545.70VSIE FUNCTIONS FOR CARBONs2p” 1 -24 11.75.?pn 1 *66 12-06C8.5412-0213-587.608.809.303.845.215.0819.4310.66Calculations on the Pd(all~1)~ molecule were performed for the two isomericforms postulated from n.m.r. results,21 the “ boat ” and “ chair ” forms of symmetryCz0 and C2, respectively, shown in fig.1. Bond lengths and angles were taken from“ Boat ”C2” “ Chair ” C2hally1 ligandFIG. 1 .-Pd(alIyl)2 isomers30 APPROXIMATE MOLECULAR ORBITAL CALCULATIONSthe X-ray structure of Pd2C12(ally1)2,22 assuming that the xy plane is that definedby the Pd and bridging C1 atoms, and using X-ray data on the substituted methallyls 23to give the orientation of the C-H bonds with respect to the carbon skeleton. Thetransformation properties of the metal orbitals for these two symmetries and theappropriate combinations of ligand orbitals are given in table 2. Ferrocene andTABLE 2.--TRANSFORMATION SCHEMES FOR “ BOAT ” (cz”) AND “ CHAIR ” (c2h)* ISOMERS OFPd(allyl)zbl P xdxz*The transformation properties of the orbitals in C2h are given in parenthesis.t s and p refer to carbon atomic orbitals, h referring to hydrogen 1s orbitals, with the numberingof the atoms as in fig.1.dibenzene chromium were taken to have symmetries DSd and Dsh respectively withbond lengths taken from ref. (24). The z-axis was chosen to be perpendicular tothe aromatic ringsI . H . HILLIER AND MRS. R . M. CANADINE 31RESULTSPd(allyl),The self-consistent atomic charges and configurations for the two isomers aregiven in table 3. In both cases the metal and four equivalent terminal carbon atomshave definite negative charges whilst the remaining two central carbon atoms areelectron deficient. The hydrogen atoms are found to be only slightly charged.TABLE 3.-sELF CONSISTENT CHARGES AND CONFIGURATIONS OF Pd(allyl)2atomPdC1CZH1H2H3charge- 0.3025 - 0.10050-257 10.03300.03 17-0.0345- 0.3 179-0.09560.2574003190.03 14-0.0338s Px0.425 0.5981.048 1.0580.998 0.9400.9670.9681.0350.437 0.5981.047 1.0580.998 0.9400.9680-9691.034cz,, SYMMETRYconfigurationPY Pz dx2-y2 dzz dxy dx, dyz0.235 -0.023 1.869 1.940 1.333 1.999 1.9271-004 0-9900.980 0-825CZJ, SYMMETRY0.226 -0.026 1.880 1.944 1.336 1.998 1.9211.003 0.9880.980 0.824The origin and relative importance of the bonding in these molecules may be assessedby examination of the interatomic overlap populations which give the amount ofelectron density associated with the internuclear region between any two atoms Aand B due to the interaction of orbital i on atom A with orbital j on atom B aswhere cb is the coefficient of the ith atomic orbital on atom A in the kth molecularorbital, Nk the number of electrons in the kth molecular orbital, and Sif the overlapintegral between i and j .When symmetry adapted ligand orbitals are used, a groupoverlap population may be defined, with the group overlap integral G i j replacingSij in eqn. (6). The sum is over all occupied molecular orbitals, and to find thetotal overlap population between A and B, may be summed over all orbitals i on Aand j on B. The major contributions (whose absolute values are greater than 0.1)to the palladium-carbon overlap populations are given in table 4 for each irreducibleTABLE 4.-GROUP OVERLAP POPULATIONS USING THE NORMALIZED SYMMETRY-ADAPTED LIGANDORBITALS OF TABLE 2overlap populationligand terminal centralorbital carbons carbons representation zi;:,SSdXYdXYP xPxPYP zP ZP ZP ZP ZP zP ZPzSS0.232 0 .1 2 50-25 1 0 . 1 1 50.2450-2450.4790-479- 0 . 1 0 9 - 0 . 1 0 40 . 1 80 0 . 1 4 8-0.111 -0.1030.167 0 . 1 5 32 APPROXIMATE MOLECULAR ORBITAL CALCULATIONSrepresentation. Although only one metal-carbon a-interaction appears in this table,there are a number of such interactions having overlap populations of greater than0.05. However, as expected, it is the ligand n-orbitals which are predominantlyinvolved in the bonding. The bond overlap populations between the metal andligand atoms, given in table 5, also show that the bonding is strongest between thePd and terminal carbons atoms.Eigenvalues for the filled orbitals of highest energyand vacant orbitals of lowest energy are given in table 6.TABLE 5.-BOND OVERLAP POPULATIONSsymmetryC," C2hatom 1 atom 2Pd c1 0.3 139 0.3 130c2 0-1 339 0.1333HI - 0.0446 - 0.0466H2 - 0.0460 - 0.0458H3 - 0.0294 - 0.0287TABLE 6.-ELGENVALUES FOR Pd(aIIyI),tboat (CZ"~representation eigenvalue (eV)5.01- 2.08- 4.80- 12.68- 13.38- 13.69- 13.74- 13.98- 16.32- 17.13- 17-31* highest occupied orbital. t The major contributions to the eigenvectors are given in parenthesis.If the free metal atom is assumed to have a filled 4d shell containing ten electrons,a closer analysis of the bond overlap populations in table 4 shows that the majorcontributions to the bonding in the C,, isomer may then be considered to occurvia electron donation : (a) from the ligand a, to metal 5s orbital ; (b) from the metal4dXy to the ligand a2 orbital ; (c) from the ligand b , to the metal 5p, orbital ; (4 fromthe ligand b2 to the metal 5py orbital.Of these, (a) and (d) involve all six carbonatoms, whilst (b) and (c) involve only terminal carbon atoms. These interactionsare reflected in the atomic configurations, table 3, the m a i n features being the relativelysmall metal 4dxy, the high 5s, 5px and 5pV and the small 5p, populations. Thesecomments apply to both isomers equally well as the eigenvalues, atomic charges andconfigurations, and the overlap populations are similar for the two molecules.Inboth cases the individual metal-ligand interactions are identical, any differences inthe bonding arising entirely from differences in the ligand-ligand inteiations. Ourcalculations yield a slightly increased metal-carbon charge separation for the '' chair "isomer (table 3). As these calculations include electron interaction terms, the totalelectronic energy cannot be calculated from the orbital energies as in calculationsof the Hiickel type. However, the sum of the energies of the filled orbitals is morI . H . HILLIER A N D MRS. R . M. CANADINE 33negative by 5.83 kcal for the " chair " form. Since this difference amounts toless than 0.1 % of the total energy of either molecule, and recalling the approximatenature of the calculation, we have not attempted to evaluate accurately the requiredelectron repulsion integrals necessary to obtain a more precise value for the energydifference.Alternatively, we have evaluated the sum of the coulomb interactionenergies arising from the calculated atomic charges, which gives the ionic attractionforces to be weaker in the " chair " form by 0.1 kcal. These estimates may becompared with the experimental energy difference of = 0.5 kcal obtained from variabletemperature n.m.r. studies.21FERROCENE AND DIBENZENE CHROMIUMThe general features of the bonding in these molecules has been previouslydiscussed,13-15* 2 5 9 26 so that we shall only note those points peculiar to the presentcalculation.Computations on these molecules were performed using eqn. (3) and(4) in which the nuclear attraction integrals were evaluated by a point-charge approxi-mation. Initial calculations including the metal 3d, 4s and 4p orbitals gave self-ferrocene and dibenzene chromium respectively. These negative populations ofthe virtual orbitals of the metal atoms arose from the arbitrary partitioning of theoverlap charge density by the Mulliken population analysis. A calculation forferrocene by the WH scheme yielded a similar anomalous result.26 These negativepopulations were taken to indicate that the 4s and 4p orbitals do not participate signifi-cantly in the bonding in these molecules, at least in the ground state. The calculationsconsistent metal configurations of d 6 n a 7 p-1*56 s-0.19 and d5.65 p-1.78 s-0.49 forTABLE 7.-sELF CONSISTENT CHARGES AND CONFIGURATIONS OF FERROCENE AND DIBENZENECHROMIUMconfiguration molecule atom chargeferrocene Fe 1 -02 diiZ2,2, dz2ioo, d;;17C -0.13 s1.03, pA.98, pA.12H 0.03 s O * 9 7di benzene Cr 0.54 d-333,2, dz2ioo, d;i4'sl.OO p 2 * O Q , pk.06 chromium C - 0.06 ¶ H 0.01 so.99TABLE 8.-EIGENVALUES FOR FERROCENE AND DIBENZENE CHROMIUM*ferrocene dibenzene chromiumrepresentation eigenvaluc (eV) representation eigenvalue (ev)a1,(M) - 7.71 a&) - 3.76ez,(M) - 11.03 e2&M) - 8.49elu(L7J - 13.47 el .(Ln) - 15.34a2&) - 18.46 el,(L,) - 17-31el g(M,Ln) - 15-52 el,(M,Ln) - 15-97Ql,(L*) - 19.1 1* The major contributions to the eigenvectors are given in parenthesis ; M = metal ; L = ligand.were therefore repeated using only metal 3d orbitals and gave the atomic populationslisted in table 7.The energies and symmetries of the m.o. containing the 18 mostloosely bound electrons are listed in table 8. In only one of these orbitals, theel, of dibenzene chromium does the 0-framework of the hydrocarbon enter to a34 APPROXIMATE MOLECULAR ORBITAL CALCULATIONSgreater extent than the n-system. The overall effect of the a-framework on themetal carbon bond may be assessed by examining the contributions to the overlappopulations (eqn. (6)). For ferrocene, the interactions involving the p x , p,,, pz and sligand orbitals contribute -0.011, -0.011, 0.115, and -0.001 to the total bondpopulation of 0.092, whilst for dibenzene chromium the corresponding figures are-0.012, -0.012, 0.108 and -0.009, the total bond population being 0.075. Thebonding in the representations elg (dxz, dyz) and e2, (dx2-y2, dxy) contributes 0.073and 0.042 to the n overlap population in ferrocene, and 0.049 and 0-061 in dibenzenechromium.In both molecules, a,, orbitals ( 4 2 ) contribute little to the bonding.The predominant mode of bonding therefore involves the ligand 2pn orbitals, thea-framework contributing a small antibonding effect. Such a result would not havebeen deduced from overlap considerations alone for the overlap matrix contains anumber of terms between the metal d orbitals and carbon a-orbitals (e.g., dyz : 2s)which are greater than 0.1. The various orderings of the molecular orbitals in thesetwo molecules, as calculated by other workers, are summarized in table 9.ArmstrongTABLE 9.-M.O. SCHEMES FOR FERROCENE AND DIBENZENE CHROMIUM"et aZ.,26 who considered the ligand a-orbitals, and F i s ~ h e r , ~ ~ both employed theWH method, whilst the remaining authors used various approximations within theformalism of Roothaan, but considered only the ligand n-orbitals. Ionization data,particularly the photoelectron spectrum of ferrocene (ref. (27), fig. 19) may becompared with the predictions of the various calculations. The 2 : 1 intensity ratiofor the components of the highest energy band at ~ 6 . 8 and 7-2 eV, may be ascribedto the e2, and a,, orbitals respectively. The only calculation to predict this orderis that of Shustorovich and Dyatkina,15 our calculation giving the a,, orbital at-7.71 eV to be of higher energy than the ez,.Further bands in the spectrum occurat ~9 eV and between 12-14 eV, but none of the calculations so far performed canconfidently assign their origin.DISCUSSIONThere are, in general, two approaches by which approximate wavefunctions formolecules as complex as those discussed here may be calculated. The first is theempirical Wolfsberg-Helmholz method with its various modifications, which do notspecify a Hamiltonian, and which utilize experimental data and adjustable parameterssuch as F in eqn. (5) to evaluate the elements of the secular determinant. As th1. H. HILLIER AND MRS. R . M. CANADINE 35approximations in such schemes are not well-defined, it is doubtful if they can beimproved to give mare reliable results.The second approach, to work within theHF formalism of Roothaan, introducing only well-defined approximations to reducethe numerical computation, has resulted in a number of computational schemes,including the one we have used here. These methods have the advantage? apartfrom their more rigorous foundation, that they may be further improved in systematicways, particularly by comparison with results obtained from more accuratecalculations.As already observed, the present method reduces to the WH scheme when onIythe leading terms in eqn. (3) and (4) are used. The present scheme, with the inclusionof the additional terms, has given encouraging agreement with experiment for aseries of transition metal halides,28 and also for SO:- and SF6.6 The m.0.energiesobtained on a series of transition metal carbonyl complexes agree well with thoseobtained from photoelectron spectroscopy. While the agreement is not so goodfor calculations on ferrocene by this or other method^,^^-^^* 2 5 * 26 our value ofthe energy of the a,, orbital of - 7.7 eV is nearer to the experimental value of - 7-2eV than the value of -9.8 eV obtained by a WH calculation.26In this paper we have shown the usefulness of approximate 112.0. schemes inassessing the contributions of the various interactions to the metal-ligand bond.Because of the simplicity of our computational scheme, it is possible to handleproblems with a large number of basis orbitals, so that we have been able to assessthe extent of participation of the a-framework of the hydrocarbon ligands in bondingto the metal.Although we find such bonding (as measured by the overlap population)to be relatively weak, it should be considered in more sophisticated calculations.The results we have given should be more realistic than those calculated by the WHmethod, but the accuracy of any m.0. scheme can only be assessed by agreementwith experiment. The differences between the calculated and experimental ionizationenergies of ferrocene from photoelectron spectroscopy measurements show theneed for further development of approximate computational methods.C. C. J. Roothaan, Rev. Mod. Phys., 1951,23,69.E. Clementi and D.R. Davis, J. Comp. Phys., 1966, 1,223.M. Wolfsberg and L. Helmholz, J. Chem. Phys., 1952, 20, 837.C. J. Ballhausen and H. B. Gray, Molecular Orbital Theory, (W. A. Benjamin Inc., 1965).R. F. Fenske, K. G. Caulton, D. D. Radtke and C. C. Sweeney, Inorg. Chem., 1966,5,951.I. H. Hillier, J, Chem. SOC. A, 1969, 878.C. K. Jsrgensen, S. M. Horner, W. E. Hatfield and S. Y. Tyree, Znt. J. Quantum Chem., 1967,1,191.J. A. Pople and G. A. Segal, J. Chem. Phys., 1965,43, S 136.* M. D. Newton, F. P. Boer and W. N. Lipscomb, J. Amer. Chem. SOC., 1966, 88,2353.lo R. M. Canadine and I. H. Hillier, J. Chem. Phys., 1969, 50,2984.l 1 J. W. Richardson and R. E. Rundle, A Theoretical Study of the Electronic Structure of TransitionMetal Complexes (U.S.A.E. Report ISC-830, Ames Laboratory, Iowa State College, Ames,Iowa, 1956).l 2 R. S. Muiliken, J. Chem.Phys., 1955,23,1833, 1841.l3 M. Yamazaki, J. Chem. Phys., 1956, 24,1260.l4 J. P. Dahl and C. J. Ballhausen, Kong. danske. Vidensk. SeEsk. mat-fysiske Medd., 1961, 33,l 5 E. M. Shustorovich and M. E. Dyatkina, Doklady Acad. Nauk S.S.S.R., 1959, 128,1234.l6 H. Basch and H. B. Gray, Theor. chim. Acta, 1966,4,367.l7 J. W. Richardson, W. C. Nieuwpoort, R. R. Powell and W. F. Edgell, J. Chem. Phys., 1962,36,no. 5.1057.J. W. Richardson, W. C. Nieuwpoort and R. R. Powell, J. Chem. Phys., 1963,38,796.E. Clementi, Tables of Atomic Functions, (I.B.M. Corp., 1965).C. E. Moore, Atomic Energy Levels, (Nat. Bur. S t a d . Circ., 467, vol. I, I1 and 111, 1949, 1952,1958)36 APPROXIMATE MOLECULAR ORBITAL CALCULATIONS2o H. Basch, A. Viste and H. B. Gray, Theor. chim. Acta, 1965, 3,458.21 J. K. Becconsall and S. O'Brien, J. Organometal. Chem., 1967, 9, P 27.22 A. E. Smith, Acta. Cryst., 1965, 18,331.23 R. Mason and A. G. Wheeler, J. Chem. SOC. A, 1968,2543 and 2549.24 Tables of Interatomic Distances and Configurations in Molecules and Ions, Chem. Soc., (Spec.2 5 R. D. Fisher, Theor. chim. Acta, 1963, 1,418.26 A. T. Armstrong, D. G. Carroll and S. P. McGlynn, J. Chem. Phys., 1967, 47,1104.*' D. W. Turner in Physical Methods in Advanced Inor.qa,anic Chemistry, (John Wiley and Sons,28 R. M. Canadine and I. H. Hillier, unpublished results.Publ. no. l l . , 1958).1968), p. 74

