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QUARTERLY REVIEWS MOLECULAR-SIEVE ACTION OF SOLIDS By R. M. BARRER D.Sc. Sc.D. (PROFESSOR OF CHEMISTRY IN THE UNIVERSITY OF ABERDEEN) MANY important phenomena in physical and biological chemistry result from molecular-sieve action including osmosis dialysis Donnan membrane equilibria (unequal ionic distributions hydrolysis potential differences) isotope separation and gaseous and solution chromatography. These secondary phenomena will not be examined in detail but instead the causes of molecular-sieve action will be considered together with some of the information which studies of permeability have contributed to the fundamental chemistry and physics of solids. Molecular-sieve action may be total or partial in its effect upon the permeability. I n total molecular-sieve separations the flow of one species into or through the medium is wholly prevented while the diffusion of a second species occurs at a finite rate.In partial separations both species diffuse but a t different velocities. Molecular-sieve action can moreover be demonstrated in two ways the diffusion medium can be in the form of foils or of beds of powdered sorbent through which different trans- mission velocities occur ; alternatively a sorbent may be exposed to a stationary molecular mixture and different sorption velocities may be found. Elastomers and other polymers inorganic glasses certain metals and also natural membranes forming the walls of plant and animal cells provide good examples of media effecting sometimes partial and sometimes total separations. Sorbent media showing ultra-porosity include porous glass charcoals and other xerogels but by far the most spectacular mole- cular-sieve effects are shown by some dehydrated zeolite crystals especially chabazite [ (Ca,Na2)A1,Si,012,6H20] gmelinite [ ( Na2,Ca)A1,Si401,,6H,0] mordenite [ (Ca,K,Na,)Al,Sil,0,4,6~H20] levynite (CaA12Si,0,0,5H20) and a synthetic zeolite ( BaAl2Si,0,,,~H2O) together with their cation-exchange modifications; and to a lesser extent by analcite (NaAlSi,O,,H,O) and harmotome [ ( Ba,K,)A1,Si,Ol,,5H2O].Other zeolites may show the pro- perty but not all have yet been investigated. First we may consider the ultra-porosity of sorbents and then that of compact membranes. Molecular-sieve Action in Porous Sorbents Some General Considerations.-When a gas is circulated through a porous (1) Because greater resistance is offered to diffusion of one species than medium different flow rates arise in two important ways 293 294 QUARTERLY REVIEWS of another ; e.g.if a large and a small molecule both pass along a capillary which is of greater diameter than that of the small molecule but constricts the large molecule then the small one diffuses more rapidly. ( 2 ) Because of differing sorption potentials within the porous medium through which both species are flowing. If the medium has a large internal surface considerable amounts of the diffusing species may be sorbed and so become largely immobilised. If the amount of sorbed material and thus the lifetimes of each sorbate in the sorption layer differ then chromato- graphic bands may develop or a major difference in the rates of transmission through a column of the porous medium may occur.For example in powdered dehydrated chabazite crystals exposed to a mixture of ethane and propane the ethane is very speedily occluded a t room tem- peratures and the propane is only slowly occluded because the very narrow intracrystalline diffusion paths greatly restrict the mobility of the larger propane molecule while allowing the ethane to diffuse rapidly. Therefore the crystals first become charged with ethane * which is thus quickly removed from the gas stream. However the affinity of propane for the crystals is greater than that of ethane and therefore for comparable partial pressures of the gases a t or near equilibrium there would be a larger interstitial con- centration of propane. By suitably adjusting the time of transit of the ethane-propane mixture through a column of chabazite so that factor (1) was dominant the effluent gas was wholly freed from ethane; for very slow rates of transmission however the effluent gas should be enriched in ethane.In the absence of appreciable sorption transmission through a porous bed a t high pressure follows Poiseuille’s law and there is no separation of the constituents of a gas mixture. At low pressure when the capillary radii are small compared with the mean free path of the gas the flow fol- lows Knudsen’s law and the separation of the constituent gases is inversely proportional to the square roots of their molecular weights. As the capil- lary radii become still smaller and the surface to volume ratio increases adsorption effects begin to become important.The adsorbed molecules may be immobilised but also may contribute to flow by surface diffusion. Mole- cular streaming in a capillary may be described in terms of the diffusion equation &/at = D ( a 2 c / W ) where c denotes the concentration of gas in the gas phase. The values which the diffusion coefficient D may then have in a given cylindrical capillary are These two effects can act in opposition a t one and the same time. D = (4r/3)dZkT/nm (molecular streaming only) . 4r2 (molecular streaming with adsorption to give D,= 3rdnm,/2k~ + an immobile film) . * (2) R. 31. Barrer and L. Belchetz J . SOC. Chem. I n d . 1945 64 131. R. M. Barrer 1’mn.s. Paraclay SOC. 194-8 No. 3 61. giving a zeolitic or interstitial solid solution. * Both ethane and propane are taken up interstitially throughout each crystallite, BARRER MOLECULAR-SIEVE ACTION OF SOLIDS 295 (molecular streaming with adsorption to give a mobile film) .(3) D - 3- I n these equations z denotes the lifetime of a molecule in the adsorbed state (z = where 7 = l / v Y is the vibration frequency in the adsorbed state and AH the heat of adsorption) ; rn is the molecular mass r the radius of the capillary k the Boltzmann constant and T the tem- perature. Us = DOe-E/RT is the surface diffusion coefficient and E is the energy of activation for surface diffusion. In deriving equation (3) it was further assumed that concentration on surface c&centration in gas phase = constant = tdkT/2nm This condition is probable for many systems a t the low pressures where molecular streaming is the principal mechanism of transmission.A still further diminution in capillary diameter results in ‘‘ surface ” diffusion becoming dominant because in capillaries with diameters of mole- cular magnitudes only the diffusing species do not leave the range of surface forces. The diffusion process now occurs by a succession of jumps in a periodically varying potential energy field due to the solid. Such periodicity arises from the discontinuous atomic structure of the medium and gives rise to the energy of activation E.* The diffusion of many gases into gas-sorbing zeolites (see p. 303) takes place according to this mechanism. Capillary size distributions have been estimated 3 by means of the Kelvin equation RT lnp/p = - 2oV cos 8/r where r is the radius of a capillary in which liquid is in equilibrium with its vapour a t vapour pres- sure p ; p s is the saturation pressure of the liquid which has molecular volume V and surface tension o.The contact angle of the liquid with the wall of the capillary is 8 and if the liquid wets the capillary 8 is zero and cos 8 = 1. From the sorption isotherm one can determine V the amount sorbed. One may subtract from V the amount Vm required to form a monolayer and attribute the remainder (V - Vm) to capillary condensa- tion. Vm can be estimated by several methods,4 and is perhaps most easily derived from B E T plots of p / V ( p - p ) against p/p when the intercept on the axis of V is l/Vmc and the slope is (c - l)/Vmc where c is a con- stant. Typical structure differentiated curves of d V/dr against r derived from isotherms by the use of the Kelvin equation are given in Fig.1. Capillary condensation in the usual sense cannot occur when the capillary radius is of the order of 3 A. G. Foster Trans. Faraday SOC. 1948 No. 3 41 ; 1932 28 245; P. H. Emmett and T. de Witt J . Amer. Chem. SOC. 1943 85 1253; P. H. Emmett and M. R. Cines J . Physical Chem. 1947 51 1248 ; S. Brunauer “ The Adsorption of Gases and Vapours” O.U.P. 1944 Chap. XI where a summary of much earlier work will be found. There are limitations to the Kelvin method. “ -4dvances in Catalysis” Vol. 1 Academic Press Inc. N.Y. 194S p. 65. * Relative movements of atoms or groups in close contact whether in chemical reaction intramolecular rotations diffusion or viscous flow all tend in this way to be jumps from one stable configuration to another via a less stable transition state.296 QUARTERLY REVIEWS one or two molecular diameters and the Kelvin equation may no longer describe the phenomena occurring. Secondly a correction to r is applied 71 28 32 34 r'A* 22 26 31 ? A . I. Benzene-Fe,O,(B). I I I . H,O-SiO (B). 11. Dioxan + AlcohoZ-Fe,O,. IV. H,O-AI,O,. FIG. 1 Iv Typical structure differentiated curues for some ge2 sorbents showing range aibd distribution of pore sixes (Foster). [Reproduced by permission from Trans. Faraday SOC. Discussion on Interaction of Water and Porous materials p . 44.1 to allow for the space taken up by the monolayer film the formation of which precedes multi-layer formation and capillary condensation. 5 Also the capillaries may not be shaped as for the Kelvin equation like test- tubes. In charcoal for example many capillaries will be the space between pairs of plate-like crystallites." Finally because many capillaries are open a t both ends the condition for forming the meniscus may differ from the condition for its equilibrium once formed.L. H. Cohan considers the condition for forming liquid in a cylindrical tube open a t botlh ends to be RT In p/p8 = - oV cos O/r which may be compared with Kelvin's equa- tion. Therefore capillary sizes and size distributions are best determined on the desorption cycle of the isotherm. In spite of these limitations the distribution curve of capillary radii in terms of Kelvin's equation may have a qualitative significance. These A. G. Foster Trans. Faraday Xoc. 1948 No. 3 44. J Amer. Chem. Soc. 1938 60 433. H. L. Riley Quart. Reviews 1947 1 65.* These laminae may have diameters no more than 20-100 A. in many gas-sorbing charcoals .' BARRER MOLECULAR-SIEVE ACTION OF SOLIDS 297 curves normally show maxima and molecular-sieve effects may become clearly defined if there is a sharp upper limit to the capillary radii or where all the capillaries are of the same dimensions (Fig. 2 curves 2 and 1). Curve 2 may be approximated to by some gel sorbents for which the isotherms have the form given in Fig. 3 curve 2. The comparatively flat upper part of the isotherms probably corresponds to condensation in a group of capillaries all of nearly the same radius followed by cessation of sorption FIG. 2 Three possible distributions of pore sizes. Curves (1) and (2) will give sharp molecuhr- sieve effects but Curve (3) will not.