ISSN:0366-9033    

DOI:10.1039/DF9694700027

出版商:RSC    

年代:1969

数据来源: RSC

摘要:

A Simplified Molecular Orbital Model for Octahedral MetalCarbonyl CompoundsBY WAYNE P. ANDERSON* AND THEODORE L. BROWN?Noyes Chemical Laboratory, University of Illinois, Urbana 61 801, U.S.A.Received 2nd January, 1969A simple model for the x-electron interaction between metal and CO groups in octahedral metalcarbonyl systems has been developed. The basis set employed includes metal dorbitals of x symmetrywith respect to the M-CO bond axes, and two orbitals of x symmetry on each CO, correspondingto the vacant x* orbitals of the CO groups. Each CO is assumed to donate 0.5 electrons to themetal via the cr bond. In compounds of the form M (CO)e-nLn, the ligands L are characterizedby a certain degree of cr donor ability, and by two acceptor orbitals of x symmetry and variableenergy.The model is sufficiently simple to permit exploration of a wide variety of ligandcharacteristics.With the aid of assumptions relating the charge distribution to bond orders, relative CO stretchingforce constants and C0,CO stretching interaction force constants can be calculated as a function ofvariable ligand characteristics. The results support the adequacy of the simple force field commonlyemployed in determining relative force constants in substituted octahedral metal carbonyl systems.Molecular orbital calculations for octahedral metal carbonyl compounds haveshown lm3 a number of important features of the ground state electron distributions.(i) Carbon-oxygen CT bonding in the CO moieties is essentially unaffected by bondingof CO to the metal.(ii) The metal p orbitals, which are the only metal valenceorbitals capable of interacting with both the CT and n orbital sets of the CO groups,are in fact involved almost exclusively in CT bonding. To a first approximationthey may be ignored in considering the 7~ bonding. (iii) the chromium atom inCr(CO)6 carries only a small net charge, which is, however, the result of large chargetransfers between metal and CO in both the CT and 17t bonding systems.There has been a substantial interest in the chemistry of metal carbonyl com-pounds, and in their derivatives with various Lewis bases.4* Physical studies ofthese compounds, and particularly infra-red spectral studies of the CO stretchingmodes in the 2000 cm-' have been employed to assess the natureof the ligand-metal interaction, and to evaluate the effects of substituents.It isnot practicable to carry out detailed molecular orbital calculations on substitutedmetal carbonyls in the same degree of complexity as employed in treating the parent,unsubstituted compound. Accordingly, we have developed a much simpler, moreempirical model for the octahedral systems in which only the essential elements ofthe n electronic network are treated explicitly. With the aid of this model it hasbeen possible to consider the effects of substituents on the bonding between metaland CO, and to develop various comparisons between calculated and observedquantities.DESCRIPTION OF THE MODELThe basis set employed in the calculations consists of a set of three metal orbitals,representing the d orbitals of tZg symmetry in the octahedral point groups, and two* National Science Foundation Trainee, 1965-1968. Present Address, Dept.of Chemistry,University of Delaware, Newark, 1971 1.i to whom inquiries should be addressed.338 MOLECULAR ORBITAL MODEL FOR CARBONYLSorbitals for each CO group or ligand L, fig. 1. Each CO group orbital respresentsone of the two n* orbitals of the CO. Similarly, each ligand L may possess twoorthogonal vacant orbitals of appropriate symmetry as shown in fig. 1. Thus, thereare at most 15 orbitals in the basis set x from which the molecular orbitals areconstructed :4 k = &jCj,* (1)iIn the group 6 carbonyls there is a total of six electrons to be distributed in the mole-cular orbitals formed from this basis set.The bonding 7-c orbitals of the CO groupsare not included in this model at all. They are assumed to form part of the " core "which determines the energies of the six electrons which are being explicitly considered.The justification for this simplification is that the energies of the CO n: orbitals liefar below that of the metal d, orbitals, which are much more closely matched tothe CO n* energies.FIG. 1 .-Schematic representations of basis set employed in x-electron approximation.We employ a Huckel formalism; the secular equations to be solved areC(HiJ(eff)- ekSij)Cjk = 0.jThe diagonal Hamiltonian matrix elements Hi, are assumed to be of the form H,, =Hg-kq, where q represents the net charge on the atom on which orbital i is centred.We employ a value of 2 eV per unit charge for k9-l The off-diagonal matrix ele-ments Hi, are evaluated using the Wolfsberg-Helmholtz approximation :Hij = FSij(Hii + Hjj)/2.(3)The scaling constant F i s given the value 1-75,Although the Q electronic system is not treated explicitly in the calculations, it isnecessary to account in some way for the effect of changes in Q donor ability of theligands on the n energy levels of the metal and the ligands. Therefore, a donorcharacter ot, corresponding to a certain fractional electronic charge transfer to themetal, is assigned to each ligand, L or CO, at the beginning of the calculation. Thisparameter is held fixed throughout all iterative cycles of the calculation.Thus WAYNE P. ANDERSON AND THEODOREqi = Qi + bi,Ni = 1q M = QM- Caiwhere q1 and qM represent the net total electronic chargesrespectively, and Ql and QM represent the correspondingL. BROWN 39(4)on ligand i and the metal,n electronic charges. TheQ value chosen for all CO groups in this work is 0.5 e-, based on the value of 0.47 e-obtained in the more complete molecular orbital ca1culation.l Values of cf forother ligands are varied from 0.25 to 0.75 e- to simulate varying degrees of Q donorability .The diagonal matrix elements for the metal and ligands are given byHM = -4*35-2qM,Hco = - 6.0 -2qcO, (6)HL = ~ , - - 2 q L .The value -4.35 for zero-charge metal is based on the VSIE for an assumed d6configuration on chromium.12 The value of -6.0 for the energy of the vacantx* orbital of the CO is much lower in energy than the virtual 2~ orbital of C0.13In keeping with the usual practice in semi-empirical procedures it is chosen tocorrespond roughly to the difference in energy of the occupied 50 level and theenergy of the (1x45a12n1) singlet configuration l4 in CO.Both values are some-what arbitrary? as is always the case in a calculation in which inter-electronic repul-sions are not explicitly considered. The available data, especially the infra-redCO stretching frequencies? indicate that the CO groups in Cr(C0)6, Mo(CO)~ andW(C0) are similarly bonded. 5a A similar conclusion applies to the mono-substitutedcompounds, e.g., (C6H&P M(C0)5, where M = Cr, Mo, W.lSb It m a y thereforebe assumed that the present model is equally applicable to compounds of thesethree metals.In order to treat other octahedral species, e.g., Mn(CO),X, or V(CO);,it would be necessary only to adjust the HMO appropriately.The overlaps between metal orbitals and the carbon and oxygen 2pn orbitals,S(d, Czpn) and S(d, 02pn) where taken from Schreiner’s results for Cr(C0) 6. l6 Valuesof the d-n* overlaps were obtained from the expression(7)The coefficients preceding the two overlap terms correspond to the coefficients ofthe carbon and oxygen 2pn orbitds in the x* orbital of CO, as given by Ran~i1.l~ Theresults obtained for the relevant overlaps are given in table 1.S(d, n*) = 0.9313 S(d7 C2,,)-0*696 S(d, 02,,)TABLE 1 .-METAL-CARBONYL AND CARBONYL-CARBONYL OVERLAPS IN Cr(CO),s(dJ*); 0.0806S(n:,n*)cis (z,z) 0*0208S(n*,n)Zis(x,y) 0-0591s ( ~ * 7~ *)trans 0.0018a This value refers to the overlap of a single metal orbital with a single CO group.b The overlap of two x* orbitals in which the constituent 2px atomic orbitals are aligned alongC The constituent atomic orbitals are in a common plane, but those of one CO group are normalparallel axes.to those of the other.Gross n-electron populations on the metal and ligands are calculated usingMulliken’s formalism40 MOLECULAR ORBITAL MODEL FOR CARBONYLSwhere the summation i is over all orbitals on centre A, and j i s over all other orbitals.The n charge Q A on A is given by Q A = ZA-PA, where ZA is the number of nelectrons on the neutral centre A (6 for the metal, zero for CO or other ligands).A set of charges Qi are estimated and used as input.The secular equation (2)is solved and a new set of charges QF obtained, using eqn. (8). If I Q:+l- QA I <0401 for all A, the self-consistency criterion is assumed to be met. Otherwise, anew set of input charges, QF2 = 0.9 QA+O.l Qi+l, is used as input, and the cal-culation is repeated. The relatively low weighting of QYi is required to avoidnon-convergency difficulties.RESULTS AND DISCUSSION(i) M(CO)5L COMPOUNDSCharge distributions for a number of hypothetical compounds of the type M(C0)5Lare given in table 2. For M(CO)6 the net charge on the metal is close to that obtainedin previous work from this laboratory for Cr(C0)6.1 When a CO is replaced by aligand which has a much lower n acceptor capacity than CO the charge on the metalbecomes more negative, although a large part of the increased negative charge isdeposited on the remaining CO groups.Thus, for a ligand with no n bonding ability,and a cr donor capacity of 0.5 e-, the charge on the metal decreases from +0-762for M(C0)6 to 3-0.645, a change of 0.117 e-. The total charge change on the fiveremaining CO groups, however, is 0.511 e-. The change in metal charge is inter-mediate between that experienced by the axial CO, 0.183 e-, and each radial CO,0*082e-. The effect on the axial CO is approximately twice that for the radial.For a ligand of given cr donor ability, the presence of n-acceptor orbitals on L doesnot appear to affect seriously the n-electron distribution for a, energy levels aboveabout -2 eV.In this connection, az represents an approximation to the n energylevel of the zero charge ligand rather than of the ligand in the complex. The latterquantity is given by HL, eqn. (6) after convergence.The manner in which the amount of n charge transferred to L varies with the crdonor character of L is revealed by noting the n electron distribution in the fourcases for which a, is - 4-5 and oL is varied from 0.25 to 0.65. The n-charge transferredto L increases by 0.166 in this series, whereas the total for the five CO groups is only0.148. This effect is due to the net charge dependencies of the Hii. The resultindicates the strong synergistic relationship between the Q donor and n acceptorcharacteristics of the ligand.(ii> MULTIPLY SUBSTITUTED COMPOUNDS, M(CO),L6-,The effect of successive replacements of CO groups by non n-acceptor ligandsis a rapidly decreasing positive charge on the metal and increasing negative chargeon the CO groups, as shown by the data in table 3 for ligands of no n-acceptorcharacter and oL = 0.50.The decrease in net charge qM at the metal and theincreased negative charges on the CO groups, are non-linear functions of the numberof added ligands. The results are consistent with the observation that successivereplacements of CO groups, especially by non n-acceptor ligands, are increasinglydifficult to effect.(G) FORCE CONSTANTSCoulson and Longuet-Higgins l8 used simple Huckel theory in arriving at relation-ships between C-C stretching and stretch-stretch interaction force constants iTABLE 2.AHARGE DISTRIBUTIONS AND DIAGONAL AND INTERACTION FORCE CONSTANTS FORM(CO)&SUBSTITUENT PARAMETERSa, =L QL-6.0' 0.50 a - 0.627030000- 2.5- 4 5- 4.5- 4.5- 4.5- 5.0- 5.0- 5.5- 5.5- 5.50.250.500.650.650.250.410.550-650.500.6500.250.65000- 0.047-0.142-0.197- 0.260- 0.308- 0.350-0.438- 0.205- 0.325- 0.58 14M QEO QEO F200.762 - 0.627 - 0.627 16.490.727 - 0.674 - 0.770 16.340.645 - 0.709 - 0.810 16.220.603 - 0.730 - 0.83 1 16.140.610 - 0.725 -0.815 16.170.742 - 0.657 - 0.7230.706 - 0.67 1 - 0.7330.676 - 0.683 - 0.7340.656 - 0.691 - 0.7350.708 - 0.664 - 0.7030.680 - 0.673 - 0.7006.396-356-3 16.286.376.340.829 - 0.614 - 0.666 16.540.778 - 0.633 - 0.670 16.470.707 - 0.653 - 0,662 16-41a These parameters correspond to those for CO42 MOLECULAR ORBITAL MODEL FOR CARBONYLSunsaturated hydrocarbons and quantities derived from the theory.In the simplestapproximation, the stretching force constant is assumed to vary linearly with mobilebond order.lg An expression of the formFco = 6*8P:O+C (9)where F', represents the stretching force constant, P,"" is the total CO x bond order,and C is a constant, was used by Cotton in relating the CO force constants inoctahedral metal carbonyls to bond order. Since charge transferred to CO inour model occupies a n* orbital, thenP,"" = 2- I QcJ2 1.(10)TABLE 3.