* Relative pressure ‘‘0 FIG. 3 Isotherms in porous solids. Isotherm (2) corresponds with pore distributions such as those in Curve 2 Pig. 2. Isotherm (3) corresponds similarly with Curve 3 Fig. 2. With pore distributions like those in Curve 1 Fig. 2 but with pore radius too small for capillary condensa- tion Langmuir-type isotherm as in Curve 1 are observed (cf. isotherms in zeolites). because no larger capillaries exist in the porous gel and because all capil- laries of smaller diameters have already been filled. Curve 1 is charac- teristic of the crystalline zeolites in which however the pore diameters are only a few A. so that capillary condensation cannot occur. Evidence of molecular-sieve effects in charcoals has been presented inter alia by Miss M. Franklin,6 who reported that apparent densities of char- coals during graphitisation depended on the dimensions of the molecules of the immersion medium (helium water carbon disulphide methanol and benzene); the larger the molecule the smaller was the apparent density owing to incomplete penetration of the pore structure.At higher tem- peratures of graphitisation all the immersion media gave the same apparent density ; but this density was - 1.7 which is lower than that of graphite suggesting sealed pores inaccessible to molecules of any medium. According to L. G. Gur- Silica gels may also show analogous effects. International Colloquium on Reactions in the Solid State Paris Oct. 1948; Bull. SOC. chim. 1949 Nos. 1 and 2 D.53. 298 QUARTERLY REVIEWS witsch's the volumes (reckoned as liquid) of all sorbates taken UP in a porous gel sorbent should be the same when the sorbent is saturated.A. G. Foster,lo however found that this was not the case in some samples of silica gel; the larger the molecule the less the volume occluded a t saturation. In one case the number of moles sorbed at saturation increased linearly with the reciprocal of the molecular volume. Intracrystalline Sorption.-Some crystalline zeolites such as those noted on p. 293 can be dehydrated by heat and evacuation without appreciable lattice shrinkage to give crystals permeated by very narrow diffusion paths no more than a molecular diameter across. Following dehydration there- fore a great internal " surface " is developed and the crystals are capable of sorbing some other molecules in place of their original crystal water.The sorptive capacity in certain cases rivals that of activated charcoals and in addition the most remarkable molecular-sieve properties are developed.ll Evidence as to the dimensions of the intracrystalline channels has been obtained both by X-ray methods and by the study of sorptive behaviour of the crystals. Structures have been obtained for the zeolites analcite,l2 natrolite,13 scolecite,13 thomsonite,13 and edingtonite.l* X-Ray studies have been made of chabazite,l5 harmotome,ls phillipsite7l7 and heulandite,lZ but detailed structures are not known. However the minerals sodalite l8 and ultramarine l9 have been successfully examined and it is believed that the aluminosilicate framework in chabazite resembles the basket-like anionic structure recurring in amodified form in these minerals.Fig. 4 shows the channels in sodalite which cross other channels a t intervals throughout the lattice. In analcite four channels diverge from each crossing point; in chabazite ultramarine or sodalite eight channels diverge from each such point. These junction points occur regularly throughout the crystal and the channels themselves are continuous although their diameter may vary periodically along their length. Zeolite crystals have been subjected progressively to more and more drastic conditions of heat and evacuation and were a t the same time exam- ined by X-ray or sorption methods.20 Taking these data together with 9 J . Russ. Phys. Chem. SOC. 1915 47 805. 10 Nature 1946 157 340. 11 J. 'CV. McBain " Sorption of Gases by Solids " Routledge & Sons 1932 Chap.V and R. M. Barrer Ann. Reports 1944 41 31 give general summaries of the sorptive behaviour of some zeolites. 12 W. H. Taylor 2. Krist. 1930 74 1. 13 W. H. Taylor C. A. Meek and W. W. Jackson ibid. 1933 84 373. l4 W. H. Taylor and R. Jackson ibid. 1933 86 53. 1 5 J. Wyart Bull. SOC. franp. Min. 1933 56 81. l6 J. Sekawina and J. Wyart ibid. 1937 60 139. 17 J. Wyart and P. Chatelain ibid. 1938 61 121. 1 8 L. Pauling Proc. Nat. Acad. Sci. 1930 16 453; 2. Krist. 1930 74 213. 1 9 F. M. Jaeger Baker Lectures Cornell University McGraw-Hill N.Y. 1930 Part I11 ; F. M. Jaeger H. G. Westenbrink anti B. A. van Melle Proc. Acad. Anisterdnm 1937 30 249. 2o W. Milligan and H. Weiser J . Physical ClLe?iL. 1937 41 1089 ; R. &I. Barror Proc. Roy. Soc. 1938 A 167 392 406. BARRER MOLECULAR-SIEVE ACTION OF SOLIDS 299 I Strength of bonding in the main direction of aluminosilicate chains greater than strength of cross-linking of chains Strength of bonding in plans of aluminosilicate sheets greater than strength of cross-linking of sheets Strength of bonding comparable in all three dimensions cleavage properties one may make a classification of some zeolite crystals in terms of the direction stability of the lattice (Table I).Only in the robust Sodalite Na,Al,S i60,,C1 FIG. 4 Some of the details of the sodalite structure showing the basket-like anionic framework and the channels running diagonally from the eight corners of the unit cell (HelzdricEs). [Reproduced by permission from Ind. Eng. Chem. 1945 37 625.1 network group of crystals is there any general tendency for other molecules to replace water within the lattice because the channels in fibrous and laminar zeolites collapse at least partly during heating and evacuation.TABLE r Crystal chemical characters of some zeolites Bonding characteristics. Type of zeolite. Fibrous Laminar Robust network cry st als Examples. Natrolite Scolecite E ding tonit e Thomsonite Heulandite Stilbite Chabazite Gmelinite Mordenite Levynite Harmotome Analcite 300 QUARTERLY REVIEWS The interstitial volume can be determined by measuring the volume of water which can be driven off from the crystals and is a substantial frac- tion of the total volume of the crystals as the data of Table I1 show. The place of water can be taken by quite small molecules only demonstrating the small cross-sectional diameter of the channels but for these small molecules the robust network zeolites are quite exceptional sorbents.In harmotome and most analcites the only molecules sorbed are small polar molecules but in the other zeolites in Table I1 and in some synthetic zeolites non-polar molecules are sorbed as we11.21 TABLE I1 Interstitial volumes in Some gas-sorbing zeolites Zeolite. Chabazite . . . Gmelinite . . . Levynite. . . . Harmotome . . Mordenite . . . Anctlcite . . . . G.c. of liquid H,O dis- placed from 100 c . ~ . of crystals (approx.). 50 50 40 35 33 20 Sorbs non-polar gases. Yes Yes Yes No; but sorbs Yes As a rule no ; but sorbs H20 NH, HC1 H2°7 NH3 The kinetics of sorption can successfully be described in terms of Pick’s law of diffusion although D the diffusion coefficient sometimes depends upon the concentration of sorbate in the crystal.21 2 2 The channels are so narrow that the sorbates move wholly in the periodic potential-energy field of the crystal and as already noted (p.295) the diffusion coefficient must then obey the relation D = Doe-E/R1’ where Do is a temperature- independent factor and E is the energy of activation for a jump from one preferred sorption site to the next (cf. Table XI). The values of E depend in a complex way upon the radius and charge of the interstitial cations and more directly upon the diameter of the diffusing molecule (Fig. 5). Thus the more closely the interstitial channel surrounds the diffusing mole- cule the larger E becomes and the smaller the diffusion rate ; until finally large molecules do not enter the crystal at all.21-25 Every degree of intra- crystalline mobility has been observed and so every degree of molecular- sieve effect can be expected.I n an early classification of a number of molecular-sieve crystals certain sorbate molecules of known dimensions were used to compare the dimensions of the intra-crystalline channels.23’ 26 *1 R. M. Barrer J. 1948 127; R. M. Barrer and D. W. Riley ibid. p. 133. 2 2 R. M. Barrer and D. Ibbitson Trans. Paraday Soc. 1944 40 206. 23 R. M. Barrer J. SOC. Chem. Id. 1945 44 1 3 0 ~ . 24 I d e m Trans. Paraday Soc. 1944 40 555. 26 I d e m ibid. 1949 45 358. 26 I d e m Ann. Reports 1944 41 31. BARRER MOLECULAR-SIEVE AC!CION O F SOLIDS 301 FIG. 6 Relation between the true energy of activation E for diffusion and the cationic radius in a series of base exchange mordenites for a given sorbate molecule.Inset is shown the relation between the true energy of activation E and the cross-sectional radius of a series of sorbate molecules for a given base-exchange mordenite here K-mordenite. [Reproduced by permission from Trans. Faraday SOC. 1949 45 363.1 "2 A ~ 4.89 A. B FIG. 6 Critical dimensions of some typical molecules used to classify molecular sieves,. Three categories of molecular sieve are described (Table 111). The critical dimensions of these reference molecules and of some other molecular species are indicated in Pig. 6. Molecular dimensions cannot of course be given 302 QUARTERLY REVIEWS with absolute certainty but are sufficiently definite to predict the sorptive behaviour of the minerals towards other possible sorbates.Thus in the series /CH2\ c1 c1 /cH2\ /cH2\ CH CH CH CH Br Br Br c1 all the molecules are of similar shape and size owing to the comparable dimensions of the C1 and Br atoms and the methyl radical. Propane is slowly occluded by chabazite and therefore the other species should also TABLE I11 Three categories of molecular-sieve zeolite Class 1. Chabazite Gmelinite Synthetic zeolite ( BaA1,Si,0,,,nH20) ClU8S 2. Mordenite (rich in Na) Class 3. Ca- and Ba-rich mc denites (prepared hydro t her mally by cation interchange) Do not occlude {so-paraffins or aromatics. Occlude n-paraffins slowly. Occlude CH, C,H, and molecules of smaller cross- Diameter of narrowest cross-section of interstitial section very rapidly. channel between 4.89 and 5.58 A. Does not occlude n-paraffins iso-paraffins or aromatics.Occludes CH and C& slowly. Occludes N, 0, and molecules of smaller cross-section Diameter of narrowest cross-section of interstitial rapidly. channel between 4.0 and 4.89 A. Do not occlude hydrocarbons including CH and C,H,. Occlude A N, and molecules of smaller cross-section. Diameter of narrowest cross-section of interstitial channel between 3.84 and 4-0 A. be slowly sorbed. In the series CHCI, CH(CH,), CHBr, since chabazite cannot occlude isobutane it is unable to occlude chloroform and bromoform molecules which are once more of comparable shape and size to isobutane. Similarly in the n-paraffin series This prediction was confirmed. CH3 /CH2\ /CH2\ etc. F2\ / CH CH CH3 CH CH /CH2\ CH CH the cross-sectional diameter of all these molecules in the fully extended configuration is the same vix.4-89 A. Since propane is slowly occluded by chabazite all the other paraffins should be sorbed. This sorption was followed as far as n-heptane and although the sorption rate decreased as the number of carbon atoms increased large amounts of these simple n-paraffins were 27 It may be inferred that diffusion occurs 27 R. M. Barrer and D. Ibbitson Trans. Paraday Soc. 1944 40 195. BARRER MOLECULAR-SIEVE ACTION OF SOLIDS 303 in a stretched-out configuration and that the ability of the chabazite to sorb the paraffin depends on the shape and cross-sectional diameter of the molecules rather than on their molecular volume (isobutane has cross- sectional diameter 5.58 A. and molecular volume 96 C.C. and is not occluded ; n-heptane has cross-sectional diameter 4.89 A.and molecular volume 145 c.c. and is occluded). Many other examples were also found in which the observed behaviour of the zeolites as sorbents was predicted from the molecular dimensions of the sorbate molecules (Table IV). The grouping of some zeolites into the three classes of molecular sieve is dependent to a marked degree upon the thoroughness of removal of water and other interstitial molecules which reduce the mobility of the sorbate. By leaving known amounts of water in chabazite crystals it was possible to transform this zeolite from a Class 1 molecular sieve into one with sorptive properties towards hydrogen oxygen and nitrogen more nearly recalling a Class 3 sorbent.28 The water molecules immobilised in the interstitial channels substantially impede the diffusion of other species freely mobile in the absence of the water.The rates of sorption are also reduced if the heat treatment during outgassing is so severe that a measure of lattice collapse occurs. In this connection chabazite has been heated to 470" while still preserving its sorptive p0wers.l Analcite harmotome gmelinite and mordenite and many of their cation-interchanged forms have been heated for indefinitely long periods at 350" without observable lattice collapse.22 2 4 9 25 The velocity of sorption is also influenced by the dimen- sions of the sorbent particles in accordance with the theory of diffusion in such crystallites.22 Apart from the considerations of the previous paragraph it has recently been shown that the grouping of certain zeolites into three categories of molecular sieve as in Table I11 is capable of considerable refinement and extension.This can be done in either of two ways (i) by working at low temperature^,^^ or (ii) by altering the intracrystalline channel dimen- sions by cation interchange or other chemical p r o c e ~ s e s . ~ ~ ~ 3O We may first consider (i) and assign to each diffusing species a charac- teristic E and Do in the relationship D = D,e-E/RT. Temperature influences the diffusion velocity differently in each individual case and other things being equal the relative sorption velocities should diverge exponentially as the temperature is lowered. This position is however modified when the varying affinities of the sorbates for the sorbents are taken into account. It has been demonstrated that in constant-volume sorption systems the interstitial concentration and concentration gradient may sometimes increase with falling temperature more rapidly than the mobilities of individual molecules decrease giving negative instead of the more usual positive tem- perature coefficients to the rates of sorption (Fig.7).25 However in those systems with positive temperature coefficients the relative sorption rates 28 P. Emmett and T. De Witt J . Amer. Chem. SOC. 1943 65 1253. Zg R. M. Barrer Nature 1947 159 508. 30 Idem ibid. 1949 104 12. X 304 CH, C,H CH,*OH CH,*NH CH3.CN CH,Cl CH,F HCN C1 QUARTERLY REVIEWS All classes of molecules in cols. 2 and 3 section (i). TABLE IV Molecules occtuded or excluded by three classes of molecular sieve Typical molecules rapidly occluded at room tempera- ture or below.Typical molecules moderately rapidly or slowly occluded at room temperature or above. Typical molecules which are not appre- ciably occluded at room temperature or above. (i) C h s 1 minerals. He Ne A H2 N2 0 c o GO2 cos cs 32 HBr NO NH3 CH,*NH2 CH,*OH CH&N HCN C1 CH,Cl CH,Br CH,F CH,C12 CH,F CH, CJ3 CBH CH20 Has CH,*SH C,H and simple higher C2H,*OH C,H,*NH C,H,F C,H,CI C,H,Br CH,Br CHJ n-paraffins 12 H I C,H,-CN C,H,*SH H*CO,Me H*CO,Et COMe CH,*CO,Me NHMe, NHEt Aromatic hydrocarbons cycZo- and iso-paraffis. Derivatives of these hydro- carbons. Heterocyclic compounds (e.g. thiophen pyrrole pyridine) . CHCl, CCl, CHCl:CCl, and analogous bromo- and iodo- compounds. Secondary strsight-chain alco- hols thiols nitriles and halides. Primary amines with NH group attached to a secondary carbon atom.Tertiary amines. Branched-chain ethers thio- ethers and secondary amines. CH,-CHCI, CHCl,*CCl, C2CI8 A HC1 NH3 (iii) Class 3 minerals. All molecules referred to in col. 3 section (ii) . usually diverge more and more as the temperature falls. Two species both rapidly sorbed at room temperature may then be sorbed a t low temperatures a t such different velocities that molecular-sieve separation should be easily possible. Fig. 8 illustrates this low-temperature differentiation between sorption velocities of oxygen nitrogen and argon in levynite.* Numerous examples of this method of magnifying the molecular-sieve effect have been observed.25 In the modification of the zeolites by cation interchange the exchange * The " foot '' in the argon curve is due to a small nearly instantaneous van der Intra-crystalline Waals adsorption upon the external surfaces of the crystal powder.sorption is almost absent for argon on the time scale of the experiment. BARRER MOLECULAR-SIEVE ACTION OF SOLIDS 305 reaction is normally effected hydrothermally in presence of excess of the exchanging salt a t temperatures which may conveniently range from 150" to 250". Simple stainless-steel autoclaves can be used as reaction vessels Ca-mordenite . . Levynite . . . . K-mordenite. . . Ba-mordenite . . FIG. 7 ExampJes of positive and negative temperature coeflcgents in trunsgent flow oj- gases in zeolitic sorbents. (a) Kr in synthetic Na-mordenite. ( c ) Curves 1 and 2 refer to N in NH4-?nordenzte and curves 3 and 4 to Kr in the same (b) A in Ba-rnordenite. sorbent. (d) A in NH,-mordenite.[Reproduced by permission from 'I'raiis. Faraday SOC. 1949 45 363.1 except in the case of ammonium salts for which the exchange is better done in glass vessels and with ammonium chloride vapour. A range of differing molecular sieves may be prepared within limits set by it given aluminosilicate framework. In Table V are given values of D/u2 for argon a t - 78" in a series of the crystals u being the mean particle radius.25 1.51 x 10-8 Na-mordenite. . . 7-6 x lo-' U-mordenite . . . 2.8 x 10-6 NH,-mordenits . . 5.5 x TABLE V DiSfusion of argon at - 78" in cation-exchanged mordenites I Crystal. II Crystal. 1 D/a* (min.-1). i D / a z ( m h - l ) . 3.8 x 10-5 4.0 x 10-4 1-35 x 10-3 306 QUARTERLY REVIEWS Widely divergent sorption rates in dehydrated levynite at - 184" due to molecular-sieve action.(Qt denotes C.C. sorbed at N.T.P./g. QCQ denotes C.C. sorbed at N.T.P./g. at equilibrium and is for 0, 10.02 ; for N, 9.77 ; f o r k 10.13.) [Reproduced by permission from Nature 1947 159 508.1 FIG. 9 Xome relative sorption rates in Ca-mordenite at - 185" a?' - 7;". At - 185" the less easily condensed gases helium hydrogen and neon show no in the rate curves but the more easily condensed guses show "feet " due to adsorption upon external surfaces of the powder. Intracrystalline diflusion for oxygen nitrogen and argon is however very slow. A t - 78" the "feet " have almost disappeared and intracrystalline diflusion rates ure much increased. foot [Reproduced by permission from Trans. Faraday SQC. 1949 45 362.1 No allowance is made in the above data for variations in the value of a but the range in the quotients exceeds anything to be expected on this count.Fig. 9 similarly shows major differences in sorption velocities for a series of gases in a given crystal (cf. also Fig. 8). For the mordenites of Table V the rate sequence Kr < A < N < 0, Ne < H < He tended, BARRER MOLECULAR-SIEVE ACTION OF SOLIDS 307 with some exceptions to be preserved. On the other hand the affinities shown by the gases for the crystals were nearly in the converse order Kr > A N, 0 > H > Ne > He. The critical dimensions of some of these molecules are shown in Fig. 6. A special method for modifying zeolites consists fist in forming the ammonium zeolite and then slowly burning the ammonia away with oxygen to form the crystalline hydrogen ~eolite.~O The procedure is effective only for crystals capable of occluding oxygen.The removal in this way of some of the interstitial substance of the crystal results in a significant increase of the interstitial channel dimensions. Separations by Total Molecular-sieve Action.-The behaviour of single species some of which are copiously occluded while others are not sorbed (Table IV) suggests that quantitative separations might be effected merely by a single exposure of a mixture to it suitable amount of the appropriate zeolite. Such quantitative separation has been termed total molecular- sieve action (p. 293). Chabazite-type crystals were successfully used to resolve the hydro- carbon mixtures in Table VI. Derivatives of hydrocarbons may also be separated from other such constituents in admixture.31 Many such separa- tions occur only very slowly but are often accelerated by a rise in tem- perature within limits set by the decreasing sorption with increasing temperature and by the thermal stability of the species.TABLE VI Quantitative separations of hydrocarbons using Class I sorbents 28 Mineral. Chabazite Synthetic zeoIite (BaAl,Si,O,,,nH,O) Mixture. Temp. of removal. 150" 185 150 210 150 216 220 212 200 _ _ _ ~ ~ - 160 160 160 160 20 Component removed by sorption. C3H8 n-c4H10 C3H8 n-C4H10 n-C4H10 n-C7H16 n-C7H16 C,Hs and n-C,H, Some generalisations emerge from the separations effected (1) Monosubstituted met,ha,nes in which the substituent groups are small (Cl CH, OH CN NH, and the like) are very rapidly occluded by chabazite while monosubstituted ethanes with similar substituent groups are sorbed a1 R.M. Barrer J . SOC. Chem. Id. 1945 44 133. 