-cALCULATED CHARGE DISTRIBUTIONS IN MULTIPLY SUBSTITUTED CARBONYLDERIVATIVEScr, "L 9M QL QkO QEOcis-M(CO)4L2co 0.50 0.502 - - 0.827 - 0.92500 0-75 0.367 - - 0.91 5 - 1.013cis-M(CO),L3m 0.25co 0.5000 0.65-4.5 0-4.5 0.25-4.5 0.50-4.5 0.54-4.5 0.65-5.5 0.25-5.5 0.50-5.5 0.650.5400.2950.1510-8560-6620.51 10.4890.4300.7970.6720-597-- 0.079-0.165- 0.305-0.331- 0.408- 0.332-0.515- 0.633- 0.930- 1.098- 1.200- 0.707- 0.806- 0.865- 0.872- 0.885- 0.684- 0.709-0.716Jones 2o has derived expressions for the interaction force constants in metalcarbonyl compounds based on molecular orbital considerations.Assuming thatthe interaction force constants are small (-3 %) in comparison with the diagonalterms, and that x bonding is the major mode of interaction, the CO-CO interactionconstants klk, may be written as ktk = - F i ( & ) k . The quantity (SJk representsthe displacement in co-ordinate ri resulting from unit displacement in r,. Sincetransfer of charge to a CO group results in a change in CO bond order,(sCO,)k = ~'(AQco,)~,k i k = -FiC'(AQc&* (11)The quantity (AQc0Jk is the change in 7c charge on the ith CO resulting from a unitincrease in bond length in the kth CO.We define a unit stretch of COk as one which lowers the n* level of c o k by 1 eV.The required quantities are evaluated by carrying out the molecular orbital calculationfor a series of values of the x* energy level of a given CO, and then assessing theslope of the relationship between the various Qcoi and aco for the kth CO.(AQCOi)khas been found by direct calculation to be linear in the magnitude of variation inthe energy of another CO in this manner. The proportionality constant C' is chosento give a fit with some chosen reference set of dataWAYNE P . ANDERSON AND THEODORE L. BROWN 43Thus, eqn. (9) and (11) provide the basis for evaluating substituent effects onstretching force constants and C0,CO stretching interaction force constants, oncethe disposable parameters have been chosen. The procedure used has been tocompare the calculated force constants with those obtained from a simplified forcefield analysis as described by Cotton and Kraihan~el,'~ and employed in variousdegrees of elaboration by others.21* 22 In this model the M-C stretching forceconstants are ignored, and only Fco and kco,co stretch interaction constants areincluded.The resulting values for the force constants are expected to differ fromthose based on a more complete force field analysis, but the manner in which theyvary with changes in substituents might be expected to follow closely the variationwhich would emerge from more complete calculations. Accordingly, values forC and C' of 5.03 mD/A and -0.317, respectively, have been chosen to give fits forFco and kcis for Cr(CO),. Using these values, the force constants for substitutedcarbonyls listed in table 2 were computed.Graham 24 has presented a simple rule, based upon Cotton's earlier considerations,for determining Q and n parameters for each ligand X or L in Mn(CO),X or Mo(CO),Lcompounds.Mn(CO),CH, is chosen as reference in the manganese series ; changesin diagonal CO force constants in Mn(CO)5X compounds are related to Q and nparameters as follows :AFa = a+2n,AF' = ~ + n .0.5 -jl0.2 5-0000O O00 0000. I 0-3nnnI 1 IFdo - 430FIG. 2.RadiaI-axial CO force stretching constant difference in M(CO)5L compounds as a functionof x-electronic charge transferred to L.The model incorporates the notion that variations in charge at the metal dueto varying Q donor character of the ligand are relayed isotropically to all remainingCO groups.The n acceptor character of X, on the other hand, is experienced totwice the degree by the axial CO as compared with the radial CO. Graham's rulespredict that the difference in axial and radial CO force constants should be a functiononly of the n acceptor character of X. Fig. 2 depicts a graph of the calculated Fdo-Fz0 against QL, the calculated n electronic charge transferred to the ligand. Assumingthat the n parameter is a linear function of the n charge transferred to L. the relation-ship should be linear, and independent of the amount of CT charge transferred. Thisis roughly the case ; for example, for QL = 0, F& - F&-, changes by only 0.02 ou44 MOLECULAR ORBITAL MODEL FOR CARBONYLSof a 0-34 total, when aL is varied from 0.25 to 0.65 e-, a change which represents alarge variation in 0 donor character. The assumption that CJ effects from the ligandsare distributed isotropically to the remaining CO groups thus appears satisfactoryon the basis of the model.We now examine the effects of not providing for a differential degree of n: bondingfor the two kinds of CO in M(C0)5L.Assuming that there is a greater degree of n:bonding to the axial CO, one might expect that this would occasion a slightly greaterdegree of cr bonding from this CO to the metal. This in turn would have the effectof lowering the n* orbital energies on that CO, increasing still further the amountof n: bonding. The effect is not likely to be great, however ; it seems best to ignorethis second-order effect, since other factors, such as polarization of the 0 bond systemcould be responsible for equally large effects of indeterminate direction.The variations in diagonal force constants calculated from the model are too smallas compared with the " observed " variation in these quantities.For example,F& - F& in MO(CO)~NH~C~H~ is about 0.74 whereas the correspondingcalculated quantity for a cr bonding only ligand is about 0.33. Similarly, the" observed " change in Fe0 from the parent carbonyl in this case is about 1.4 mD/&whereas the calculated decrease is 0.62 mD/A. Some of the relatively low responseto ligand change in the theoretical model probably results from using a low value of2eVlunit charge for the charge corrections to the H i .A part of the apparentvariation in force constants calculated from the observed spectra is due to variationsin the M-C a bond strength. Thus, if the net charge on the metal is considerablymore negative after replacement of CO by an amine, for example, the metal-carbonylQ bonds may be weakened. This would result in a lowering of the CO stretchingfrequencies independently of changes in the ?t bonding to CO. As argument againstthe importance of this hypothesis, 0 bonding effects should be isotropic, and thusnot affect the F& - F'o difference. Finally, the assumed relationship between Qcoand stretching force constant, eqn. (9), may be in error. To reproduce properlythe variations in Fco calculated from the observed spectra using the simplified forcefield, the force constant dependence on bond order should be about 2.2 times largerthan provided for in eqn.(9) and (11). With this scaling, variations in both Goand Fto are well reproduced.It is of interest to examine the effect of variation in Fc, with variable cr donorcharacter, for a given value of a,. Fig. 3 and 4 shows graphs of F& and F60 againsta, for three different a,. Also,the dependence of the force constant on Q donor character of L depends on then-acceptor level in L. Angelici and Malone 2 5 have noted that in a series of W(CO),Lcompounds, graphs of F;o and Fto against pK, for the ligands have the same slopesfor amines and phosphines. If pK, can be taken as a measure of 0 donor character,these graphs have the same character as those in fig.3 and 4. The vertical separationbetween the plots for the amines and phosphines 2 5 is substantial for Fzo, and smallfor F&. This fact suggests that there is some n acceptor character operative inthe phosphines, but the similarity in slopes noted by Angelici and Malone poses aproblem if the n-acceptor character of the ligands is thought of in terms of a,. Themore meaningful quantity to consider may be QL, the amount of charge on the ligandin the complex. If points of equal QL are connected together, lines such as thedotted lines shown in fig. 3 and 4 (for Q = 0.205) result. These lines have essentiallythe same slope as the line for the a-only ligand. It appears, therefore, that theexplanation for Angelici and Malone's result is that the quantity of 7t charge residenton the phosphines in the complexes is roughly constant, and not that the phosphinesare incapable of 7c-acceptor character.26The two behave differently as a function of a,WAYNE P.ANDERSON AND THEODORE L . BROWN 45For the interaction force constants the calculations support the rough adequacyof the assumptions frequently made, that the trans interaction force constant istwice the cis value. When L differs drastical!y from CO, e.g., a, not included,0.25 0 . 5 0 0.75mFIG.. 3.Variation in F& with cs donor stength of ligand L in M(CO),L, for different x-acceptorlevels of L.I 1 0!750.25 0 - 5 0OLFIG. 4.-Variation in F& with a donor strength of ligand L in M(CO)5L. for diEerent x-acceptorlevels of L46 MOLECULAR ORBITAL MODEL FOR CARBONYLSoL = 0.50, the ratio k,/k,, is about 1-6, significantly less than 2, as assumed in thesimplest model.The ratio R = k,/k,, where k, is an average of the two kinds ofcis interaction force constants, appears to depend on QL, and is given by an expressionof the form(14)To apply this relationship to analysis of observed vibrational data it is necessary toreplace QL by a quantity which can be determined from the data. One might dothis by computing a n parameter for a ligand using Graham's rules, eqn. (12) and(13), and assuming R = 2. Then it might be assumed that Q L = nL/nco x 0.627.From this, R could be determined, a new set of force constants calculated, and theprocedure iterated to convergence.In his considerations of the interaction force constants in unsubstituted metalcarbonyls, Jones 2o made the simplifying assumption that the n-electronic chargeon the metal remains constant during a stretching vibration of a single metal carbonylgroup.This assumption is based on the notion that as one CO is stretched, leadingto transfer of n-electron density to that CO, there is a simultaneous contraction ofthe other CO groups, with a resultant transfer of n charge to the metal, such thatthe metal charge remains constant.the change in metal x-charge due tostretching of thejth CO, and the change in n charge on the ith CO due tostretching of the jth CO, were computed for several substituted metal carbonyls(table 4). Although is indeed small in comparison with the total chargeon the metal, it is actually larger than the corresponding changes occurring on theCO groups cis to the perturbed one.Therefore the assumption of a constant chargeon the metal is not satisfactory. From the results described earlier, the relationshipsbetween the cis and trans CO-CO interaction force constants are consistent withthe qualitative arguments based on the simple n-electronic considerations first putforward by Jones, and do not depend on the assumption of a constant metal x charge.Stone and co-workers 21 have put forth relationships between the interaction forceconstants in the substituted metal carbonyl compounds which appear to dependon the assumption of constancy of metal n charge in the CO stretching process.The results of our calculations support neither the assumptions themselves nor thegeneral character of the force constant results calculated from the observed spectrausing those results. In particular, in the n electron approximation, the stretchingforce constant for the CO trans to the substituent is always less than for the radialCO groups if L is a weaker 7c acceptor group than CO.However, the present ap-proach does not allow for a trans polarizing effect in the 0 system.R = k,/k, = 2-0.5 (0.627- QL).To test this assumption, values ofTABLE 4.-EFFECT OF A " UNIT DISPLACEMENT " OF ONE CARBONYL GROUP ON THE CHARGE ONTHE METAL IN METAL CARBONYLScompounds UL QM (~~263 (AQM)WCO) 6 - 3.762 0.042 0.067Cr(C0)5LCT 0-25 3.468 0.055 0.075(a, = a) 0.65 3.753 0.058 0-074Cr(CQ5L 0.25 3.492 0.05 1 0-073la = - 4 .5 ) 0.65 3-806 0 . 0 4 9 0.068This research was supported by a grant, GP 6396X, from the National ScienceFoundation47 WAYNE P . ANDERSON AND THEODORE L . BROWNA. F. Schreiner and T. L. Brown, J. Amer. Chem. SOC., 1968,90,3366,5947.K . G. Caulton and R. F. Fenske, Inorg. Chem., 1968,7,1273.N . A. Beach and H. B. Gray, J . Amer. Chem. SOC., 1968,90,5713.T . A. Manuel, Adv. Organometal. Chem., 1965, 3, 181.I. Wender and P. Pino, editors, Organic Syntheses via Metal Carbonyls, (Interscience Publishers,New York, N.Y., 1968), vol. 1.F. A. Cotton, Inorg. Chem., 1964, 3,702.D. J. Darensbourg and T. L. Brown, horg. Chem., 1968,7,959.F. A. Cotton, Rev. Pure Appl. Chem., 1966,16,175.lo L. L. Lohr and W. N. Lipscomb, J. Chem. Phys., 1963,38,1607.l 1 P. C. Van Der Voorn and R. S. Drago, J. Amer. Chem. SOC., 1966,88,3255.l2 H. Basch, A. Viste and H. B. Gray, Theor. Chim. Acta, 1965,3,458.l 3 B. J. Ransil, Rev. Mod. Phys., 1960, 32,245.l4 J. D. Simmons and S . G. Tilford. J. Chem. Phys., 1966, 45, 2965.15a G. Bor, Spectrochim. Acta, 1962, 18,817.15b F. A. Cotton and C. S. Kraihanzel, J. Amer. Chem. Soc., 1962, 84,4432.l6 A. F. Schreiner, Ph.D. Thesis, (University of Illinois, 1967).l7 R. S. Mulliken, J. Chem. Phys., 1955, 23, 1841.Is C. A. Coulson and H. C. Longuet-Higgins, Proc. Roy. SOC. A, 1958, 193,456.'O L. H. Jones, J. Mol. Spectr., 1960, 5, 133 ; 1962, 9, 130.21 J. Dalton, I. Paul, J. G. Smith and F. G. A. Stone, J . Chem. SOC. A , 1958, 1195.22 H. D. Kaesz, R. Bau, D. Hendrickson and J. M. Smith, J. Amer. Chem. SOC., 1967, 89,2844.23 F. A. Cotton, Inorg. Chem., 1968, 7, 1683.24 W. A. G. Graham, Inorg. Chem., 1968,7,319.25 R. J. Angelici and M. D. Malone, Inorg. Chem., 1967, 6,1731.26 R. P. Stewart and P. M. Treichel, Inorg. Chem., 1968, 7,1942.' W. D. Horrocks and R. C. Taylor, Inorg. Chem., 1963,2,723.S. Bratoz and S. Besnainou, J. Chern. Phys., 1961, 34,1142