308 QUARTERLY REVIEWS considerably more slowly. Both classes of solute can be quantitatively removed from molecules listed in col. 3 section (i) of Table IV. (2) In mordenite monosubstituted methanes were slowly occluded but monosubstituted ethanes were excluded. Separations were then obtained of substituted methanes with substituents C1 CH, OH CN or NH from similar ethane derivatives or carbon compounds with three or more carbon atoms. (3) Ca- and Ba-mordenites which did not even sorb simple methane derivatives appreciably separated only simple inorganic molecules (NH, H20 HCI) from organic species. Separations by Partial Molecular-sieve Action.-Table IV shows in a qualitative way the great extremes in rates of intracrystalline sorption.For example sorbates in col. 1 are occluded rapidly but those in col. 2 are only slowly occluded and with a great diversity of velocities even inter se. Thus in mixtures of such species it is possible to obtain separations by partial molecular-sieve action. Separations have been carried out both by exposing the mixture to the zeolite in a static system or by gaseous or liquid chromatography.l 3l One sorbate is occluded and finally removed in the time interval during which the other constituent is still largely unsorbed. Partial or complete separations were found in the mixtures C,H,-C,H ; CH,*CN-CH,Br ; C,H,*OH-CH,Br ; C,H,~OH-n-C,H, ; CH,*OH-C2H5Br ; the molecule first mentioned being removed first The most rapidly occluded molecules should preferably be more polar than those remaining.The more polar the sorbed molecule the greater the affinity between it and the zeolite tends to be. The affinity between sorbent and sorbate can exert IL strong influence upon the intracrystalline sorption velocity 25 as well as lessening the possibility of poisoning the sorbent by blocking of the intracrystalline diffusion paths by the bulkier and less mobile second component. Scope of the Molecular-sieve Method.-The molecular-sieve method can be one of unusual power which sometimes supplements other sepasation techniques. Mixtures may be resolved quantitatively which cannot be dealt with by distillation either because the species have practically the same boiling point (n-heptane and isooctane) or because azeotropic mix- tures are formed (H,O-C,H,*OH ; CH,*OH-CH,*CO*CH ; H,O-dioxan ; CS,-CH,*CO*CH ; C,H,-OH-n-heptane ; C,H,*OH-toluene).Many indi- vidual separations should be typical for groups of analogous compounds. For instance mordenite removes methyl from ethyl alcohol and ought therefore to remove it from all alcohols. Ca- or Ba-mordenite since they dry methyl and ethyl alcohol or acetone should dry all alcohols and ketones. Select'ivity in sorption shown by molecular-sieve zeolites is often the reverse of t)hat normally shown in more open capillary structures such as silica gel. In gels porous on this more macro-scale the affinity between sorbate and sorbent often increases with increasing molecular dimensions for such homologous series as the paraffin hydrocarbons.In the gas- sorbing zeolites on the other hand the sorbate-sorbent affnity drops BARRER MOLECULAR-SIEVE ACTION OF SOLIDS 309 sharply to zero once a certain limit in molecular size and shape is exceeded. Possibilities already realised with mordenite of modifying the zeolites by cation interchange or in other ways and so of altering the molecular- sieve behaviour as desired raises the question of the availability of natural zeolites as raw materials. Except perhaps in the case of mor- denite large natural deposits are not known so that practical developments must be limited until synthetic crystals can be made cheaply. Crystals of m~rdenite,~~ a n a l ~ i t e ~ ~ harmotome,3* and a chabazite substitute 21 have been successfully grown on a laboratory scale. Gmelinite levynite chaba- zite gismondite and other zeolites which may be valuable have not been prepared.Molecular-sieve Properties of Membrmes Metals.-We have now to consider selective transmission of gases and vapours through compact membranes. Some metals act as total molecular sieves being permeable to one or two gases but to no others. In all such cases there is a specific interaction between the diffusing gas and the metal. Transition metals often have the power of forming interstitial solid solutions with some elements of the first short series of the Periodic Table. These elements must be small enough to enter the interstices in positions where their co-ordination number may be either 4 or 6 (so called tetrahedral or octahedral sites). Such phases occur only when the radius ratio of non- metallic to metallic elements is about 0.6 or less and when the difference in electronegativity between non-metal and metal is not too great.For instance though hydrogen carbon and nitrogen form a considerable num- ber of these phases oxygen forms only a few and fluorine does not form a n ~ . ~ b The interstitial atoms often show considerable mobility within the metal and if the metal is in the form of a membrane an atmosphere of the alloying* gas may be transmitted through it from the high- to the low- pressure side.36 The rate of permeation is not usually large but such membranes have been suggested as slow controlled leaks for hydrogen (using palladium) or for oxygen (using No other gases are trans- 32 R. M. Barrer J. 1948 2158. 33 Cf. idem Trans. Puratdag Soc. Discussion on Crystal Growth 1949 No.5 326. 34 Idem unpublished data. 35 For two discussions of properties of these phases see R. E. Rundle Actu C'ryst. 1948 1 180 and R. M. Barrer Tram. Fa.raday SOC. Discussion on Physical Chemistry of Metallurgical Processes 1948 No. 4 68. 36 A discussion of the solution and diffusion of gases in metals will be found in C. Smithells " Gases and Metals " Chapman & Hall 1937 ; and in R. M. Barrer " Diffusion in and through Solids " C.U.P. 1941. 37 E. L. Jossem Rev. S c i . Instr. 1940 11 164 * The term " alloy " is in many ways an appropriate description of tliese interstitial solid solutions because they retain metallic conductivity often show super-conductivity and have a metallic lustre. Moreover as in many alloys the composition of the phase may vary continuously within limits.The nature of the bond has received considerable attention. 3ii 310 QUARTERLY REVIEWS mitted by these metals. Similarly iron transmits both hydrogen and nitrogen under conditions where the corresponding interstitial phases form. There are two experimental procedures for studying the diffusion of gases in metals. In one the metal is used in the form of a hollow cathode. This method has hitherto been limited to the diffusion of hydrogen.38 In the second procedure one side of the metal membrane is exposed to the gas a t high temperature the other side being maintained under a near- vacuum. In the electrolysis method surprisingly rapid transmission may 70 4/1 FIG. 10 Exponential increase in permeation velocity P with temperature ( P = P,e-E/RT) [Reproduced by permission from Barrer “Diffusion i n and through Solids ” C.U.P.1941 p . 164.1 occur even a t room temperatures but in the thermal method temperatures upwards of several hundred degrees Centigrade are ne~essary.~g 36 Per- meation velocities increase exponentially with temperatures (Fig. lo) 40 and some typical permeability constants are given in Table VII.41 The non- 38 E.g. M. Bodenstein 2. EZektrochem. 1922 28 517 ; G. Borelius and S. Lindblom Ann. Physilc 1927 82 201 and see also ref. (36). 39 R. M. Barrer Trans. Paraday Soc. 1940 36 1235. 40 Idem ref. (36). 41 The data are taken from Barrer ref. (36) Table 42. BARRER MOLECULAR-SIEVE ACTION OF SOLIDS 311 metallic element always diffuses in the form of atoms so that sorption of diatomic molecules is preceded by dissociation.The essential features of the diffusion process may be illustrated by a diagram in which characteristic periodic variations of energy are shown as a function of the positional co-ordinate of the diffusing species relative to the membrane (Fig. 11). Distance - x i o FIG. 11 bounded by the planes x = 0 and x = 1. A diagrammatic representation of energy relations in diffusion through a membrane Adsorption is an exothermal process the heat AH being given by the depth of the trough in the potential-energy diagram just outside the sur- faces of the membrane. E, E, and 3 are very closely related" respec- tively to the energies of activation for penetration from the surface to a point just inside the metal for " jumping " from one interstitial site to the next and for passing from just within the surface to the surface itself.E and E are similarly related t o the energies of activation for adsorption and desorption. The heat of occlusion is AH, and may be either positive or negative. The phase-boundary reactions occurring with activation energies E and E are sensitive to modification in the nature and extent of the sur- face due to sintering or because of adsorbed films of foreign atoms.36 The metal surfaces show some of the variability in activity which is shown by the same surfaces functioning as catalysts as the data in Table VII illus- trate. *l Phase-boundary reactions may then become much slower than diffusion within the metal. Palladium shows extreme variations due to this cause. A partial separation of isotopic species hydrogen and deuterium can be effected when these are simultaneously transmitted through palladium or platinum.On account of their different masses the isotopic atoms have in their interstitial environment different zero-point energies that of the lighter element being the greater. For this reason energies of activation for diffusion heats of solution and solubilities of the isotopes will differ. * A more exact relation is energy of activation = height of energy barrier minus the zero-point energy of the diffusing atom. 312 QUARTERLY BEVIEWS System. TABLE VII Permeability constunts P for several metallic membranes P = Poe-E/R1' Po (c.c. per see. per cm.* per mm. thick per atm. pressure). H2-Pd 2.3 x lo-' 3.0 x lod2 I -I-___ P,/P,. ~ _ _ _ _ _ 1.84 1.40 1.36 1.24 H2-Ni Temp. 20" 106 131 186 1.3 x 0.85 x 10-2 1-4 x 1-05 x lop2 1.44 x 1.55 1.50 1.35 1.36 1.27 E (cals./g.-atom).System. ll _ _ _ _ ~ 550" 650 750 850 950 15,420 11 H,-Pt 13,800 13,400 _____ 13,260 I 0,-Ag I1 13,860 i Po (c.c. per sec. per per mm. thick per atm. pressure). 2.3 x 10-3 1.5 x 10-3 ________ 1-41 x 1.18 x 10-2 2.6 x 10-2 3.75 x 10-2 2.06 x B (cals./g.-atom) 16,600 18,700 19,600 18,000 19,800 22,600 22,600 - In the steady state and for a simple diffusion through a membrane into a vacuum in absence of slow phase boundary processes PH/~D = DHCH/DDCD where the P's D's and C's are respectively permeability diffusion coeffi- cients and concentrations just within the ingoing surface. Some experi- mental values of PH/PD for platinum 42 and palladium 43 are given in Table VITI. TABLE VIII Permeability ratios for hydrogen and deuterium Analytical for P / P .Analytical expression for PH/PD. Silica and Silicate Glasses.-Membranes of silica glass transmit helium neon and hydrogen and to a lesser extent oxygen nitrogen and argon. The diffusion process occurs without dissociation of the diffusing molecule and without specific interaction between gas and membrane as is the case when gases diffuse in metals.44 Selectivity is considerable but total molecular-sieve action does not occur (Table IX). The diffusing molecules 42R. Joum J. Phys. Radium 1936 7 101. 4 3 A. Farkas Tram. Farday SOC. 1936 32 1667. 4 4 For a discussion see R. M. Barrer ref. (36). BARRER MOLECULAR-SIEVE ACTION OF SOLIDS . . . . . . . . Gas. He. Hz. Ne. Na. A. Relative permeability * . 100 18 3-3 2.6 1.6 Apparent energy of activa- tion (cals./g.-atom or ~ ~ g.-mol.) .. . . . 5600 10,100 9500 I 29,900 32,100 I 313 oa. ~ - 31,200 TABLE IX Relative permeabilities of silica g h s at 900" I Glass. I I Relative Apparent energy of activation (cals./g.-atom or g.-mol.). * There are considerable differences between permeabilities obtained when using different examples of silica glass. 44 or atoms move in channels which sheathe them very closely. In silicate glasses the diffusion paths become still more restricted and possibly fewer in number as indicated by a rising energy of activation or decreasing permeability towards helium (Table X). The apparent energies of activa- tion in Tables IX and X are probably very nearly true energies of activation because the heats of solution of the gases in silica glass at least are very sma11.45 Silica .. . . . . . . Pyrex . . . . . . . Leadglass (283') . . . Jena 16111. . . . . . Thuringian glass. . . . Soda glass (283"). . . . 100 12.1 0.31 0.117 0.114 0.027 5700 8700 - - 8720 11,200 True and apparent energies of activation for intracrystalline diffusion in various zeolites are recorded in Table 25 By contrast with some silicate glasses and zeolites helium cannot under any conditions diffuse through metals and in increasing order of openness of diffusion paths we may write the series Metals < silicate glasses < silica glass < levynite mordenite < chabaeite In all these systems the diffusing species move entirely within the range of action of the interatomic forces of the solid. The condition described by equation (3) (in which diffusion occurs in part as above and in part as a molecuIar streaming) requires appreciably wider channels.46 G. A. Williams and J. B. Ferguson J . Amer. Chem. SOC. 1924 46 635 ; H. Wiistner Ann. Physik 1915 48 1095. 314 QUARTERLY REVIEWS TABLE XI Some true and apparent energies of activation for intracrystalline diffusion in zeolites Crystal. Chabazite Natural crystals Mordenite Ba-rich crystals Ca-rich crystals Na-rich crystals Li-rich crystals NH,-rich crystals K-rich crystals Levynite Natural crystals Diffusing gas. 2 P C2H6 CH,CN A Kr A Kr A Kr H2 0 2 N2 A Kr Ne A Kr True activation I A>parent activation energy energy (cals./g.-mol.). (cals./g.-mol.). I 6600 9800 8100 11,500 - - 9300 11,000 7300 7600 7000 9000 2500 4400 4800 8400 10,000 2600 9400 12,000 4500; 6700 8900; 8600 7100; 6800 11,100 ; 11,400 ; 9600 5900 7300 N 40,000 Organic Membranes.-Pronounced molecular-sieve effects arise when gases and liquids diffuse through organic foils.Some of these membranes such as cellulose and some proteins are largely crystalline ; others includ- ing natural and synthetic elastomers are normally amorphous. The elastomers provide an interesting new feature in the diffusion process because individual molecules of the elastomer possess considerable flexi- bility and (within limits) mobility which play an important part in the diffusion of solutes within the medium. Each unit diffusion process is believed to involve not only the solute molecule but also segments of polymer chains adjacent to the solute in a zone of activation in which BARRER MOLECULAR-SIEVE ACTION OF SOLIDS 315 co-operative movements of polymer segments and solute result in a successful place change of the solute r n ~ l e c u l e .~ ~ ~ 47 SUbhUF %. FIG. 12 Permeability constants of some hydrocarbons in natural rubber vulcanisates ( P in C.C. of gas at N.T.P. passing per second through 1 cm.2 of membrane 1 mm. thick under a pressure diflerence of 1 cm. of Hg). [Reproduced by permission from J. Polymer Sci. 1948 3 554.1 TOO 80" 80 e Sulphur % . FIG. 13 Diffusion coeficients of some hydrocarbons in natural rubber vulcanisates (D in cm. 2sec.-1>. [Reproduced by permission from J. Polymer Sci. 1948 3 554.1 Number of carbon atoms FIG. 14 Solubility constants u for some hydrocarbons in a rubber vulcani- sate containing 1.7% of combined sulphur. The upper curve was obtained at 40° the middle curve at 60",and the lowest curve at 80".G i s measured in C.C. at N.T.P. dis- solved in 1 c . ~ . of rubber at 1 atm. pressure. [Reproduced by permission from J. Polymer Sci. 1948 3 566.1 - _ _ _ _ _ _ _ _ _ ~ __ . ~ _ _ 4 0 R. M. Barrer Trans. Faraday SOC. 1939 35 628. 47 R. M. Barrer and G. Skirrow J . Polymer Sci. 1948 3 549 564. 316 QUARTERLY REVIEWS It is interesting to trace the effects of systematically reducing the fiexi- bility and mobility of elastomer chains upon the permeability of a group of solute molecules which are themselves of regularly increasing molecular volume. Cross-linking by vulcanisation was used to reduce the mobility of elastomer chains and the n-paraffins from methane to n-butane were used as solutes. Several of the effects observed are illustrated in Figs.12 13 and 14. There was no marked change in the solubility of a given hydrocarbon as the amount of combined sulphur increased but the dif- fusion coefficients decreased steadily and the decrease was greatest for the solute of greatest molecular TABLE XI1 Relative permeability (P) and diffusion coeflcients (D) at 40" 100 73 27 17 17 I Natural rubber (1.7% combined S). I Natural rubber (11.3% combined S). 1 1.00 100 2-17 - 4.16 15.5 3.46 4.95 10.2 4.41 Gas. Relative P. I 1.00 2.90 7.4 13.0 31.8 Relative D. I Relative P. I Relative D. Typical relative permeability and diffusion coefficients are shown in Table XII. The relative value of P is proportional to the product of the diffusion coefficient D and the solubility constant. The solubility constant increases rapidly with increasing molecular weight of the paraffin (Fig.14) whereas D decreases (Fig. 13) so these two effects influence the permeability constant in opposite senses. The net effect is however that P becomes larger as the molecular weight of the diffusing hydrocarbon becomes greater (Fig. 12). However increasing the combined sulphur in the rubber decreases D proportionately more for higher molecular-weight paraffins while leaving the solubilities largely unaltered. Hence the molecular-sieve action towards the paraffins as measured by the relative value of P becomes smaller in the vulcanisate of larger combined sulphur content. On the other hand for the permanent gases where the solubility con- stants are all of the same order P usually decreases with increasing mole- cular weight.In this case the molecular-sieve action of the rubber towards the gases increases as combined sulphur content of the vulcanisate increases (Table XIII) .48 Clearly in ebonite a highly cross-linked three-dimensional network molecular dimensions exercise a dominant influence. As a final example of selective transmission one may give some relative permeabili- ties towards polar and non-polar gases of foils of celluloid rubber and gelatin (Table XIV).49 The greater permeability towards polar gases is due to their much larger sorption in the polar media. Natural-rubber membranes normally contain polar impurity such as protein. 50 48 R. 35. Barrer Trans. Faraday SOC. 1940 36 644. 50 C. Boggs and J. Blake Ind. Eng. Chern. 1926 18 224. Idem ref. (36). BARRER MOLECTTLAR-SIEVE ACTION OF SOLIDS Molecular Relative permeability in diameter,.A. low-sulphur rubber i at 250. 317 Relative permeability in high-sulphur rubber (ebonite) at 67’. TABLE XI11 Relative permeatbilities of gases in rubber and ebonite 100 3.0 1 161 4.08 I 25.8 He. . . . . H,. . . . . N . . . . . ~ Gas. 100 30.0 0-53 TABLE XIV Relative permeabilities towards polar and non-polar gases Gas . . . . . . . . . Celluloid . . . Gelatin . . 100 100 100 247 286 100 898 6160 906 1 2500 1 3600 1 413 3190 9520 Molecular-sieve Processes in Solution Although it is not possible here to discuss in detail data on selective transmission of ions and dissolved species through solids nevertheless some reference to the phenomena noted should be made. Glass membranes functioning as hydrogen electrodes are normally regarded as permeable to hydrogen ions; and other glasses may function similarly as sodium potassium silver or zinc electr0des.5~ Membranes composed of clay crystallites can also function as electrodes reversible with respect to ions which they may take up by cation interchange.62 Crystalline zeolites ultramarines sodalite-hauyne minerals as well as amorphous zeolite gels such as ‘‘ permutit ” or ‘‘ doucil ” also interchange cations comparatively freely.63 b49 55 Sometimes there is a high selectivity in these exchange reactions analcite (hTaA1Si206,H20) will exchange Na+ for K+ or NH,+ but not for Li+ Cs+ Ca++ or Ba+’+ save to a minor extent ; leucite (KA1Si206) similarly exchanges K+ for Na+ or NH,+ but not for the other ions.Such selective exchanges indicate the possibility of preferentially 51 See W.M. Clark “ Determination of Hydrogen Ions ” Baillisre Tindall & Cox 1928 3rd Edn. p. 430 for a summary of such observations. C. Marshall and C. Krinbill J . Amer. Chem. SOC. 1942 64 1814; C. Marshall and W. E. Bergmann ibid. 1941 63 1911 ; J . Physical Chem. 1942 46 527 326. 63 H. F. Walton J. Franklin Inst. 1941 232 305. 5 4 R. M. Barrer International Colloquium on Reactions in the Solid State Paris 66 S. B. Hendricks I f i d . Eng. Chem. 1945 57 625. Crystals and glasses are often highly selective in transmitting ions. Oct. 1948 ; Bull. SOC. chim. 1949 Nos. 1 and 2 D.71. 318 QUARTERLY REVIEWS interconverting sodium potassium or ammonium salts by hydrothermal chromat~graphy.~~? 