ISSN:0366-9033    

DOI:10.1039/DF9694700037

出版商:RSC    

年代:1969

数据来源: RSC

摘要:

Correlation between Crystal Structure and Carbonyl-BondStretching Vibrations of Methyl Benzene Transition MetalTricarbon y 1sBY H. J. BUTTERY, G. KEELING, S. F. A. KETTLE, I. PAUL AND P. J. STAMPERDepartment of Chemistry, The University, Sheffield, S3 7HFReceived 20th February, 1969The site and factor group approaches to the interpretation of solid state infra-red and Ramanspectra of metal carbonyls are compared. A factor group analysis of seven arene chromium tri-carbonyls indicates that the effective symmetry of the vibrational repeating unit may be higher thanthat of the crystallographic point group.There has been considerable interest in the correlation between the infra-redspectra of transition metal carbonyl complexes in the 2000 cm-l region and the con-figuration of the molecule in dilute so1ution.l Owing to the general success of thismethod it seemed appropriate to extend the approach to the corresponding solidstate structure and spectra.In the present paper we report the results of a systematic study of a series of methyl-substituted benzene chromium tricarbonyls.These complexes were chosen for thefollowing reasons. (i) The local carbonyl environment in solution approximatesclosely to C3u.2 (ii) The magnitude of interaction constants is such that in solutioiithe Al and E carbonyl stretching vibrations are fairly well ~eparated.~ (iii) Allmembers of the series axe readily prepared and relatively table.^ (iv) Crystal struc-ture data is available for the benzene and hexamethylbenzene compounds.EXPERIMENTALCompounds were prepared by the method of Nicholls and Whiting 4; analytical data aregiven in table 1.Raman spectra were recorded using a Spex 1401 spectrometer with helium-neon laser excitation; finely ground samples, enclosed in capillary tubes, were used. Fre-quencies were reproducible to &l cm-l. The infra-red spectra were recorded using aTABLE 1m.p.benzene chromium tricarbonyl 160-11 ,Zdimethylbenzene chromium tricarbonyl 88lY2,3-trimethylbenzene chromium tri-carbonyl 103-4lY2,4,5-tetramethylbenzene chromium tri- 94-4.5carbonylpentamethylbenzene chromium tricarbonyl 157-8hexamethylbenzene chromium tricarbonyl 227methylbenzene chromium tricarbonyl 77-848analysislit. found calculatedC H C H162-3 ' 50.7 3.1 50.3 2-880 * 52.7 4.3 52.6 3.588-90 55.2 4.5 54.6 4.1102-4 56.5 5.0 56.3 4.798-9 57.7 5.1 57.7 5.2101-2 59.3 6.1 59.2 5.6232 a 60.3 6.0 60.5 6.BUTTERY, KEELING, KETTLE, PAUL AND STAMPER 49Unicarn SP100(Mk 11) spectrometer. Samples were ground with KBr for up to half anhour (to optimize band profiles) before pressing into discs.Frequencies were reproducibleto within IfIO.5 cm-l. Space groups were determined from the systematic absences in equi-inclination Weissenberg photographs. Data for the f h t three layers along one axis wereusually obtained.RESULTSIn table 2 we detail the observed infra-red and Raman frequencies for benzene andsix methylbenzene chromium tricarbonyl species. Also given in this table are thebenzene chronium tricarbonylmethylbenzene chromiumtricarbonyl1 ,2-dimethylbenzenechromium tricarbonyl1,2,3-trimethylbenzenechromium tricarbonyl1,2.4,S-tetramethylbenzenechromium tricarbonylpentamethylbenzenechromium tricarbonylhexamethylbenzenechromium tricarbonylTABLE 2spectra space groupinfra-red frequencies Raman frequencies (cm-1)Pzl,m Cfh 1966, 1879, 1858, 1829, (C13) 1945(~), 1887(~), 1865(~~)P212121 Dt 1961, 1875, 1865, ca.1855(sh), *1938(w). 1892(s). 1878(vw), 1867(vw)I833((=13) 1853(~~), 1835(C13)(*possibility of sh. at high energy)P ~ ~ 2 ~ 2 ~ D$ 1960, 1864, ca. 1860(sh), 1944(sh). 193%~). 1883(vs) 1872(m),1830(C13) 1861(wm), 1845(wm)P Z ~ , ~ C;, 1949, cu. 1935(sh), 1870, ca. 1944(w), 1934(w), 1884(v~), 1871(vs),P21,c C;, 1944, ca.1940(sh), ca. 1946(w), 187O(vs), ca.1854(sh), 1848, 1819(C13) 1851(m)1875(sh), 1864. 1858, ca.185O(sh), CU. 1826(~h)(C13)1867(sh), 1855(m)D i i 1945,1927, ca. 1873(sh), 1854, 1931(m). 1878(m), ca.(at least two unresolvadpeaks) 1822(C13)1854, 1849, ca. 182O(sh,C13),1865(sh). 1857(vs), 1848(s), 182O(C13)Pbca of: 1943, 1925, CU. 1868(~h). 1928(~m), 1873(m), 1 8 6 5 ( ~ ~ ) ,1851(vs), 1846(s), l82l(Cl3)8 1 W 1 3 )probable space groups of the compounds, either as determined by us or, for benzenechromium tricarbonyl and hexamethylbenzene chromium tricarbonyl,6 from dataavailable in the literature.DISCUSSIONIt is convenient to consider fist benzene chromium tricarbonyl. The crystalstructure of this compound has been determined by Bailey and Dahl 5 ; it crystallizesin the P2,1m(C5,) space group with two molecules in the unit cell, each moleculehaving C, site symmetry.There are two approaches which have commonly been used to discuss the infra-redspectra of crystals, the site group and factor group approaches.The former isessentially an isolated molecule approach, the spectrum being considered to arise fromthe vibrations of uncoupled molecules having, however, the symmetry of their crystalsite and not that of isolated molecule. The factor group approach includes inter-molecular couplingand the fundamental vibrating unit is regarded, rather, as composedof all of the molecules in a unit cell.Benzene chromium tricarbonyl allows a ready assessment of the relative merits ofthe two approaches and indicates which method it would probably be most profitableto employ for the interpretation of the solid state spectra of metal carbonyls.Thesite group approach predicts that the reduction in molecular symmetry from CSy(isolated molecule) to C,(crystal) will lead to three coincident infra-red and Rama50 MBTHYL BENZENE TRANSITION METAL TRICARBONYLSbands. However, the factor group approach, whilst also predicting three infra-red andthree Raman bands requires that the two sets should show no coincidences (the factorgroup is isornorphous with the C,, point group and possesses the set of operations i . T,where T is the set of translation operations, which is analogous to the presence of acentre of symmetry in an isolated molecule.The genesis of these predictions areshown in table 3. Comparison with table 2 shows that there are no coincidencesbetween the infra-red and Raman spectra of benzene chromium tricarbonyl indicatingthe inadequacy of the site group approach. For all the cases we consider, the sitegroup approach leads to a prediction of too few infra-red and Raman peaks. Wetherefore confine our attention to the use of the factor group method.TABLE 3factor group ( ~ 2 ~ ) site group (C,) isolated molecule (C3")A' (i.r. and Raman Al (i.r. and Ramanactive) active)A'+A" (both i.r. and- E (i.r. and Ramanand Raman active) active)77Au (i.r. active)Ag (Raman active)Au+Bu (both i.r.Ag+Bg (both RamanThe relationship between modes of the isolated benzene chromium tricarbonyl moleculeThis diagram neglects the mixings which occuractive)active)and the site and factor group predictions.in either site or factor group approaches.We next consider the molecule hexamethylbenzene chromium tricarbonyl, thecrystal structure of which has been determined by Bailey and Dahl.6 The compoundcrystallizes in the Pbca( 042) orthorhombic space group with eight molecules in the unitcell.By arguments analogous to those used above, the factor group approach leadsto a prediction of nine infra-red and twelve Raman peaks which should show nocoincidences (the site group approach predicts three, coincident peaks in each spec-trum). Inspection of table 2 reveals that only five infra-red and five Raman peaks areobserved.It would appear that there are possibly four coincidences (within the limitsof error). However, the presence of a centre of inversion in the unit cell establishes thatthese coincidences are accidental and result from the relatively small magnitude of theintermolecular coupling constants (compared, say, with those in benzene chromiumtricarbonyl). These observations force the conclusion that in the solid state thepresence of a centre of inversion is not incompatible with such coincidences.The discrepancy between the predicted and observed number of infra-red andRaman peaks in the spectra of hexamethylbenzene chromium tricarbonyl may beaccounted For on the basis of a potentially general simplification. A study of thecrystal structure of hexamethylbenzene chromium tricarbonyl reveals that, althoughthe molecules occupy general positions, with site symmetry C1, each Cr(C0)3fragment may, within the limits of experimental error, be regarded as havirg C,symmetry with the mirror plane almost parallel to the b glide plane. Further, thesepseudo mirror planes will be essentially parallel for all of the molecules within aunit cell.Now, the factor group approach is applicable because the wavelength ofthe incident radiation is much greater than the size of a unit cell (for infra-red spectraby a factor of ca. 104), so that vibrations within well-separated unit cells are excited inphase, i.e., the excitation processes are insensitive to the existence of translation of theorder of magnitude of the unit cell.A consequence of this is that '' accidental BUTTERY, KEELING, KETTLE, PAUL AND STAMPER 51mirror planes as apparent in the crystal structure of hexamethylbenzene chromiumtricarbonyl will be indistinguishable from real ones, i.e., spectrally, the molecules willbehave as if they are located at special (C,) rather than general positions. From agroup-theoretical viewpoint this means that the crystallographic unit cell of hexa-methylbenzene chromium tricarbonyl is twice the size of the primitive (inf'ra-red andRaman) unit cell.The effective coincidence between the molecular and glide planes requires a similarrelationship between a twofold axis and the original screw axis along the principalaxis. Consequently, the effective space group is not P b , , ( D ~ ~ ) but Pbcm(Dirf).The newfactor group leads to a prediction of six non-coincident infra-red and Raman peaks.PENTAMETHYLBENZENE CHROMIUM TRICARBONYLThis compound has infra-red and Raman spectra similar to those of the hexa-methyl compound discussed above, suggesting that they may be isomorphous. Thissupposition was confirmed by X-ray single measurements which showed that theyhave both the same space group and unit cells which are almost identical in size. Asthe two compounds are isostructural the pentamethyl derivative presumably has thesame pseudo mirror plane as the hexamethyl compound.METHYLBENZENE CHROMIUM TRICARBONYLThis complex crystallizes in the P212121 space group ( D i ) , the four molecules in eachunit cell occupying general positions.The normal factor group analysis leads to aprediction of twelve peaks in the Raman spectrum, nine of which should also be seenin the infra-red spectrum. However, experimentally, we observe four infra-red andfive (possibly six) Raman bands. If we assume an effective site group symmetry of C,for the methylbenzene chromium tricarbonyl molecules, the appropriate supergroup isDz x C, = D&. Within this group the molecules occupy pseudo special positions.The new factor group leads to the prediction of six infra-red and six Raman bands.This is in accord with the observed spectra (table 2).1,2-DIMETHYL CHROMIUM TRICARBONYLThis compound is isomorphous with methyl benzene chromium tricarbonyl,crystallizing in the P212121 (Dl) space group, each of the four molecules in the unit celloccupying general positions.Moreover, the infra-red and Raman spectra of the twocompounds are similar. Consequently, a D;, supergroup is again indicated.1,2,3-TRIMETHYLBENZENE CHROMIUM TRICARBONYL AND 1 , 2 , 4 , 5 - T E T R A -METHYLBENZENE CHROMIUM TRICARBONYLAs the discussion of these two complexes is similar we consider them together.Both crystallize in the P21,c space group (C;,) with four molecules in the unit cell.Because, compared with the factor group (Cq,) predictions of six Raman and six infra-red bands, they show five and six infra-red absorptions respectively and both mayshow five Raman bands, there is no necessity to invoke a vibrational space group ofhigher symmetry.CONCLUSIONThese analyses suggest the following generalization.When the infra-red and Ramanspectra are both simpler than those predicted by the factor group approach this ma52 METHYL BENZENE TRANSITION METAL TRICARBONYLSimply the existence of eflectiue symmetry operations ia the unit cell, and thus a vibrationalspace group of higher symmetry.However, the above generalization is a necessary, but not sufficient, condition forthe existence of effective symmetry operations, since there could be other reasons whyfewer than the predicted number of peaks may be observed, such as accidental degen-eracy or low intensity. Nevertheless, it should be of value in the interpretation ofsolid-state vibrational spectra. Ultimately, in a solid state vibrational analysis onemust use a vibrational space group, which may, or may not, be identical to thecrystallographic space group (just as for magnetic problems one recognizes theexistence of magnetic space groups).We are indebted to the Science Research Council for maintenance grants (H.J. B.and G. K.) and a post-doctoral fellowship (I. P.). One of us (P. J. S.) is indebted toStaffordshire County Council for a postgraduate award. We are grateful to Prof.D. A. Long and Dr. A. R. Gee for the provision of Raman facilities.see, e.g., J. Dalton, I. Paul, J. G. Smith and F. G. A. Stone, J. Chern. SOC. A, 1968,1195, andreferences cited therein.L. E. Orgel, Inorg. Chern., 1962, 1,25.F. A. Cotton and C. S. Kraihanzel, J. h e r . Chem. SOC., 1962, 84,4432.B. Nicholls and M. C. Whiting, J. Chem. SOC., 1959, 551.M. F. Bailey and L. F. Dahl, Znorg. Chern., 1965,4, 1314.M. F. Bailey and L. F. Dahl, Inorg. Chern., 1965, 4, 1298.E. 0. Fischer and K. CPz!;, ,"hem. Ber., 1957, 90, 2532.forsch., 1958, 13b, 458.G. G. Ecke, US. Patent no. 3,135,776 ; Chem. Abstr., 1964, 61, 13344g.* E. 0. Fischer, K. Ofele, H. Essler, W. Frohlich, J. P. Mortensen and W. Semmlinger, Natur-lo C. N. Matthews, U.S. Patent no. 3,117,983; Chem. Abstr., 1964, 60, 6870f.l1 J. T. Price and T. S. Sorensen, Can. J. Chem., 1968, 46, 515

ISSN:0366-9033    

DOI:10.1039/DF9694700048

出版商:RSC    

年代:1969

数据来源: RSC

摘要:

Interactions of Carbonyl Groups in Compounds ContainingMetal-Metal BondsBY M. L. N. REDDY AND D. S. URCHChemistry Department, Queen Mary College, Mile End, London E.l.Received 20th February, 1969Infra-red spectra of compounds of the type ((CO), . Fe[X . CsYs])2, (X = S, Se or Te; Y = H orF), have been studied in the carbonyl stretching region. Four bands were observed. The inter-actions between the carbonyl groups were assumed to take place via iron 3d orbitals. A model todescribe the spectra was developed in which a three-fold axis was retained at each Fe(CO)3 groupand between the two iron atoms. It was concluded that the carbonyl-carbonyl interaction routebetween the iron atoms was via direct 3d-3d overlap and not via the d or p orbitals of X. Interactionforce constants were calculated and their variation with X and Y discussed. The model was extendedto explain the spectra of related compounds.Compounds of general formula (CO),Fe(X.c6Y5)(x'. c6Y5). Fe . (CO),,(X,X = S , Se or Te; Y = H or F) can be prepared by reacting xZ(c6Y5)2 with ironcarbonyls.' The basic structure is shown in fig. 1. The diamagnetism of these com-pounds may be explained by postulating a " bent " iron-bond. The t2s orbitals (assum-ing, for descriptive purposes Oh symmetry at each iron) on iron atoms are completelyFIG. 1.filled and presumably " back-bond " to the empty anti-bonding orbitals on the carbonylgroups. Since these 3d orbitals will also interact with d andp orbitals on the X atomsof the bridge and also with each other, two possible routes are available for the car-bony1 groups at the ends of the molecule to interact with each other.Carbonyl stretching infra-red spectra are shown diagrammatically in fig.2 foreleven compounds. Quantitative data for these and some related compounds areshown in table 1. Certain interesting features may be noted. The substitution ofphenyl by perfluorophenyl groups causes a general shift to longer wave numbers; thedifference between the average frequency of the two high frequency and the two lowfrequency bands (column A in table 1) is almost constant (-55 cm-I); the highestfrequency band is the weakest, the remainder are all strong; the two lowest frequencybands are often close together; also, four bands are observed.The moleculesbasically have C,, symmetry, in which case five bands might have been expected. On5TABLE 1principal carbonyl stretching frequencies (cm-1)V1 (moderate)2073207320782079208720752073208 1206920752082206620722080206620562070206220602055(CO), . co- ‘kb(co)3low temp. formy2 (v. strong)2036203820412045205820382037204220322041205520292038205 12033201920412026v 3 (strong)200320032009200820232003ZOO0200519962004202420002000201619891978200719962020 1998201 8 19942071 2004v 4 (strong)1995200120042010199219911990200220081988200419691997198819791966L---2042vl+vz2054205520592062207220562055206 12050205820682047205520652049203720552044204020362209 1y3+yq1999200320052006201619971995200519932003201 61994200020101989197320021992198819802or v 32043A555254565659605657555253555560645352525648*this work: A = c1;v2)-c3:v4) M.L. N. REDDY AND D. S. URCH 55the other hand, the molecule might retain much three-fold symmetry so that for thecarbonyl-carbonyl interactions the effective point group would be C3" (if no inter-action across the iron-iron bond) or DSk. In the former two and in the latter, threebands should be observed.I I I II I II2 0 8 0 2 0 6 0 2040 2 0 2 0 2000 1960cm-1FIG. 2.-Carbonyl stretching frequencies.MODELThe main features of the observed spectra may be explained if it is postulated thatthe CO stretching force-constant for carbonyl groups trans to the X atoms is different(k,) from the carbonyl groups trans to the iron-iron bond (k,) and also that local C3"symmetry is retained at each Fe(CO), site.Assuming that it is possible to treat thecarbonyl stretching modes independently of any other vibrations in the molecule, thenthe following normal coordinate functions can be formulated:al = (3)-0.5.c12 = ( 3 ) - 0 ' 5 .a3 = (2)-Oa5.a4 = (21-0-5.a5 = (6)-O*'.a6 = ( 6 ) - O S 5 .[CO( 1) + CO(3) + CO(5)],[ C W ) - COC5)],ICO(4) - C0(6)1,[C0(2) + CO(4) + CO(6)],[2 . CO(1) - CO(3) - CO(5)],[ 2 . C0(2)-C0(4)-C0(6)].al and a2 will both belong to representations a, in CJV symmetry and (a3, a,) and(a4, a6) will have representations e.In order to consider the interactions between the carbonyl groups in a morequantitative way, the approximate method of Cotton and Kraihanzel may be used.First, let us take the interactions of the carbonyl groups attached to the same ironatom and let the perturbation force constant for two carbonyl groups cis to eachother be k,.The appropriate force constants for the normal coordinate functions arexi, a2 : +(k, + 2kx + 6kc) = +ky + +kx + 2kc~ 3 , a4 : H2kX-2kc) = kx-k,U S , a6 : &(4ky+2k,-6kc) = 3k,++kx-kc.Next, carbonyl-carbonyl interactions across the Fe(X.C 6Y 5)2Fe region can be con-sidered. The simplest assumption is that the interaction between any two carbonylgroups across the bridge region gives rise to a perturbation force constant of kd,irrespective of the relative positions of the carbonyl groups. This is equivalent t56 INTERACTIONS OF CARBONYL GROUPSassuming that those orbitals that carry the interactions have threefold symmetry andthat, to a first approximation, for the carbonyl groups, the system has DJh symmetry.Normal coordinate functions for all six carbonyl groups can now be constructedand their force constants written down.The only effect of this '' bridge " interaction is to cause a big splitting of forceconstants for (a, +a2) and (a, -a2), whereas all other force constants are unaltered.Thus, four distinct force constants are found.If the symmetries of the correspondingfunctions are determined it will be possible to determine which will give rise to inf'ra-red active frequencies.Although D3h symmetry was assumed, the actual symmetryof the molecules is CZ0 or less. Table 2 therefore shows which functions would beinfra-red active (indicated by *) in both these symmetry groups. If it is assumed thatthe distortion from DSh to cZu symmetry is small, then a frequency forbidden in D3hbut permitted in Cz, would be expected to be weak whilst frequencies permitted underboth symmetries would be strong. The various stages in the development of thismodel axe summarized diagrammatically in fig. 3.TABLE 2functions irreducible representationsD3h c2uDISCUSSIONThis model explains the observation of four bands in the carbonyl-stretching regionand also why the highest frequency band should be weaker than the others. Theother special features of the spectra may also be rationalized.The difference betweenthe average of the two high and the two low frequency lines is related to the cisperturbation force constant only (3k,). Changing the nature of X or Y should notchange this factor greatly and so it is reasonable that the difference should be more orless constant. The relation between force-constants (mdynes A-') and frequency(cm-l) is k = v2 4-0383 x Thus, k, is 0.31 mdynes A-l, comparable to thevalues given by Cotton and Kraihanzel.' The observation of only four lines in theinfra-red spectra is a direct consequence of the coefficients used for the functions a3---.6.Since these coefficients result from the assumption of local C,,symmetryat the Fe(C0)3sites, the deviation from this symmetry caused by postulating different force constantsk, and k, is slight. This, in turn, suggests that the two low frequency Lines should beclose together as is observedM . L . N. REDDY AND D . S . URCH2 000-2020-57X-Y-rl10 E2 0 4 0 -2 0 6 0 -I \ \\,-\ \ 11- - \ "i,B C D C B- X- YAFIG. 3. -Carbony1 interactions for [ Fe( CO) S( C H 41 2.A, fundamental frequencies vx, vy; B, normal coordinate function frequencies derived from A; C ,effect on B of " cis " carbonyl interaction in an Fe(CO)3 group; D, effect on C of including carbonylinteractions across the Fe(X .C6Y&Fe bridge.We now calculate values for k, and k,,. The corresponding:frequencies have beencalculated in table 1 in columns I and 11. Two possible situations can be envisaged, v,is either greater or less than v,, the former case is given in I, the latter in 11.v x = v 3 + j { ( + ( T ) ] 1 v,+v,v3+v4vy = v, +(v3 - v4),The general formulation of the problem is the same for both I and I1 and we nowdetermine which case obtains for the compounds. It seems reasonable to connect thedifferences between CO groups 1 and 2 and the other CO groups with their positionsrelative to the (XC,Y,) groups. d-Orbitals on the X atoms will interact with the 3dorbitals of the iron atoms and the n-orbitals of the phenyl or perfluorophenyl groups;this will provide a conjugated route for the transmission of effects due to changes ineither X or Y .A simple consideration of orbital overlap suggests that such change58 INTERACTIONS OF CARBONYL GROUPSshould have a more profound effect on the CO groups trans to X than on those transto the iron-iron bond. Since fluorine is an electronegative atom the effect of replacingC6H5 by C6F5 will be to withdraw electrons through the conjugated system, i.e.,backbonding to the carbonyl groups will be reduced and so k, will be increased. Thisresults in the general shift to higher frequencies observed originally. Similarly, whenS is replaced by Se or Te, the 3d orbitals are replaced by 4d and 5d orbitals which willbe less efficient at n-bonding.8 The iron-X resonance integral will decrease and soback-bonding to carbonyl groups will be increased. This will cause k, to decrease andexplains the shifts to lower frequencies observed when the chalcogen increases inatomic number.Variations in X and Y will therefore cause larger changes in v, than v,,.Uponexamining columns I and Il it is often possible to find a trio of compounds in which asY is varied (for constant X), v, is more or less constant but in which v, increases asfirst one and then two phenyl groups are replaced by perfl uorophenyl groups. Thevalues of v, and v, that follow from this simple rule are in bold type in either I or 11.Table 3 shows the corresponding values of k, and k,, for (Fe(C0)3.S(C6Y,)),.TABLE 3bridge componentsFe(C0) ~[bridge]Fe(CO) 3force constants mdynes A-1kx kYWhen two phenyl groups are present v,>v,, the situation is reversed for two perfluoro-phenyl groups and when the compound contains one group of each kind v,-v, SOthat the separation between v 3 and v4 is very small.As evident from table 1 the generalpattern of bonding suggested in this model can be extended to compounds of relatedstructure containing bridging elements other than members of group V1 and also tothe low temperature form of CO,(CO)~.In this model the interaction across the metal-metal bond is measured by thesplitting between the two high frequency lines. Using the equation above, k, for((CO)&S(C6H5))2 is 0.09 mdynes Hi-' and is weaker than the cis interaction as mightbe expected for a long range effect. Changing phenyl for perfluorophenyl has theeffect of reducing kd somewhat. This is in accord with the postulated increase inFe-X x-bonding attendant upon this change that permits an increase in the carbonylstretching frequencies. In the same way as back-bonding from the iron to carbonylgroups is reduced, so also will the bonding between the iron atoms due to 3d orbitalsbe reduced, thus kd will be slightly reduced.The authors acknowledge the cooperation of Dr. A. G. Massey in the preparationof this paper.E. Kostiner, M. L. N. Reddy, D. S. Urch and A. G. Massey, J. Organometal. Chem., 1968,15, 383.H. Hieber and W. Beck, 2. anorg. Chem., 1960, 305, 265.S. Kettle and L. Orgel, J. Chem. Soc., 1960, 3890.H. Hieber and T. Kruck, Ber., 1960,95,2027.B. E. Job, R. A. N. McLean and D. T. Thompson, Chem. Comm., 1966, 895.G. Bor, Spectrochim. Acta 1963, 19, 1209; K. Noack, ibid., 1925.F. A. Cottonand C. S. Kraihanzel, J. Amer. Chem. Sac., 1962, 84,4432.D. S. Urch, J. Inorg. Nucl. Chem., 1963, 25,771

ISSN:0366-9033    

DOI:10.1039/DF9694700053

出版商:RSC    

年代:1969

数据来源: RSC

摘要:

GENERAL DISCUSSIONProf. J. N. Murrell (University of Sussex) said: The paper by McWeeny, Masonand Tow1 presents a theory which predicts that ligand geometries should be relatedto the geometries of their excited triplet states. I think this is based on doubtfulassumptions. In the first place, if one argues from the standpoint of separated-statecontributions to the ground state wave functions, then the molecular orbital expressionis a bad starting point. Thus, the expansion in expression (3) of their paper containsno terms corresponding to excited singlet states of the ligand, A(laLQMg), whereasthese must arise to represent the physical situation of the ligand being polarized bythe field of the metal. Both this state, and ionic states like AM(-L+), will have lowerenergies than the double triplet kf(3@D,@M) which they consider important, and thematrix elements between these and the ground state A(@,,QMg) will probably belarger (the matrix element between the ground state and the double triplet is secondorder in overlap).From a separated state theory, one is more likely to deduce that the ligand geo-metry is close to the geometry of the ligand excited singlet state or the ligand negativeion than that it is close to the triplet state geometry.As excited singlet and tripletstates that are related will have similar geometries it is difficult to use the experimentaldata to differentiate these theories.Prof. R. Mason (Shefleld University) said : In reply to Murrell, it is true that a mol-ecular orbital description would be a bad starting point in describing weakly interact-ing systems, for which a separated-state wave function might be more appropriate.Inthe present situation, however, overlap is considerable and we are particularly con-cerned with the overlap-dependent terms characteristic of covalency effects : the M.O.function has been used as a starting point simply to examine the types of separated-state contributions which arise when it is re-expressed in separated-state language.We think it significant that the function, mentioned by him is overlapindependent and therefore more appropriate in describing polarization effects at longrange.Without making a full variation caldation on a particular complex, we cannot becertain about the relative weights of the various ionic and locally excited contributionsreferred to in our expansion, but our view is that the ionic functions will predominatein complexes where the metal is in a high formal oxidation state, while the neutralexcited state functions will be important in complexes where the metal has a lowoxidation state (0 or + 1, say).As for the relative importance of singlet and tripletexcited states, it is true that the geometry of the excited ligand would be very similar :we think the triplet is most important for two reasons : (i) the expansion in powers ofoverlap gives the triplet term three times the weight of the singlet, and more signifi-cantly, (ii) when fragments in triplet states are coupled to a singlet, overlap is accom-panide by a stabilization effect (which would enhance their weights in a variationalcalculation), whereas fragments in singlet states are mutually repulsive as overlapincreases (indicating a diminishing variational importance).The structural results on several butadiene and oxygen complexes support ourbelief that the ionic functions are relatively unimportant.Thus, for butadiene,560 GENERAL DISCUSSIONCooper and McWeeny give the following n-bond orders (complete CI (M.O. andV.B.)) for the lowest triplet state and anion respectively.2 0.727 3 2 0-580 30.380 0.638 0.638 /7 I + - - n 0=148--- 4 q--0 . 3 8 004091--* 4Bu- 3 B ~ *There is no doubt that a number of butadiene complexes have geometries whichapproximate more nearly that of 3 B ~ * than of Bu- or Bu2- ; this extends even to the" squeezing " together of atoms 1 and 4 in the complexes (note the positive long-range bond order) compared with their separation in cis-butadiene.Again the bondlengths of the 0- and 02- ions are well characterized as 1.31 and 1.48 A respectively ;these are distances which are reproduced in a variety of bridged binuclear cobalt(II1)complexes but the 0-0 bond length of 1.65 A in the [(Ph2PCH2CH2PPh2)2Lr02]cation must imply, in our view, that the electron distribution in the ligand is muchmore nearly that of the 3C; neutral excited state than of the anion. In M.O. termsthese conclusions suggest that a population analysis would show the coordinate Iigandto be roughly n37r*3 rather than n47P3 or 7 t 4 ~ * 4 .Mr.A. F. Orchard (Oxford University) (partly communicated): I believe that theapproximate molecular orbital method employed by Dr. Hillier and Mrs. Canadine,whilst undoubtedly a considerable advance on the Wolfsberg-Helmholz approach,is unsatisfactory in its present form. I am particularly concerned that the quantitycfu (in eqn. (3) of the paper) is approximated simply as an atomic IP (or VSIE). Ifthe model is to be related to the LCAO-MO SCF theory in a reasonably well definedmanner then E ~ , must have the general formwhere = (iu I 7'- Vlore I iu), and nku is the effective occupancy of the atomicorbital Xku. Expressions of the type (1) may be obtained by applying either zerodifferential overlap (ZDO) or Mulliken-style approximations to the Roothaanequations.Such procedures are highly approximate, but unless they are pursuedconsistently the simple m.0. treatment will suffer all the ambiguity inherent in Huckelmet hods.According to eqn. (l), Eiu is a one-electron energy (for the a.0. xi, in the valencestate of the atom u) which includes the self-repulsion term, +nfuJlu,iu. In other words,for a closed-shell molecule the n,, electrons occupying xfu in the valence state mustbe treated as +niu of each spin type, a and p, and these repel each other. Now thispoint is a familiar one [e.g., ref. (6)], but deserves additional emphasis here, particularlyin connection with the reported ferrocene calculation.Suppose Xi, belongs to the atomic I-subshell a, which is symmetrically occupied inthe valence state.With modification of notation (and dropping the u subscript),Cooper and R. McWeeny, J. Chem. Phys., 1968,49,3223.Churchill and Mason, Adv. Orgunometallic Chem., 1967, 5, 93.e.g., Gerloch and R. Mason, Proc. Roy. SOC. A , 1964, 279, 170. Churchill and R. Mason,Proc. Roy. SOC. A , 1966, 292, 61.4 J. A. Pople, D. P. Santry and G. A. Segal, J. Chem. Phys., 1965,43, S129.5 J. W. Richardson and R. E. Rundle, A Theoretical Study of the Electronic Structure of TransitionMetal Complexes, U.S.A.E. Report ISC-830, Ames Laboratory, Iowa State College, Ames,Iowa, 1956).6 R. E. Watson and A. J. Freeman, Phys. Rev. A , 1964,134, 1526GENERAL DISCUSSION 61the average one-electron energy iswhere o, is the degeneracy of the subshell, and where Oab = (.Iik- Kik) is a fullyaverged two-electron interaction.Now a space and spin randomized IP (cf. V0IP)lis a different quantity from If we neglect orbital rescaling effects (as in Koop-mans' approximation, for example) then the fully averaged atomic IP from thesubshell a isIy[na,nb.. .] = - ,ye- (na - lIeixa - nbeab* (3)b#aAt this level of approximation, we therefore haveE , = - 1; + [ 1 - (na/2ua)]Oaa, (4)[cf. ref. (2)) Thus, E , may be identified with -lav only when the subshell a is fullyoccupied.In the general case, the second term in eqn. (4) is by no means trivial. Its magni-tude will often be comparable with that of the penetration integrals that Hillier andMrs. Canadine are much concerned with (eqn. (3) of the paper).There will besimilar difficulty with the off-diagonal elements of the simplified Fock matrix.The role of the self-repulsion terms in the valence state is in fact more complexthan my eqn. (4) indicates, because the degeneracies of the atomic 2-subshells arelifted in sufficiently low molecular symmetries. The concomitant self-consistenteffects must then be carefully taken into account. Let us consider the ferrocenecalculation reported here. Arguing directly from eqn. (l),Ed(eZg)-Ed(alg) = '&nal,(FO- 16F2-39F4)-+ne2@(Fo - 36F2 61F4) - 5neIn(F2 - 5F4), ( 5 )where the Fk are the usual Slater-Condon parameter^.^ If we now use the numericalresults of Hillier and Mrs. Canadine, with the same choice of metal 3d wave-functions,we obtain &d(e2g)-&d(~1g)N3'8 eV.The ad hoc adjustment of the reported m.0.eigenvalues then leads to an e2,-a,, energy separation of about +Om5 eV, and thee2g molecular orbital energy becomes about -7.2 eV. These figures are in moresatisfactory agreement with the photoelectron spectrum of ferrocene determined byTurner.4 A similar consideration of the chromium dibenzene results also suggestsa reversal of the relative positions of the mainly-metal ez, and a,, levels. We hopeshortly to measure the photoelectron spectrum of chromium dibenzene in order toexamine this point.The above arguments are incomplete, and the numerical features insubstantial :and, the excellent agreement with the ferrocene photoelectron data is fortuitous.However, it does seem likely that the self-repulsion terms represent an importantfactor determining the relative energies of the eag and a,, molecular orbitals.Itwould be interesting to see what effect these additional terms produce in a repeat ofthe full self-consistent calculation. Certainly my own experience with calculationsH. Basch, A. Viste and H. B. Gray, Theor. chirn. Acta, 1965,3,458.Press, 1963).P. Day (Wiley, 1968).2 L. C. Cusachs and J. W. Reynolds, J. Chem. Phys., 1965,43, S160.3 E. U. Condon and G. H. Shortley, The Theory of Atomic Spectra, (Cambridge University4D. W. Turner in Physical Methods in Advanced Inorganic Chemistry, ed. H. A. 0. Hill an62 GENERAL DISCUSSIONon simple transition-metal complexes is that the self-repulsion energies are a crucialaspect.For example, without their inclusion, one cannot reproduce the subtleeffects manifest in are cent, “ exact ” LCAO-MO SCF treatment of hypotheticalsquare-planar NiFz- to which Hillier has referred.A much improved methodology is needed if an approximate molecular orbitaltheory of the extended Huckel type is to contribute seriously in areas such as metallo-organic chemistry. Apart from the problems discussed above, some attention mustbe given to neutral penetration integrals, which are particularly significant in highlycovalent molecules. These are ignored in the treatment advocated by Hillier andMrs. Canadine. And there clearly remain difficulties with regard to the satisfactoryhandling of diffuse valence a.0. such as transition metal (n + l)s,p or d orbitals.Similar problems are apparent in the use of 3d valence orbitals for atoms belongingto the second short period.Dr.I. H. Hillier and Mrs. R. M. Canadine (University of Manchester) (communi-cated) : Orchard has suggested that our computational scheme may be improved bythe introduction of self-repulsion terms into the Etu of eqn (3). Any semi-empiricalscheme that is to retain a large measure of computational simplicity and thus besuitable for the treatment of large inorganic systems, must contain a number of grossapproximations and may be criticized for their introduction. However, the usefulnessof such schemes should be judged by comparison with more accurate calculations andwith experimental data for a wide range of compounds. For a series of transitionmetal halides, and carbonyl complexes we find our method gives encouraging resultswhere such comparisons are possible, although it is naturally not expected to repro-duce the subtle effects of ab initio calculations.Such comparisons with ab initiocalculations are limited by the small number which have been performed. For thisreason we have underway in our laboratory such calculations on a number of inorganiccomplexes, which will hopefully suggest ways of improving semi-empirical schemes.Such improvements may be attempted by reducing the approximations involved, butat the expense of additional computation which must be weighed against the improve-ment obtained in the wavefunction. It would thus certainly be interesting to see ifOrchard’s suggestion does lead to a significant improvement for a range of molecules.The other problems referred to by him, such as the treatment of the penetration terms,and the difficulty of handling diffuse valence atomic orbitals are well-known and donot require further comment, except to suggest that ab initio calculations referred topreviously should throw light on their correct inclusion.Prof.J. N. Murrell (University of Sussex) said: The C=O stretching bands ofcomplex carbonyls can be fitted adequately by assuming a harmonic force field withdiagonal C=O force constants and off-diagonal C=O, C=O interaction constantsk’. However, if these empirical force constants are to be interpreted in terms ofvalence calculations, then a more precise definition of k’ is required.In the system, O * L M - k b , the effective k’ between the carbonyl vibrationsdepends on all the interaction constants k14, k12, k34, k13, k24 and k23, where wedefine k14 = (d2V/dRlaR4) etc.The precise form in which these contribute tok’ depends on the G-matrix, i.e., on the masses of the atoms and the geometry. Tointerpret the experimentally determined k’ as k14, as has been done by Anderson andBrown, is an oversimplification.‘H. Basch, C. Hollister and J. W. Moskowitz, Application of MotecuIar Orbital Theory toProb[ems in Inorganic Chemistry, 154th Meeting, Amer. Chem. SOC., (Chicago, Illinois, 1967).A. F, Orchard, unpublished resultsGENERAL DISCUSSION 63Dr. D. S . Urch (Queen Mary College) said: Unfortunately reference to the workof Hayter was omitted from our paper.The relevant data are :v1 v2 v3 v4(Fe(C0)3P(CH3)2)2 2050 210 1977 19622043 2004 1973 1961(Fe(C0)3BrP(CH3)2} 2 2080 2038 2010 - {Fe(C0)3AS(CH3)2)2( Fe (CO) 3 €3 r As (CH 3) 2 1 2 2072 2034" 2008{Fe(C0)31As(CH3)2) 2 2066 2028 2006* given as2054 in solution but 2038 in mull. It seems reasonable to suppose that 2054 is a misprintfor 2034.The four frequencies reported for the first compounds are in accord with the modelpresented. In the last three compounds the Fe-Fe bond has been broken byhalogenation and the only remaining route for Fe(CO)3-Fe(C0)3 interaction isacross the chalcogen bridge. Under these circumstances only one high frequencyline is observed,- confirming that theformation of two. high-frequency linesis due to interactions associated with adirect Fe-Fe bond.The differencebetween the average of v3 and v4 andthe single high frequency line is stillca 50-55 cm-l. This is the magnitude ofsplitting expected in an isolated Fe(C0)3group, and is also strong evidence thatthe low-frequency splitting is due only tothe inductive effect of the bridge groups.Dr. J. R. MilIer (Essex University) said :The infra-red spectrum of [Fe(CO),SEt],in fig. 1 in cyclo-hexane solution wasmeasured by K. Edgar at ManchesterUniversity. Unlike the spectra reportedby Urch, it shows five strong bands anda number of weak ones. We thoughtinitially that the strong bands would bethe C-0 stretching fundamentalallowedby the Czu selection rules and that theweak bands could be attributed tonaturally occurring 3C0 species.However, in view of the commentsmade by Bor, it seems at least as likelythat this spectrum is of a mixture ofsyn-anti isomers.One of the maincm-l2100 2 000 1900I 1 IFIG. 1 .-Infra-red spectrum of [Fe(CO),SEt], inthe 2000 cm-' Region (cyclohexane solution).features of interest in these compounds is the interaction between C-0 oscillators atopposite ends of the molecule; Urch has suggested that this interaction may be thesame between any pair of C-0 groups on different iron atoms.R. G. Hayter, Inoi-g. Chem., 1964, 3, 71464 GENERAL DISCUSSIONOur spectra, if they be of one pure isomer, show that there is a detectable splittingof the low frequency bands into three components ; if they be of isomeric mixtures,then they show that the isomers do have different frequencies.Both of these possi-bilities, we would suggest, lead to the conclusion that the CO-CO interaction isdependent upon the path across the molecule. In order to clear up the interpretationof these spectra it is desirable that measurements and proper assignments should bemade on the naturally occurring 3C0 bands undoubtedly present, and the appropriatecalculations made.Dr. D. S. Urch (QueeB Mary College) said: I find it hard to understand how thespectrum presented by Miller could be that of a pure compound, and would concurwith his opinion that it must be of a mixture. A mixture of syn- and anti-isomers(at least?) seems reasonable, bands A , C, E, G (and possibly P) could comprise onespectrum and BD together with other bands in the EFG region could be the spectrumof another isomer.The latter would then be related to the former by a shift to lowerfrequencies of about 3-5 cm-I. As suggested in the reply to Greenwood there is noreason why a change in conformation should not manifest itself as a small changein the inductive effects of the XC6Y5 groups.It is not clear whether P must be regarded as due to an impurity or associatedwith the BD spectrum or whether ACEFG is a true five line spectrum. If the latteris found to be so then this may be easily understood as due to some deviation fromlocal trigonal symmetry by the carbonyl groups in X(CO),.Thus, neither thepossibility of slight shifts due to conformational isomers nor the observation of afive line spectrum is reason for doubting the basic validity of the simple theorypresented the original paper. It is interesting to observe that ACEG fits quite wellwith this theory.King has reported the infra-red spectra of two isomers of the related compound,(Fe(C0)3SCH3)2 :(i) 2085 2050 2000(ii) 2075 2040 2000 1995.A slight conformational shift can be observed as well as a slight change in the inductiveeffect due to substituted chalcogens, as suggested above the explanation of Miller’sresults.Dr. G. Bor (University College, London) said : The spectrum of [Fe(CO),SEt],given in Miller’s remark, and Urch’s proposed interpretation to it, prompts somefurther comments.Based on the published results on the i.