56 Other zeolites such as chabazite or mordenite or zeolite gels interchange a much greater diversity of cations.Crystalline zeolites show cation interchange freely in the range 150-250" and less rapidly below 150"; zeolitic gels organic exchangers and some clays however interchange ions freely even at room temperature. Both organic and inorganic gel membranes may serve as ultra-filters for dissolved species whether ionic or non-ionic. Membranes may there- fore establish and maintain differences in potential pH or concentration between opposite faces. They may bring about hydrolysis effect dialysis and permit also the establishment and measurement of osmotic pressures. Reasons for the selective transmission giving rise to these effects may be considered. Membranes which selectively transmit cations include polyacrylic acid co-deposited with acetyl cellulose ; oxycellulose ; cellulose impregnated with dyes containing acid groups ; and to a lesser degree nitrocellulose and acetyl cellulose which both contain some free carboxyl groups.On the other hand membranes may be prepared which contain both anionic and cationic groups for instance by condensation of triethanolamine and phthalic acid. Here there are tertiary amino-groups and carboxyl groups. I n alkaline media where the ionisation of the amino-groups is suppressed and that of the carboxyl groups promoted the medium is cation-permeable. However if the tertiary amino-groups are converted into quaternary groups by reaction with methyl iodide the membrane is anion-permeable under all conditions. Protein membranes are also amphoteric and according to the pH of the solution they transmit anions (in acid media) or cations (in alkaline media).Basic membranes sorb acid dyes acid membranes sorb basic dyes. Clearly electrostatic forces govern in part the selective trans- mission in such systems.57 In membranes without ionisable groups and of very open character the mobility of ions is substantially the same as in aqueous solution. As the capillaries become more and more restricted the mobilities become increasingly different from their values in " free " trans- port. In membranes which also contain ionisable groups these molecular- sieve effects may be superposed upon the further specificity due to electro- static attractions or repulsions between charged groups in the organic net- work and the ions diffusing. A quantitative interpretation of the over-all transmission is due to T.Teorell 58 and to Meyer and Sie~ers.5~ The relative permeation velocity is NJN, where N and N are the numbers of cations and anions transmitted per unit time. This ratio in the steady state depends on the product of the ratios U/V and C,/C, where U and 7 are mobilities of cation and anion respectively and Cc and S. Green and C. McCarthy I d . Eng. Chem. 1944 36 412. 67 K. H. Meyer " Natural and Synthetic High Polymers " Interscience 1942 68 PTOC. Soc. Exp. Biol. Med. 1935 33 282. 69 Address to the Assoc. Chim. de GenGve 1935; Helv. China. Acta 1936 19 p. 631. 649 665 987 ; K. H. Meyer H. Hauptmann and J. F. Sievers ibid. p. 948. BARRER MOLECULAR-SIEVE ACTION OF SOLIDS 319 Ca are the concentrations of cations and anions just inside the ingoing surface of the membrane.As an example one may consider an uni- ivalent electrolyte which is fully dissociated. Let the concentration in the solution be C and let X denote the concentration of non-diffusible fixed ionic groups in the membrane expressed in g.-equivs. per litre of imbibed liquid; X is a constant characteristic of the membrane at any one liquid content. Also Y denotes the concentration of imbibed electro- lyte in g.-equivs. per litre of imbibed liquid. In a membrane where the fixed ionic groups are anionic the concentration of diffusible anions is Y and of cations X + Y . The Donnan membrane equilibrium condition yields * C2 = Y(Y + X ) which may be combined with the relation Nc/Na = U ( Y + X ) / V Y = UCc/VCu to give N u 1 / 4 m 2 + x v 1/4c2 + x2 - x _ _ _ - Na In this equation one sees separately the effect of the mobility ratio and that of electrolyte concentration and concentration of fixed anionic groups upon the permeability ratio.According to the equation as C becomes very small the network of the anionic membrane contains only diffusible cations and is permeable only to cations. At higher electrolyte concentrations both ions are transmitted. Membranes with graded pore sizes have been prepared from cellulose nitrate or acetate. These membranes can be used in the separation and characterisation of colloidal particles bacteria and viruses.60s 61* 62 The ester dissolved in solvents such as acetone was poured on to a suitable surface and the solvent allowed partly to evaporate. The foil still con- taining solvent was further precipitated with coagulating agents such as dilute acetic acid.By varying the conditions of formation (nature of solvent amount of solvent in partly dried film and amount of coagulant) the texture of the porous membrane was altered and a series of graded ultra-filters obtained which transmitted particles of up to 500 mp. in diameter. Such membranes must be stored in water to prevent the irre- versible shrinkage which would follow drying. Standard particles such as 80 H. Bechold and K. Silbereisen Biochem. Z. 1928,199 1 ; H. Bechold Kolbid-Z. F. Erbe {bid. 1932 59 32 ; 1933 63 277. 1934 66 329; 1934 6'7 66. 6 2 W. J. Elford Trans. Faraday SOC. 1937 33 1094. * This equation in terms of activities should be written c2 = Y(Y + X)YcYu/Y+Y- = Y(Y + X ) R where 'ye and ya are the ionic activity coefficients of the imbibed ions and y+ and y- me theso coefficients in the surrounding electrolyte.The correction factor R may then be introduced. Y 320 QUARTERLY REVIEWS hzmoglobins were employed to calibrate the membranes. It is considered that the particles being separated should be more or less spherical and that this molecular-sieve method is not suitable for separating chain polymers. The Reviewer acknowledges gratefully permission to reproduce illustrations as follows the Faraday Society Figs. 1 5 7 and 9 ; the American Chemical Society Fig. 4 ; Messrs. Macmillan & Co. Ltd. Fig. 8 ; the University Press Cambridge Fig. 10 ; and Interscience Publishers Inc. Figs. 12 13 and 14. He also thanks the Faraday Society and the Aberdeen University Press Ltd. for the loan of blocks for Figs. 5 6 and 9.

ISSN:0009-2681    

DOI:10.1039/QR9490300293

出版商:RSC    

年代:1949

数据来源: RSC

摘要:

RECENT STEREOCHEMISTRY OF THE GROUP VIII ELEMENTS By R. S. NYHOLM M.Sc. A.R.I.C. (UNIVERSITY COLLEGE LONDON) THE Group VIII elements are of special interest in the Periodic Table for the number of valencies that they display and for the ease with which they give rise to co-ordination compounds. The ammines of cobalt platinum and rhodium in particular have been studied for many years but more recent work on metals other than these and with hitherto uncertain valency states suggests that a survey of the group as a whole would be profitable. Attention is directed here chiefly towards work which has been done during the last few years. This shrvey leads to the broad conclusion that although many formerly doubtful valency states have been verified and the stereo- chemistry elucidated yet many other problems -await solution.The properties of these elements are bound up with their special elec- tronic arrangements. In each of the triads Fe Co N i ; Ru Rh P d ; Os Ir Pt (referred to henceforth as the Ni Pd and Pt triads respectively) the d orbitals are being filled leading to a total of 18 electrons in the outer- most shell. This means that d s and p orbitals all of similar energy are available for bond formation. Complex formation may thus involve the use of a mixture of d s and p orbitals with excellent opportunities for hybridisation which together with the large number of valencies of these elements give us the greatest possibilities for stable complex formation. Before discussing the stereochemistry of these elements in their different valency states it is helpful to outline the more prominent chemical relation- ships of the group as a whole.The nickel triad differs from the other six elements in two main ways although it is very similar to them in the com- plexes which they form. First iron cobalt and nickel form free ions much more readily than do the other elements and as a consequence their salts differ from those of the palladium and the platinum triad. Thus the halides of Fe Co and Ni are true salts and give rise to simple hydrated cations whereas the properties of the other halides are essentially those of covalent complexes. This greater tendency towards complex formation manifests itself in such ways as the formation of very stable anions and in peculiar magnetic behaviour. Secondly although Fe Co and Ni show variable valency the stability of higher-valency states increases sharply on passing from the Ni triad to the Pd and the Pt triad.Towards chlorine the maximum valency is two for all elements in the first row except iron but it increases to four for the remainder ; and the double halides M1,MIVC1, which are so characteristic of the Pd and Pt elements cannot be isolated with Fe Co and Ni. Towards oxygen ruthenium and osmium show valencies as high as eight. It must be emphasised that although horizontal similarity is most evident between Fe Co and Ni in their simple salts yet vertical similarity is the rule when considering complexes. A few examples will illustrate these vertical relationships. Fe Ru and 0s in the bivalent state all form highly coloured stable tris-o-phenanthroline and trisdipyridyl complexes 321 322 QUARTERLY REVIEWS like [Fe(dipy),]CI and give rise to the isomorphous series of cyanides K4M(CN),,3H,0 where M = FeII RuII 0sT1.In the tervalent state com- plexes of the type K,M111(C,04) (MII1 = FeIII RuIII) are known but relatively few tervalent osmium compounds have been described. In the tervalent state Co Rh and Ir are very much alike in their ammines alums and complex nitrites. However as we shall see later whilst bivalent cobalt gives rise to four-covalent complexes bivalent rhodium and iridium (except in carbonyl complexes) are invariably six-covalent. Bivalent Ni Pd and Pt may be either four-covalent or six-covalent in their complexes with a very marked preference for the former ; in fact aix-covalent complexes of bivalent platinum and palladium are rare.Typical of the four-covalent complexes of these 'elements are the complex cyanides like K,Ni(CN) ; the quadrivalent state is exemplified by com- plex halides of the type K,PtCl and K,PdCl, but the corresponding nickel compound is unknown. In Table I are shown the valencies (and usual co-ordination numbers) of the Group VIII elements which have been established beyond any reasonable doubt. An example is given of each valency ; numbers in brackets refer to co-ordination numbers which are only found very rarely. Except for occasional references carbonyls have been omitted from this article as they have been fully discussed in an earlier Review.l TABLE I Group VIII elements valencies and co-ordination numbers __ Fe. I1 I11 VI Ru. I I1 I11 IV V VI VII VIII 0s. I1 I11 IV VI VIII ~ __ - Co-ord 1 no.? 6 6 ? ? ? 