-r. spectra ofpure ant- andsyn isomers of the same compound and on Cl80 enrichment studies with the ana-logous methyl comp~und,~ the following straightforward assignment of the publishedspectrum may be made :A, C , E, G : fundamental C-0 stretches of the anti-isomer,B, C, F, G : fundamental C-0 stretches of the syn-isomer,D, I, J (wavenumbers for the CH,-compound : 2 028.2, 1 956.4, and 1 943.3 cm-I,respectively 2, : 13C-0 bands of the anti-isomer.R. B. King, J. Amer. Chem. SOC., 1962,84,2460.G. Bor, J. Organometal. Chem., 1968, 11, 195.P. W. Robinson, private communicationGENERAL DISCUSSION 65The C frequencies of both isomers coincide and the G bands are separated onlyby 2 cm-1 resulting in the broader shape of this band if the spectrum is recorded on the“ natural ” mixture of the isomers.The 13CO-satellite band of A is overlapped inthis spectrum by the highest C-0 fundamental B of the syn-isomer. The muchlower concentration of the syn-isomer does not enable one to detect the isotopebands of this isomer from the published spectrum.Dr. J. R. Miller (Essex University) said: I would suggest that the term “forceconstant ” be reserved for quadratic potential coefficients of the general type[C3’V/drtC3rjl0, and that this term should not be used for the type of experimentalparameter under discussion. A convenient naming for these latter quantities wouldbe CK parameters (after Cotton and Kraihanzel).Concerning the relationship between these experimental parameters and the trueforce constants,05 4if one assumes that the vibrational force-field of a carboayl molecule is harmonic(which is open to criticism), then the true force constants for stretching co-ordinatesin a symmetric dicarbonyl molecule, as defined in the figure, may be related to theCK parameters in the following way :~ C I C = kz + (2*3/7)ki - @/7)k12,k& = ki, 4- (2’3/7)k23 4- (8/7)k13,where kCK and k& are the C-0 stretching and interaction parameters respectively.These equations are derived with the knowledge that so far as C-0 stretchingvibrations are concerned, there is negligible mechanical coupling across the metalatom (this is not true of M-C stretching vibrations).This result can be extendedto any system containing one set of symmetry-equivalent carbonyl groups and theinteresting feature is that, for a given interaction parameter, the component forceconstants are all of the off-diagonal type and are those which follow the same paththrough the molecule as the interaction parameter concerned. Thus in the octahedralcase,k&,(trans) = k,,(trans) + (2.3/7)kz3(trans) + (8/7)k13(trans),khK(cis) = k,,(cis) + (2*3/7)k,,(cis) - (8/7)k13(cis).Dr. G. Bor (University College, London) said: I would comment on the paper ofReddy and Urch. Our observations are based on the spectra of the same type ofcompounds as well as the cobalt complexes Co,(CO),(RC = CR’)2 having strictlyanalogous structures to the RS- (RSe- or RTe-) bridged compound^.^ Weconfirm that the organothio-bridged Fe2(CO),(SR), compounds have in fact onlyfour C-0 stretching bands, even if R = alkyl (having usually narrower bands andthus giving better resolution), or if the syn and anti forms are studied in pure formG.Bor, J. Organometal. Chem., 1968, 11, 195.G. Bor, Chem. Ber., 1963,96,2644.W. G. Sly, J. Amer. Chem. SOC., 1959,81, 18.L. F. Dahl and C. H. Wei, Inorg. Chem., 1963, 2, 328.G. Bor, Proc. Symp. Co-ord. Chem., Tihany (Hungary), Sept. 1964, p. 361.66 GENERAL DISCUSSION(cf. fig. 1 in ref. (1)). On the other hand, the acetylenic dicobalt hexacarbonylshave$ve C-0 stretching bands if the spectra are taken with good resolution (cf.fig. 1 and table 1 of ref. (2), and fig.3 of ref. (5)). The possibility that the lowest-frequency band of the Co,(CO),(acetylene) type compounds, being always weak,may be a 13C0 isotope band has been excluded by 13C0 enrichment studies. Weneed an explanation to account for all of these observations.If we construct the F matrix elements (in a factored off C-0 stretching model)without any a priori assumptions or constraints between the interaction constantsthen we have two direct and four indirect interaction (“ perturbation ”) constantsthe latter ones acting between the two halves of the molecule (ijk acting betweenthejth and kth CO ligand, according to the numbering scheme of the authors).Then if i36 # i34 there is a “splitting” between the a, and b, modes. Andfrom similar studies with other binuclear carbonyls these indirect interactions betweenCO ligands bound to different metal atoms are mainly distance-dependent and acis or cisoid type of interaction is usually 0.08 to 0.20 mdyn/A higher that the transinteraction.This difference results thus in a separation b2-a2 = 5-12 cm-’. Butsince the a, species is i.-r.-forbidden this does not appear in the spectrum of eitherof these types of compounds.On the other hand, the authors have assigned the entire splitting due to “ kd ”to the higher roots of the two second-order species a, and b, which generally is trueonly if iI2 = i14-again this very unlikely. But since i34 and i12, on the one hand,and i14 and i36, on the other, may be expected to have nearly equal values, thev2(al)-vv,(b,) separation may be very near to the value of the v,(b,)-v,(a,) splitting.Thus the alternative model is the following : the local symmetry of the M(CO)3groups is not C3, but C, which should give rise to a three-band spectrum, the magni-tude of the lower a’(v,) -v3(a”) splitting reflecting the deviation from the threefoldsymmetry.The secondary splitting due to the ( 0 C ) M . . . M’(C0)‘ coupling islarge (32-40 cm-l) for the v , ( L z , ) - v ~ ( ~ ~ ) separation, and smaller (7-18 cm-’) for thetwo lo w-frequency separations.Our view is that the number of five bands (being normal for point group C2, andobserved experimentally for the alkyne-bridged cobalt complexes) is reduced to fourfor the RS-bridged compounds by the overlapping of the weak band v,(bl) throughthe strong v6(bZ).If the (hypothetical) v,(a’) -v3(a”) splitting of the monomericM(CO)3 unit is small as compared with the “ dimerization splitting ”, this overlappingdoes not occur. Clearly the conditions for this case are the better given the nearerthe local symmetry of the M(CO)3 groups is to C3,. This may be the case for thecobalt compounds. If, on the other hand, the two types of splittings are nearlyequal, v5 should be overlapped by vg, or by the v 2 f v 6 doublet, which seems to bethe case with the iron compounds.Dr. D. S. Urch (Queen Mary College) said: I must disagree with the rather complexinterpretation of the infra-red spectra proposed by Bor. Let us consider the possi-bility that different carbonyl-carbonyl perturbation force constants may exist due todifferent interaction routes across the “ bridge ” ; these will then be, assuming C,,symmetry, and numbering the carbonyl groups as in fig.1 :3-4 = 5-6 = k,,4-5 = 3-6 = kp,1-2 = k,1-4 = 1-6 = 2-3 = 2-5 = kgGENERAL DISCUSSION 67Then the normal coordinate functions in table 2 have the corresponding forceconstants :a1 + a2 6-1 *(2ky +4kx+ 12k, + 4k, + 4ks + 8ka + 2ky),a1 - 6-' (2ky + 4kx + 12kc -4ka-4ka - 8ka-2ky)ya3 + 12-1 (8ky + 4kx- 12kC-4ka - 4ka + 16ka - 8ky),as + a6 12-1 (8ky +4kx- 12k,+ 4ka +4ka - 16ka + 8ky),a3 - a4a5 -a64-1 (4kx - 4kc - 4k, -E 4kp),4-1 (4k,- 4k,+4kU-4kp).Furthermore there will be some interaction between the two coordinates of symmetryal and the two coordinates of symmetry 6,.This has been ignored in the above tablebut the magnitude of the cross-term may not be negligible :for a,, 6-1 (4ky-4k,-4k,-4ka+4k,+4kd),and for bl, 6-' (4ky - 4kx + 4k, + 4ks - 4k, - 4kd).If all the " bridge " perturbation force constants are different and vary both withchalcogen and with the group attached to the chalcogen and with the relative orienta-tions of the groups attached to the chalcogens, it is difficult to see how any regularpattern could emerge from the infra-red spectra of a wide variety of compounds.And yet remarkably regular and constant features are observed. In all the four linespectra discussed above it was observed that, no matter what the compound, thedifference between the average of the two high frequency lines and the average of thetwo low frequency lines was a constant (-55 cm-').This is clearly associated withsome invariant feature of the molecules, e.g., an X(CO)3 group. It is of interest tonote that the splitting in carbonyl stretching frequencies associated with three carbonylgroups cis to each other is about 55 cm-l.An examination of the force constants for al +a2 and al -a2 in both the " simple "and " complex " theories show that the splitting here is associated with " bridge "interactions. However, in its " complex " form the theory suggests that the low-frequency lines will also be shifted, and as can be seen from the force constants, in amost complicated way. It is difficult in this case to see why the spectra should showa total of four and not five lines and why any simple relationship about averages offrequencies should ever hold.It therefore seems necessary to suppose that the low-frequency lines are not shifted by interactions across the " bridge " and this in turncan be achieved by postulating that ka = kp = ky = ka. I am surprised that thisis so. It is interesting to note that if all the " bridge " perturbation force constantsare equal then the coupling term between the two al and the two bl coordinatesreduces to (2/3) (ky-k,.). This is very small indeed (table 1) and explains why thiscoupling was safely ignored in the original paper.It should be remembered that the reason why the low frequency lines are notshifted by " bridge " interactions in the simple theory is because of the coefficientschosen for the carbonyl normal coordinate modes at each X(CO)3 site.Thesecoefficients depend on local C3" symmetry. Thus any deviation from this localsymmetry would result in slight shifts in the low frequency region and in particularmight well result in a weak line due to (as-a6) being observed (coincident witha,+&, in the simple theory). If kx>ky, then a g - c t g might still not be observedsince it would be displaced to lower frequencies in the region of strong absorptiondue a3+a,. On the other hand, when k,<ky, then a5-a6 would be the lowes68 GENERAL DISCUSSIONfrequency of all and might be most easily discerned as a shoulder or as an independentband.Thus the simple model presented above can easily be adapted to explain thepossible observation of five bands in some compounds but this does not affect thetype of bonding proposed in the bridge region but merely reflects the degree ofdistortion from local CSv symmetry at each X(CO)3 site.Dr.G. Bor (University ColZege, London) said : The spectra of [C1*0] enrichedsamples of the A isomer of di-p-methylthiodi-iron hexacarbonyl, Fe,(CO),(SME),,have been studied recently.2 The isotopic frequencies observed cannot be explainedon the basis of the model suggested by Urch and Reddy; moreover, they led to thedetermination of the two unobserved frequencies (species A2 i.-r. inactive ; speciesB1 obscured by the lowest observed strong band) which do not coincide with theCO stretching frequencies observed directly.This offers additional evidence forthe interpretation given in the discussion.Prof. N. N. Greenwood (Newcastle upon Tyne) : said With regard to Urch’s paper,in addition to the diamagnetism there exists strong evidence from Mossbauer spectro-scopy that compounds of the type shown in fig. 1 have a bent metal-metal bond, thusmaking the co-ordination symmetry around each iron atom octahedral. Thisevidence comes from the quadrupole splitting of such compounds which is in therange 0.65-1 a05 mm sec-l typical of &coordinate low-spin iron complexes ratherthan in the range 2.0-2.6 mm sec-l typical of 5-coordinate low-spin iron.There is also a question concerning the configuration of the bridging groupsand its effect on the symmetry arguments used in the paper. Geometric models ofsuch compounds show that the pendant groups on the bridging chalcogens can adopteither the cis or trans configurations and this can have a pronounced effect on severalphysical properties, e.g., colour, thermal stability etc.We have found that theMossbauer spectrum of cis-(OC),Fe(SMe),Fe(CO), has 6 = 0-2S5 and A = 0.895mm sec-l, whereas the trans form has 6 = 0.279 and A = 1434 mm sec-I. Otherexamples could be quoted. Were the compounds studied by Urch cis or transisomers, or were they mixtures of the two forms?Dr. D. S . Urch (Queen Mary CoZZege) said: May I thank Greenwood for pointingout that Mossbauer data also provides excellent confirmation of the structure proposedin fig. 1. Typical values of A, the quadrupole splitting constant, in Fe2(CO),*s&&)(c6Y5), are : Y = Y’ = H, 1.01 ; Y = Y’ = F, 1.33 ; Y = H, Y‘ = F, 1.16,suggestive of octahedral coordination about the iron (ref.(l), our paper).In reply to Greenwood’s question about the conformation of the groups attachedto the bridging chalcogen atoms the answer is that we do not know. In the compound[Fe(C0),S(C6F,)I2 there is some evidence (n.m.r.-ref. (l), our paper) for the exist-ance of syn- and anti- isomers in the ratio 1 : 6, but for the most part extensive work,lin an attempt to resolve syn- and anti-isomers, was without result. Thin layerchromatography of reaction products typically gave two or three spots, only one ofwhich corresponded to the required compound.The model discussed in our paper describes only one facet of bonding in themolecules [Fe(C0)3XC6Y5]2 so that it may well be there are differences in manyR.B. King, J, Amer. Chem. SOC., 1962, 84,2460.P. W. Robinson, Dept of Chemistry, University College, London, private communication.T. C . Gibb, R. Greatrex, N. N. Greenwood and D. T. Thompson, J. Chern. SOC. A , 1967,1663GENERAL DISCUSSION 69physical properties of the syn- and anti- isomers. The contention is that part of theelectronic structure which is relevant to the transmission of carbonyl group stretchingeffects involves mainly the interaction of iron 3d orbitals and is not greatly perturbedby the conformation of the groups attached to the chalcogens. This does notpreclude the possibility of slight conformational effects manifesting themselves via apurely inductive effect (see also later my reply to Miller).Dr. A.J. Rest (Cambridge) (communicated) : We have obtained the infra-redspectra of a number of transition metal carbonyl compounds in argon matrices(dilution 1 in 200) at 15°K. We find that the matrix isolated spectra, unlike thesolid state spectra described by Kettle, are analogous to those obtained in the gasphase. The absorptions are sharp with half-widths at half-height in the range 3-8cm-l.Dr. D. S. Urch (Queen Mary College) said: Kettle has raised the interesting questionof the orientation of the planar part of an olefinic molecule relative to the metal atomin complex formation. In Dewar’s model for the silver-olefinic complex the silveratom is situated above the double bond.It is interesting to speculate if this is theonly possible configuration.Gas chromatographic columns in which the active substrate is silver nitratedissolved in polymethylene glycol are effective at separating cis and trans isomersof olefins; cis isomers are retained about four times as long as the correspondingtrans isomer. Presumably this is because the cis isomer can form a more stablecomplex with Ag+ than the trans isomer. Could it be that the structure of the ciscomplex is different from the trans? Indeed, might not it be possible that the cis-isomer forms a complex with the silver ion in the plane of the olefinic atoms (sidecomplex) rather than above this plane (top complex)? (see fig. 1). From the symmetrypoint of view the side complex is just as viable as the top complex. (In the followingzC- HHI Y’top’ complex ‘side‘ complexFra. 1.argument I shall ignore the hybridization of orbitals which merely makes potentialoverlap more intuitively obvious.) Ag+ has the configuration 4dI0, 5s0, 5p0. Ifthe olefinic-Agf line is taken as the z-axis and the x-axis is parallel to the olefinicband then in the side complex the 5s orbital can receive electron by donation from theolefinic o-bond and the 5p, by donation from the n-bond. Effective back donationcan be established between 44, and the antibonding n*-orbital. All four lobes of4dx, and n* can interact. By contrast the overlap potential in the top complex ismore restricted. 5s and 5p, are now available to receive electrons from the 0 and nbonds but only one half of the x bond can be used. Similarly, 4dx, can back-bondto only half of the n* orbital. Thus, at a very naive level, it might be arguable tha70 GENERAL DISCUSSIONa side complex should be more stable than a top complex, provided the silver ion isnear enough to the olefin bond. Clearly steric requirements will be of great import-ance. The replacement of just one hydrogen by a methyl group may mean that thesilver ion cannot come as close as is necessary for the side complex to be more stablethan the top complex: such steric arguments would not apply to top complexes,(which is why some modification to the original Dewar proposal would seemnecessary to explain the differences in stability between cis and trans complexes).Thus, it is proposed that ethylene, propylene, 1-enes and cis-olefins could form themore stable side complex whilst other olefins would form the less stable top complex