4 6 (4) I co. I1 I11 Rh. I1 111 IV VI Ir. I I1 I11 IV VI iIrC1L Yo-ord no. ~ 4 6 6 ~ Ni. 0 I I1 I11 VI Pd . 0 I1 I11 IV Pt. I1 I11 IV VI K,Ni( CN) K,Ni(CN) K,Ni(CN) NiBr, 2PEt BaNiO PtC1,,2NH3 PtC1 K,PtCl KpPtO Co-ord 110. 4 ? 4 6 5 ? 4 4 $6) 6 [All of the Group VIII elements except Pd and Pt form carbonyls in which the metal atom is formally zero-valent.] 1 J. S. bderson Quart. Reviews 1947 1 331. NYEOLM RECENT STEREOCHEMISTRY OF GROUP VIII ELEMENTS 323 Fe++ (ion) . * Theoretical Basis of Stereochemistry The modern theory of stereochemistry as applied to these elements owes much to the work of Pauling and his school. A brief summary only is given here for many adequate reviews of the subject are By making use of the directional properties of s p and d orbitals it is possible to predict the shape of the simpler molecules.To do this a know- ledge of the orbitals used for bond formation is required ; these are some- times obvious but with the transition elements the magnetic behaviour is often of great help. As electrons are progressively added to an atom the order of increasing energy in any given level is s < p < d < f corresponding to azimuthal quantum numbers of 0 1 2 3 respectively. In any given level a maximum of two s six p and ten d electrons may be accommodated. When elec- trons are added to a p or d'sub-group Hund's rule requires that the maxi- mum number of orbitals shall be singly occupied before pairing of spins takes place; when only five electrons are available for a d sub-group for example this means that all five spins will be parallel.Using oppositely directed arrows to indicate opposite spins the iron atom in the ground state (ls22s22~63s23p63d64s2) and the Fe++ ion (ls22s22p63s23p63d6) would be represented as in Table 11. Unless far-reaching reorganisation of these levels takes place an example of which is the ferrocyanide ion the four electrons in the 3d shell of the Fe++ ion remain unpaired and give rise to the paramagnetic moment characteristic of the iron-group elements. 3 9 J-f-1 .1 .I q-(j TABLE I1 Electronic arrangement of iron compounds 3d. 4s. The shape of simple molecules can be deduced from a knowledge of the facts that s orbitals are spherically symmetrical whilst the three p orbitals are mutua,lly perpendicular to one another. Since s orbitals have no directional properties one might expect a combination of an s orbital with a p orbital to give two bonds at the maximum angle to one another i.e.180". Thus we find that mercuric chloride is linear since the forma- tion of two covalent bonds from the mercury requires unpairing of the two 5s electrons to give a bivalent mercury atom using one 5s and one 5p orbital. Two p orbitals however give rise to a V-shaped molecule the L. Pauling " Nature of the Chemical Bond " N.Y. 1945. 2nd edn. G. E. Kimball J . Chem. Physics 1940 8 194. C. A. Coulson Quart. Reviews 1947 1 144. 324 QUARTERLY REVIEWS commonest example being water ; here the oxygen atom (ls22s22p4) has two singly occupied 2p orbitals and pairing of these with the electrons from two hydrogen atoms gives a molecule with an angle at the oxygen atom slightly greater than go" the widening being attributed to partial ionic character of the 2p bonds or some s character therein.In the same way ammonia is a pyramidal molecule because of the use of the three 2p orbitals of the nitrogen atom whildb boron trichloride is planar since the boron atom uses one 2s and two 2p orbitals for bond formation. Using symmetry theory G. E. Kimball has summarised the possibilities of stable bond formation which arise from various combinations of orbitals; the results are given in Table 111. (approaching the problem from a different viewpoint) have confirmed several of these and have amplified the five-covalent structures by considering in more detail d orbitals in combination with the s and p . We are specially interested in the four- five- and six-covalent com- plexes in which a mixing of s p and d orbitals takes place.Conditions are ideal for many possible combinations with these elements because the d sub-group and the s and p sub-groups of the next higher principal quan- tum number are very similar in energy. The more important combinations are the sp3 hybridisation resulting in the tetrahedral configuration the dsp2 hybridisation which gives rise to planar (square) four-covalent com- plexes and the a " ~ ~ hybridisation which leads to octahedral six;covalent complexes. Double-bond formation appears to increase the strength of certain of these bonds ; Pauling has suggested that in the case of the ferrocyanides for example the three remaining 3d electron pairs from the iron atom may be used to give double-bond character to the d2sp3 bonds.The recent structure determination of [Fe(CN*CH3),]C1,,3~~0 shows an Fe-C bond length of 1-85 A. ; and since the sum of the single-bond radii for these elements is 2.00 A, this is usually taken to indicate about 50% double-bond character arising from the use of the three 3d electron pairs of the iron atom resonating among the six d2q3 bonds. Kimbal13 has discussed the cases in which these strong st bonds might be expected. It should be mentioned however that the need for care in the assignment of double-bond character to short bonds has been discussed by several workers.' 8 s Unfortunately these theoretical methods go no further than indicating possible spatial arrangements and as N. V. Sidgwick and H. M. Powell lo have pointed out " the chemist would be glad to infer the stereochemical type from some property of the molecule with which he is more familiar ".It is often difficult to decide whether d s or p orbitals in an atom are being used for bond formation; but magnetic data are sometimes useful. M. R. Daudel and A. Bucher J . Chim. physique 1945 42 6. H. M. Powell and G. W. R. Bartindde J . 1945 799. A. F. Wells J . 1949 65. G. M. Phillips J. M. Hunter and L. E. Sutton J . 1945 146. H. A. Skinner and L. E. Sutton Trans. Furaday SOC. 1944 40 164. lo Proc. Roy. Soc. 1940 A 178 153. NYHOLM RECENT STEREOCHEMISTRY OF GROUP VIII ELEMENTS 325 TABLE I11 Shpe of bond orbituls Jo-ord no. Configuration. Configuration. g P 9 dP Arrangement. Arrangement. dasp3 Linear Angular Octahedron Trigonal prism p a ds d 2 sp2 dpa d2s d 3 Trigonal plane d3p3 Trigonal anti -pr ism 3 -__ 4 5 Unsymm.plane d3sp2 d6s d4p2 Mixed dSP P3 d2P d %p3 d %p ZrF,3- Structure Trigonal pyramid sp3 d3s Tetrahedral d4sp2 d4p3,d6pz TaF,2- Structure dsp2 dapa Tetragonal planar d 4sp Dodecahedron 8 Irregular tetrahedron d6p3 Anti-prism d4 Tetragonal pyramid d Face-centred prism dsp3 d3sp Bip yr amid d2spa d4s d2p3 0% Tetragonal pyramid Pentagonal plane d3p2 d6 Pentagonal pyramid Doubly filled orbitals contribute nothing to the spin magnetic moment but singly filled orbitals give rise to the paramagnetic moment of dn(n + 2) Bohr magnetons (B.M.) where n is the number of unpaired electrons. This assumes that for the iron-group elements the moment may be attributed to spin only an approximation which appears to hold for a wide range of compounds.The use of magnetic data to indicate bond type is well illus- trated by the complexes of bivalent nickel. The electronic configuration of nickel in tetrahedral and planar four covalent complexes is : 326 QUARTERLY REVIEWS 3d. 4s. 4P. Thus four-covalent planar bivalent nickel complexes with asp2 bonds are diamagnetic but when tetrahedral complexes are formed whether ionic or covalent using sp3 bonds the two unpaired electrons in the 3d shell will give rise to paramagnetism. This has been confirmed for a large number of nickel complexes the magnetic moment of the tetrahedral compounds being slightly larger than 3 B.M. Magnetic data are limited in their application for Hund's rule does not seem to apply to the palladium and the platinum triad and rearrangement of the electrons usually takes place so as to give the minimum moment.In these two triads the only cases where a moment greater than 1.73 B.M. (corresponding to one unpaired electron) has been observed are with the complexes of quadrivalent ruthenium. For K,RuCI the value 2.83 B.M. has been reported.ll This is the moment expected for the d 2 q 3 octahedral configuration but the anomalous behaviour of these elements is illustrated by the fact that the similar osmium compound K20sC1, has a moment of only 1.3 B.M.ll The majority of molecules with an even number of electrons are diamagnetic owing presumably to complete quenching of both orbital and spin components by molecular or crystalline forces. D. P. Mellor 12 has reported that even complexes of Rhrr are diamagnetic in spite of the fact that bivalent rhodium contains an odd number of electrons.The literature has been summarised by P. W. Sel~ood.*~ Before the magnetic data of the Pd and the Pt triad can be of much help the mag- netic behaviour of many of the less common valency states needs further investigation. In Table IV are shown the predicted number of unpaired electrons and magnetic moments for the planar tetrahedral and octahedral configurations. The predicted value is 2-83 B.M. Co-ordination Number Four Although the Group VIII elements show many valencies the number of different stereochemical types is small. The co-ordination number of 2 which is so characteristic of the univalent state of the neighbouring elements copper silver and gold has not been reported except possibly for the compound RuBr,CO. Among these elements univalency is rare There is no known example in Group VIII of the co-ordination number 3 the elements being usually 4- or 6-covalent.Recently a few 5-covalent com- plexes have been obtained but the only compound in which the co-ordina- tion number exceeds 6 is the octavalent compound OsF,. We shall discuss the various shapes in turn and the valencies where they arise. 11 W. P. Groves and S. Sugden (W. P. Groves Ph.D. Thesis 1941 London). 12 J . Proc. Roy. SOC. N.S.W. 1943 77 145. NYHOLM RECENT STEREOCHEMISTRY OF GROUP VIII ELEMENTS 3'27 TABLE IV Predicted moments for Group VIII elements in covalent complexes Tetrahedral ionic or sp8 bonds. Octahedral d%p* bonds. Unpaired electrons. Unpaired electrons. 1 2 Moment. Moment. 1.73 .- ___ 2-83 - Ni Pd Pt Ni Pd Pt Co Rh Ir Univalent Bivalent Univalent o I Diam.