ISSN:0366-9033    

DOI:10.1039/DF9694700059

出版商:RSC    

年代:1969

数据来源: RSC

摘要:

Extension of the Woodward-Hoffman Rules toOrganomet allic SystemsBY R. PETTIT, H. SUGAHARA, J. WRISTERS AND W. MERKDept. of Chemistry, The University of Texas at Austin, Austin, Texas 78712Received 3 1st January, 1969It has been demonstrated that in the presence of catalytic amounts of certain metal ions or metalcomplexes, various strained derivatives of cyclobutene can undergo an extremely facile disrotatoryring opening to yield derivatives of butadiene. An explanation for the " apparent " break-down ofthe Woodward-Hoffman rules is offered in terms of the formation of intermediate organometallic picomplexes and consideration of the subsequent energetics involved for the ring opening reaction ofthese complexes.Our interest in the application of the Woodward-Hoffman rules to organometallicsystems arose from the observation that dimerization of benzocyclobutadiene proceedsin different directions depending on whether the reaction is carried out in the presenceor absence of silver i0ns.I Cava and Nenitzescu and their coworkers hadshown that dimerization of benzocyclobutadiene proceeds to give the C1 6H12 hydro-carbon (I) and the plausible pathway below was proposed.However, we observedthat when benzocyclobutadiene was liberated from its iron carbonyl complex in thepresence of silver ions, then the isomeric material (11) was then only C1 6H12 productformed. Similar reactions when conducted in the absence of silver ions gave the" normal " dimer (I). Further experiments along these lines indicated that thesilver ion was allowing a disruption in the pathway leading to (I) and the nature ofits involvement became the subject of this study.(1) UWOODWARD-HOFFMAN RULESThere have been developed a set of powerful rules for determination of thenature of organic reactions which proceed in a concerted manner.3 Three suchreactions which are of present interest are (a) the thermal isomerization of cyclobutene772 EXTENSION OF THE WOODWARD-HOFFMAN RULESto butadiene, (b) cyclic addition of two ethylene units to give cyclobutane derivativesand (c) 1,3-hydrogen shifts in olefin isomerizations.The following discussionindicates that the rules governing these reactions are to be modified when the electronicrearrangements also include participation by metal atoms.CYCLOBUTENE-BUTADIENE INTERCONVERSIONSThe Woodward-Hoffman rules state that the thermal isomerization of cyclobuteneto butadiene will proceed preferentially via a conrotatory process (eqn.(1)) ratherthan by a disrotatory process (eqn. (2)). The reason for the distinction is that theRRsymmetry of the systems involved is such that one cannot proceed smoothly fromthe electronic ground state of cyclobutene to that of the ground state of butadienein process (2); whereas it is possible for process (1). The fact that the disrotatoryprocess is forbidden is evident in the following argument developed by Longuet-Higgins and Abraham~on.~ In the disrotatory process the molecular species involvedin the rearrangement at all times possess a plane of symmetry bisecting the molecule.Those molecular orbitals of cyclobutene and butadiene which becomes involved inthe chemical change are classified as to whether they are symmetric or antisymmetricwith respect to this plane and listed in fig. 1.The electronic ground state of cyclo-butene is 02z2 and if the symmetry of the orbitals is to be maintained, this wouldSymmetric 6,a *Antisymmetric 6, 7T 4 - %?a*)/ k2' P4FIG. 1 .-Classification of the molecular and atomic orbitals involved in cyclobutene-butadienecatalyzed isomerizations.lead an electronic state of butadiene ($ f $ $) which is of much higher energy than theground state ($ f $22). The energetics of this disrotatory process are schematicallygiven in fig. 2. The reaction, if forced to follow a disotatory process (dotted line,fig.2a) will involve at least a high activation energy.Consider now the isomerization of the silver complex of cyclobutene to the silvercomplex of butadiene via a disrotatory process. The ground state of the cyclobutene-Ag+ complex can be satisfactorily described as oz, (7c - ~ 4 2 ) ~ (n* -pXdx,J2 while that oR . PETTIT, H . SUGAHARA, J . WRISTERS AND W. MERK 73thesilver complex of butadienewillbe($l -s&)~, ($2 -pxdxz)4, (t+h3 - p , ~ 5 ) ~ ( $ ~ - dxy).2The atomic orbitals of the silver ion which are involved are also classified as to theirFIG. 2a FIG. 2bsymmetry properties in fig. 1. It is seen from fig. 1 that the transformation 02, (n - ~ d , 2 ) ~(n* -pxdxy)2-+($1 - ~ 4 2 ) ~ ($2 -p,dx,)2 ($3 -p,,dJ4 (+ dXJ2 is a symmetry allowedprocess.Now the difference between this allowed product and the description ofthe ground state of the butadiene-Ag+ complex mentioned above is whether twoelectrons are placed in a pydyz or a pxdxz hybridized atomic orbital of silver. Atworst this difference in energy will be very small; hence, as indicated in fig 2(b), theactivation energy will be small and the process becomes " allowed ". Argumentsalong similar lines have also been developed by Mango and Schachtschneider toindicate that the thermal cleavage of cyclobutanes to two olefinic units is also allowedin the presence of metals.Similar application of orbital symmetry rules indicate that the disrotatory con-version of the silver complexes of benzocyclobutene to o-xylylene (eqn. (3))'.is anallowed process ; whereas in the absence of metal ions, it is forbidden.-c b+CR (3)RConfirmation of these arguments are found in the isomerization of dibenzotri-cyclooctadiene(II1) to dibenzo-cyclooctatetraene(1V). The direct thermal ringopening of the central cyclobutane ring in (III) to yield(1V) directly is a forbiddenH(I11 1 CIV)process. Likewise ring opening to yield the oxylylene derivative 0, and subsequentisomerization to (IV), would have to be a disrotatory process and this also is forbidden.Hence despite the large amount of strain present in the system, the molecule onlyslowly undergoes thermal isomerization, the activation energy being approximately23 kcal/rnol. However, if silver nitrate is added to solutions of (III), the isomeriza-tion is complete almost instantly at room temperature ; even at - 24" the reaction i74 EXTENSION OF THE WOODWARD-HOFFMAN RULES50 % completed within 30 sec.The activation energy of the silver catalyzed reactionis found to be approximately 8 kcal/moI.Addition of maleic anhydride to compound (111) produces no reaction ; however,if silver nitrate is then added to the mixture, there ensues immediate precipitationof the Diels-Alder adduct (VI). This confirms that the isomerization of (Ill) to(IV) proceeds via the o-xylylene derivative (V). The role of the Ag+ is to complexwith the benzene ring and make the disrotatory ring opening to the hydrocarbon (V)an allowed process. The driving force behind the facile reaction is the relief ofsteric strain.co(VI)Similar types of catalyzed isomerizations have been observed with other strainedcyclobutene derivatives.One of particular interest is that of the isomerization ofbenzotricyclooctadiene (VI) to benzocyclooctatetraene (VII). As indicated, the metal-catalyzed isomerization of (VI) to (VII) can in principle proceed by two differentcr X Ipathways, one involving a benzocyclobutene-o-xylylene isomerization (VIII) whilethe other a cyclobutene-butadiene isomerization (IX) ; both processes involve dis-rotatory ring openings. Again the isomerization is strongly catalyzed by silver ions.When conducted in the presence of maleic anhydride, this reaction leads to thecoformation of the adduct (X), thus indicating the intermediacy of the o-xylyleneintermediate (VIII).Thus although the benzene ring is destroyed in this pathway,whereas it would not be if the reaction proceeded via (IX), there are presumablysteric factors present in the molecule which make this the lower energy process.However, the situation appears to be reversed when nickel is used as the catalyst.The isomerization of compound (111) to compound (IV) is not catalyzed by biscyclo-octadienyhkkel, this being consistent with the observation that aromatic compoundsdo not tend to displace the olefinic ligands from (XI). However, the isomerizationof the olefinic compound (VI) to (VLI) is catalyzed by the complex (XI). Presumablythen the isomerization with nickel proceeds via the intermediate (IX).Ring openinR. PETTIT, H . SUGAHARA, J . WRISTERS AND W. MERK 75from this same end is also indicated in the thermal isomerization of the solid palladiumcomplex (XU) to the palladium complex of benzocyclooctatetraene.A limited attempt has been made so far to determine the rmge of metal atomswhich can influence the disrotatory ring openings discussed above. As well assilver ions, we find that cuprous ions and the complexes PdC12(4CN), and[C2H4PtCl2I2 and metallic palladium on charcoal also have a catalytic influence onthe isomerization of benzocyclooctatriene to benzocyclooctatetraene. Silver ionsare also found to catalyze the analogous reactions indicated in eqn. (4), (5) and (6).__I_c IlllnSyn and Anti(5)The results now allow for a reasonable explanation for the effect of silver ions onthe dimerization of benzocyclobutadiene noted earlier.The intermediate species(Ia), in the presence of Ag+, undergoes rapid isomerization to (XIII) before itrearranges to (1). The intermediate (XIII) isomerizes further to (XIV) which thenundergoes an intramolecular Diels-Alder type addition to yield the observed product11.-wCYCLOBUTANE-ETHYLENE INTERCONVERSIONSThe Woodward-Hoffman rules allow one to conclude that concerted cyclobutane-ethylene interconversions (eqn. (7)) are forbidden. Hogeveen and Volger discoveredRho complexes enhance greatly the rate of isomerization of quadricyclane (XV) to/I + I1 (7)norbomadiene and Mango and Shachtschneider have produced analogous argumentsto those given above showing that in the presence of metal atoms the cyclobutaneto diolefin rearrangement now is an allowed process.One particular reaction of interest in this area concerns the metal-catalyzeddismutation of olefins (eqn.(8)). Thus treatment of 1-butene in benzene with atungsten catalyst (produced by reduction of WC16 with Et,Al) gives within secondsat room temperature a mixture of ethylene and 3-hexene. The presumed mechanis76 EXTENSION OF THE WOODWARD-HOFFMAN RULES(XV)of this unusual reaction involves the " allowed " metal-cycloadditon reaction to givea cyclobutane intermediate, reversal of this process giving the observed dismutationproducts. In order to test this hypothesis we have treated all cis-tetramethylcyclo-butane (XVI) under the same condition with the tungsten-containing catalyst.CHS CH, CH2 = cw2CH =cy II + I1CH CHI IE t Et W(8)+Et Et/ - --H-- - Et 5 EtHowever, no reaction occurred, in particular 2-butene was not formed (eqn.(9j).One possible explanation for this failure is that whereas the cyclobutane ring ineqn. (8) is formed in intimate contact with the metal, under the conditions used ineqn. (9), the preformed cyclobutane would need to displace a molecule of coordinatedbenzene before the cyclobutane-metal interaction could give rise to olefinic products.me me(9) - )1 + ,[meme meW(XVI)In agreement with this we find that the gas phase reaction of tetramethylcyclobutanewith molybdenum on alumina, a catalyst system which also effects the dismutationof olefins,8 readily yields 2-butene and subsequent products derived therefrom.Weconsider then that the dismutation of olefins does involve an allowed metal-participating cycloaddition reaction of two ethylenic units and that the failure of thecyclobutane to cleave in the presence of the tungsten resulted from the inability toposition the metal atom and the saturated hydrocarbon in close proximity.1 : 3 SIGMATROPIC SHIFTSThe above discussion suggests that there may well be other types of reactions inwhich the usual orbital symmetry rules become altered upon involvement of a metalatom. One such reaction would appear to be the metal-catalyzed reactions ofolefins. As indicated in eqn. (lo), the thermal supraf'acial concerted 1,3-hydrogenshift is not an allowed reaction according to the Woodwad-Hoffman rules.-4-Earlier suggestions for the mechanism of metal-catalyzed isomerization of olefinshave involved either addition of metal hydride bonds across the olefinic linkage(eqn.(1 1)) or involvement of intermediate n-ally1 metal hydrides (eqn. (12)).M-H -MHR--CH2-CH=CH2+R-CH2-CH-CH2 +R--CH=CH--CH2 (11)1R . PETTIT, H . SUGAHARA, J . WRISTERS AND W. MERK 77While this may be correct in some cases it cannot be in all, for Orchin has shown thatthe isornerization of 3-phenyl propene to 1-phenylpropene with DCO(CO)~ leadsto a product with only very small incorporation of deuterium. A mechanism suchCH /---\IR-CH2-CH=CH2-+R-CH' CHZ+R-CH=CH-CHs (12)M-H1Mas in eqn.(1 1) or (12) should lead to large incorporation of deuterium. Perhapsthen a meclianism such as indicated in eqn. (13) where the presence of the metalatom now allows the suprafacial 1,3-hydrogen shift to occur is operative. An addedH + rn mfeature to be then considered is whether the hydrogen atom on the same side of themetal is the one which shifts or whether it is the one on the opposite side. A recentexperiment by von Rosenberg and coworkers may here be significant. Theseworkers have found that Fe(CO), catalyzes isomerization of the alcohol (XVIl) tothe ketone (XVlII) but the epimeric alcohol is unaffected. It is reasonable to assumethat the iron atom only interacts with the olefin through the outside face of the mole-cule ; hence the hydrogen which is on the same side of the olefin as the attached metal( X I X Iatom is the one which migrates.A transition state as indicated in formula (XIX)is then suggested in such a concerted reaction78 EXTENSION OF THE WOODWARD-HOFFMAN RULESCONCLUSIONThere is now good evidence to suggest that involvement of certain metals candrastically affect the rules governing the concerted reactions of otherwise purelyorganic reactions. The implications of this effect could have important consequencesin synthetic organic chemistry as well as being of interest in its own right and thiscould develop into another important aspect of organometallic chemistry.W. Merk and R. Pettit, J. Amer. Chem. SOC., 1967,89,4787.M. P. Cava and D. R. Napier, J. Amer. Chem. SOC., 1957,79,1701.C. D. Nenitzescu, M. Avram and D. Dinu, Ber., 1957, 90,2541.R. Hoffmann and R. B. Woodward, Acct. Chem. Res., 1968,1,17.H. C. Longuet-Higgins and E. W. Abrahamson, J. Amer. Chem. SOC., 1965,87, 2045.F. D. Mango and J. H. Schachtschneider, J. Amer. Chem. SOC., 1967, 89,2484.H. Hogeveen and H. C. Volger, J. Amer. Chem. SOC., 1967,89,2486.1967,34, 3327.R. L. Banks and G. C. Baile, I. and EC. Product Res. Dez)., 1964,3 (3), 170.J. L. von Rosenberg, personal communication.' N. C. Calderon, E. A. Ofstead, J. P. Ward, W. A. Judy and K. W. Scott, Tetrahedron Letters

ISSN:0366-9033    

DOI:10.1039/DF9694700071

出版商:RSC    

年代:1969

数据来源: RSC

摘要:

Some Bonding Questions Prompted by Studies of the FluxionalMolecule TriscyclopentadienylnitrosylmolybdenumBY F. A. COTTONDept. of Chemistry, Massachusetts Institute of Technology, Cambridge,Massachusetts 02 139, U.S.A.Received 20th January, 1969The compound (C5H5),MoN0 has been prepared. Its proton magnetic resonance spectrumin solution has been studied as a function of temperature down to - 1 10". The crystal structurehas been determined with relatively high precision at 22". The combined results of these studieslead to some new observations concerning the manner in which cyclopentadienyl rings may bebonded to metal atoms. Specific conclusions are (a) that a trihapto (i.e., x-allylic) form of bondingis not a real possibility, and (6) unsymmetrical relationships of ring to metal, which cannot be readilydescribed in terms of any simple limiting structure, are possible under certain conditions.The molecule (C,H,),(NO)Mo was prepared in these laboratories in early 1968and has been subject to two physical studies.(i) The p.m.r. spectrum in the tempera-ture range - 110 to + 14" has been recorded ; the molecule is shown thereby to bea fluxional one.2* (ii) A single-crystal X-ray structure determination at ca. 22°Chas been carried The results of these studies raise several significant questionsabout bonding in this and related molecules. As a basis for discussion, the back-ground and experimental results will be briefly summarized.Synthesis of (C5H5)3(NO)M~ was prompted by the idea that it might have astructure of the type shown in fig.1, in which there are three different (but eachFIG. 1.-A hypothetical structure for the (C5H,)3MoN0 molecule in which there is a pentahapto(IT) ring, a trihapto (x-allylic) ring and a monohapto (0) ring.well-defined) forms of C,H,-to-metal bonding, viz., monohapto (or a-), trihapto(or " n-ally1 ") and pentahapto (n- or '' sandwich "). This was considered plausibleon the basis that the Mo atom has six electrons, a h5-CSH5 ring contributes fiveand the NO ligand formally contributes three. Thus, in order to achieve the comple-ment of 18 electrons which usually affords maximum stability, the remaining twoC5HS rings together would have to supply 18-6-5-3 = 4 electrons to bondingorbitals. It seemed plausible in the beginning that this would be achieved by having780 TRISCYCLOPENTADIENYLNITROSYLMOLYBDENUMone ring act as a 1-electron donor and the other as a 3-electron donor, with thegeometric relationship of these to the metal atom and its other ligands being asdepicted in fig.1. The only part of this overall structure for which there was nodistinct precedent was the trihapto- (or n-allyl) type ring. However, the idea ofsuch a metal-ring interaction had been mentioned previously in the literature, specifi-cally for the closely related compounds (CSH,),Mo(NO)I and (C,H&NO(NO)CH~,~and it did not seem to be an unreasonable idea at the outset.SUMMARY OF EXPERIMENTAL DATAThe main results of the n.m.r. study are the following. (i) At room temperatureall 15 protons together give a single sharp line, but even before the temperaturereaches 0" this line broadens appreciably.(ii) By about -30" the spectrum of aslowly moving h1-C5H5 ring appears and becomes a well-resolved AA'BB'X patternby about - 50°, while the line due to the remaining 10 protons simultaneously becomes3 4 5 67FIG. 2.-The proton n.m.r.spectrum (60 mHz ; 4 . 2 M in CS2) of (CSH5)3MoN0 at various tempera-tures (reproduced by permission from ref. (1 >).sharp again. (iii) Between - 50" and - 110" two things happen simultaneously.The line of intensity 10 broadens, separates into two lines of equal intensity and theselines then become sharp. At the same time the AA'BB'X pattern is transformedinto an ABCDX pattern. These changes are shown in fig.2.Clearly, observation (i) means that intramolecular rearrangements occur rapidlyenough at 25" to give all three rings time-average equivalence. Each ring passeF . A. COTTON 81from one to another of the available environments rapidly on the n.m.r. time scale.Observation (ii) clearly shows that the slowest processes are entering and leaving theh1-C5H5 environment and the eccentric rotation (" ring-whizzing ") of a ring whenit is in this environment. Beyond this, however, the interpretation is not entirelyunequivocal, although it might appear to be so.It is true that observations cited under (iii) are consistent with the notion that theinstantaneous structure is that shown in fig. 1. Thus, we might assume that at-50" the h3-C5H5 ring is still exchanging roles rapidly with the h5-C5H5 ring (andpresumably also executing its own form of eccentric rotation).We might then assumefurther that this exchange is gradually slowed between - 50" and - 110" so that theh3 and h5 rings become distinguishable, although the five protons within each oneremain indistinguishable due to rotations. Because of the dissymmetry of thepostulated (h5-C,H5)(h3-CSH5)(NO)Mo moiety, the A and A' and also the B andB' protons of the h1-CgH5 ring would not actually be equivalent and as the rate ofinterchange of the (hS-CSH5) and the (h3-C5H5) rings slowed, this non-equivalenceshould be revealed, thus accounting for the changes in the AA'BB'X spectrum.However, this is not the only possible interpretation of the observations and,conversely, the observations do not prove that the structure shown in fig.1 is correct.All of the spectral changes which are seen below - 50" could be accounted for simplyby slowing the rotation about the bond from Mo to C1 of the h1-C5H5 ring; noparticular kind of difference-or any dzflereizce at all-in the way the other tworings are bound to Mo is necessarily implied.In order to establish more definitely the structure of the (C5Hs),MoN0 molecule,an X-ray crystallographic study of the crystalline compound was carried out. Fig. 3, I ! ,'- - - - - A+FIG. 3.-A projection of the structure of the (C5H5)3MoN0 molecule as it occurs in the crystal.and 4 show the results. It is dear that the ring consisting of C(11) to C(15) is agenuine h1-C5H5 ring.Also, the NO group is boundin the expected manner (Mo-N,1.75 l(3) A ; N-0, 1.207(4) A ; angle Mo-N-0, 179.2(2)") [or a three-electrondonor. The relationship of the other two rings to the Mo atom is novel. We believethat it is, in fact, unprecedented. First, each of these two rings is related in essentiallythe same way to the metal atom. There is no possibility of regarding one as a82 TRISCYCLOPENTADIENYLNITROSYLMOLYBDENUMhS-C5H5 and the other as an h3-C5H5 ring. Secondly, each one has a curiousorientation relative to the metal, in which there are two short (2.32-2.35a), onemedium (2.42-2.44 A) and two long (2.58-2.68 A) Mo-C distances.* J 1-00 70.93FIG. 4.-Some bond distances in the (CSH&MoNO molecule.Figures beside bond lines giveC-C and C-H bond lengths in A. Figures beside carbon atoms give Mo-C distances in A.The e.s.d. of C - 4 distances are 0.005-0-008 A ; the e.s.d. of Mo-C distances are 0.003-0-005 A.If this structure (or something essentially similar) is the instantaneous structurein solution, then it is clear that the spectral changes below -50" must indeed beattributed to slowing rotation about the Mo-C bond as mentioned above.DISCUSSION OF CSHS-MO BONDINGThe absence of a trihapto or Ir-allylic C5H5 ring, especially when this apparentlyrequires that the presumably favourable pentahapto arrangement for the other ringalso has to be sacrificed, prompts a closer examination of the concept of a (trihapto-cyclopentadienyl) metal bond, which in turn leads to the conclusion that this is amythical concept.Tt is first to be noted that a large number of (n-allyl) metal bonds have now beenstructurally characterized.' Unless there is some great disparity among the otherligands also bound to the metal, which could be expected to cause a skewing of theallyl plane, the three metal-carbon distances are equal, or nearly equal.Mostimportant, the distance from the metal atom to the centre carbon atom of the allylgroup is usually equal to, and never more than 0.1 A less than the distances to theouter carbon atoms.If we assume that in the postulated (h3-C5H5)M group the ring would remain aregular plane pentagon then by simple geometry it follows that a metal atom whichis equidistant from three carbon atoms is, in fact, equidistant from all five.Inshort, (h3-C5H5)M is geometrically indistinguishable from (h5-C5H5)M. It is notlikely that the ring could deviate very much from planarity, nor are the angles likelyto become greatly distorted in real cases. Hence this conclusion concerning theidealized system should apply in a practical sense.Implicit in the above argument is the fundamental assumption that if an atom Alies a distance d from two other identical atoms B and B' and A is considered to bebonded to B, it must also be considered to be equally bonded to B'. This notioncan be generalized to the idea that, under proper conditions, interatomic distances(especially those which are near but not quite as short as normal bond distances)give a measure of bond energy.While this is not the place to conduct a detaileddiscussion of this point, it does seem applicable to the present case in that thereappears to be appreciable bonding between the Mo atom and all ten carbon atomsof the two comparable rings in (C5H,),MoN0. Thus, according to a previouslyproposed * correlation of bond lengths and bond orders for Mo-C bonds, theshort Mo-C distances indicate approximately single bonds, the medium ones slightlyweaker ones, while the distances 2-58-2.68 A suggest bond orders of ca. 0.6F . A . COTTON 83While none of the foregoing estimates is intended to have literal significance, thepoint is that we have here a very unsymmetrical interaction in which all five ringatoms participate in varying degrees. It does not appear possible to give any simpleelectronic description of this bonding however. Evidently, the flexibility of C5H5rings in respect to bonding with metals is very great and less reliance should beplaced on the simple prototype modes than might previously have seemed safe.F. A. Cotton and P. Legzdins, J . Amer. Chem. SOC., 1968, 90,6232.F. A. Cotton, Chemistry in Britain, 1968, 4, 345.idem, Acct. Chem. Res., 1968, 1,257.J. L. Calderon, F. A. Cotton and P. Legzdins, J. Amer. Chem. SOC. in press.This relatively new notation is explained in F. A. Cotton, J . Amer. Chem. SOC., 1968,90,6230.R. B. King, inorg. Chem., 1968, 7,90. ’ Recently reported structures are found in (a) R. Mason and A. G. Wheeler, J. Chem. SOC.,1968,2543,2549 ; T. G. Hewitt and J. J. deBoer, Chem. Comm., 1968, 1413 ; B. T. Kilbourne,R. H. B. Mais and P. G. Owston, Chem. Comm., 1968, 1438. * F. A. Cotton and R. M. Wing, Inorg. Chem., 1965,4,314; cf., fig. 2

ISSN:0366-9033    

DOI:10.1039/DF9694700079

出版商:RSC    

年代:1969

数据来源: RSC