- - Diam. Tervalent Bivalent Univalent Ni Pd Pt Co Rh Ir Fe Ru 0 s Ni Pd Pt Co Rh Ir Fe Ru 0 s 3 4 3.88 Quadrivalent Tervalent Bivalent 4.90 5-92 Quadrivalent Tervalent Co Rh Ir Fe Ru 0 s 1-73 5 Fe Ru 0 s 4 4-90 4 I 4.90 2 2.83 Quadrivalent The Planar Configuration When there are four bonds to a metal atom (in this group) the mole- cule is either tetrahedral or planar. In the shaded parts of Table V are shown the valencies for which the tetrahedral and planar configurations occur. The three elements of Group Ib which form planar d q 2 bonds have been added for comparison. The onIy other well-established square complex outside this table is the compound KICl4,l3 the iodine atom presumably using d2p2 bonds. For criticism of the claims advanced for the square arrangement for certain other metals see D.P. Mel10r.l~ The planar configuration was postulated in 1893 by A. Werner l5 to explain the existence of the two isomeric diammines of platinous chloride. Before this hypothesis was universally accepted there was much con- troversy owing to several observations which were difficult to reconcile with the planar arrangement. The evidence for the square arrangement has been reviewed very thoroughly by Mellor l4 and will only be summarised here. Two problems need to be recognised first the proof of the exis- tence of the square configuration for bivalent platinum complexes and secondly the establishment of the circumstances in which it is expected l 3 R. Mooney 2. Krist. 1938 98 377. l4 Chem. Reviews 1943 33 137. l5 8. anorg.Chem. 1893 3 267. 328 QUARTERLY REVIEWS TABLE V Square conf igupation Tetmhedpal configur -ation ( a ) In forced configurations only. ( b ) This vaIency occurs in the compounds (pyH),RuCl 46 and H,OsI 4 7 9 48 about They are possibly some form of tetrahedron; but in their ( c ) The compounds OsO, RuO, and K+[OsO,N]-. (d) It has been claimed that K[Au(CN),,dipy] is plctnar.ls ( e ) Fe,C16 in the vapour state or in non-polar organic solvents. which little is known. complexes RuII and OsII are usually octahedral. to arise. Here only the planar and tetrahedral shapes are considered ; arguments against less likely arrangements have been given elsewhere. l 4 The literature of this subject is so extensive that only a few references will be given here for detailed references see Mellor’s review.l4 The two isomers PtCI,,(NH,), which have been the subject of many chemical investigations have been known for over 100 years.Much con- fusion occurs in the literature in discussing these owing to the inconsistent use of the symbols a and ,8; we shall avoid these here and refer to the isomers as cis and tram. M. Peyrone’s l7 reaction NH NHS PtC1,- -+ [Pt’(NH3)Cl& + [PtCl,(NH3)2] leads to the cis-isomer but Jorgensen’s reaction c1- c1- Pt(NH3),++ IC [Pt(NH,),Cl]+ -+ [PtCI,(NH,),] leads to the trans-isomer. The conductivity of both forms in water is small and indicates that they are non-electrolytes undergoing slow reaction with the water itself. The large number of chemical investigations lead to the definite conclusion that one isomer certainly has the trans-planar configuration but for a long time many chemists considered that the other isomer was either tetrahedral or arose from structural isomerism involving %covalent platinum.The problem was difficult to settle partly because the insolubility of the ammines prevented suitable accurate physical measurements and partly because of the isolation of possible third isomers. Claims that certain bivalent platinum complexes showed optical activity also threw doubt on the planar hypothesis.lg In addition to chemical l6 H. J. Dothie F. IAleweIIyn IFr. Waxdlaw and A. Welch J. 1939 426. l7 Annalen 1845 51 15. l 8 S. M. Jorgensen J. pr. Chem. 1886 33 489. l9 H. Reihlen and W. Huhn Annulen 1931 489 42. NYHOLM RECENT STEREOCHEMISTRY OF GROUP VIII ELEMENTS 329 investigations the evidence for the planar arrangements includes K.A. Jensen’s 2* electric dipole moment work ; the resolution by W. H. Mills and T. H. Quibell 21 of a bivalent platinum cation which could be optically active only if the four bonds to the platinum atom were coplanar or square- pyramidal ; the isolation of a large number of geometric isomers ; certain optical properties ; the results of Raman spectral investigations ; and finally X-ray diffraction studies. The planar arrangement has been rigidly established by the detailed X-ray analysis of K2PtC1 22 and [Pt(NH3),]C1,,H,0 23 among other platinum compounds. A survey of the mass of data shows that with the exception of a few compounds such as pyrromethene 24 and triaminotrimethylamine 25 compounds where the bonds must be forced out of the plane the four bonds to a bivalent platinum atom are invariably coplanar.It is of interest to see where else the planar arrangement should arise. L. Pauling 26 showed that 4-coplanar square complexes arise from the use of dsp2 bonds and magnetic data may often be used to decide whether or not these bonds are employed. Table IV shows the number of unpaired electrons expected should any given valency state assume the planar arrange- ment. Some elements e.g. nickel give rise to both tetrahedral and planar complexes; the factors leading to the change from one configuration to the other are not clear but the relative electronegativities of the metal and the attached groups are certainly important. Univa1eacy.-No example is known of a planar complex of a univalent element in this group ; complexes of univalent cobalt rhodium and iridium its yet unknown would most likely show this arrangement.Biva1ency.-In addition to PtII the square arrangement is found with Ydll NiII CoII and FeII. Table IV shows that planar 4-covalent com- plexes of PtII PdII and NiII will be diamagnetic. In view of the marked tendency of the Pd and the Pt triad to form covalent compounds with the minimum magnetic moment the invariable planar arrangement of the bonds around PdII and PtII is not unexpected. All complexes of palladium and platinum so far investigated have been shown to be diamagnetic. The complexes of palladium are less stable than those of platinum and the isolation of the &-isomers is more difficult. Cases where this has been achieved include the preparation of the cis- and trans-forms of palladium with benzylmethylglyoxime 27 glycine 28 and the diammines.29 Bivalent nickel shows both the tetrahedral and planar arrangements and the isola- “O 2.anarg. Chem. 1936 229 225. 31 J . 1935 839. 32 R. G. Dickinson J . Amer. Chenz. SOC. 1922 44 2404. 2 3 E. G. Cox J . 1932 1912. 24 C. R. Porter J. 1938 368. 25 F. G. Mann and W. J. Pope J . 1926. 2675. 26 eJ. Amer. Chenz. Soc. 1931 53 1367 ; 27 F. I?. Dwyer and D. P. Mellor ibid. 1938 57 605. 2 s F . W. Pinkard E. Sharrat W. Wardlaw and E. G. Cox J . 1934 1012. 29 A. Grunberg and V. M. Shul’man Compt. rend. Acad. Sci. U.R.S.S. 1933 136 1932 54 994. 143; F. G. Mann D. Crowfoot D. Gattiker and N. Wooster J. 1935 1642. 330 QUARTERLY REVIEWS tion of cis-planar complexes is difficult as with palladium. The circum- stances in which nickel gives tetrahedral complexes are discussed later.With cobalt the magnetic data 30n 31 indicate the planar arrangement for many of the internal complexes. Planar complexes should contain one unpaired electron and the tetrahedral three. It is interesting to compare the non-ionic trialkylphosphine complexes of bivalent Pt Pd Ni and Co which have the general formula M11X,,2PR,. Platinum 20 forms very stable complexes which may be isolated in cis- and trans-forms; these are sufficiently stable to permit of physical measurement without ready isomerisation. From palladium 32 only the trans-planar isomer has been obtained the groups being more labile than with platinum. Nickel 33 forms both planar and tetrahedral complexes ; the chloride and bromide complexes with triethylphosphine are diamagnetic with a zero dipole moment and are thus the trans-planar derivatives.However the nitrate Ni(N0,),,2PEt3 has a dipole moment of 8-85 D. and a magnetic moment of 3.05 B.M. showing that it is tetrahedral. Finally with cobalt 34 the only complexes obtained even of the chloride or bromide are strongly paramagnetic with a large dipole moment; CoC1,,2PEt3 has a dipole moment of 8.7 D. and a magnetic moment of 3.5 B.M. which are consistent with the tetrahedral arrangement. From bivalent iron no trialkylphos- phine complex could be obtained ; even with other co-ordinating groups square complexes are formed from this element only in forced configura- tions. Thus the tendency to form square complexes decreases in the order Pt > Pd > Ni > Co > Fe. It will be noticed that when compounds of the type MX,Y are obtained as square complexes in only one form it is usual to assume that these are the trans-forms.This is not always true for platinum ; the only form of the complex PtC1,,2TeR2 so far isolated is the ~is-isomer.~5 It is found also that the stability of cis-isomers increases in the sequence PtI,X < PtBr,X < PtCl,X,. No square complexes have been obtained from bivalent Ru Os Rh or Ir. Terva1ency.-The claim by G. Grube and G. Fromm36 that cis- and trans-planar isomerism of a tervalent ruthenium compound has been observed in solution must be treated with reserve. These workers attri- buted the green and yellow colours of aqueous solutions of RuCl, prepared by different methods to cis-trans-isomerism in the cation of [Ru(H20),C1,]+C1-.Their claim rests largely on colour differences and on the fact that electrical conductivity shows only one ionised chlorine in each case; however the colour differences may well arise from cis-trans- isomerism in an octahedral complex ion. 30 D. P. Mellor and D. P. Craig J . Proc. Roy. SOC. N.S.W. 1940 74 495 ; E. D. Barkworth and S. Sugden Nature 1937 139 374; L. Cambi and L. Mslatesta Gazxetta 1939 69 547. 31 D. P. Mellor and C. D. Corycll J . Awaep. Chem. SOC. 1038 60 1768. 32 I?. G. Mann and. D. Piirdie J. 1035 1549. 3 3 K. A. Jensen 2. anorg. Chem. 1936 229 266. 34 Idem ibid. p. 282. 36 2. Electrochem. 1940 46 661. 35 Idem. ibid. 1937 231 365. NYHOLM RECENT STEREOCHEMISTRY OF GROUP VIII ELEMENTS 331 The Tetrahedral Configuration This configuration has been assumed rather than proven in many cases and with the bbalent complexes of cobalt and nickel for example it is assumed wherever the dipole moment and magnetic data exclude the planar arrangement.There appears to be no case of an optical resolution of a tetrahedral element in Group VIII. Of the few compounds whose struc- tures have been fully determined one should mention Cs,CoCl (by X-ray 37) and Ni(CO) (by electron diffraction 38 39). Zero-valent and Univalent State.-Although the carbonyls are a group of compounds wherein the metal atom is formally zero-valent special interest attaches to the isolation of the first salt-like substances in which the metal atom shows zero-valency. By reduction of the bivalent cyanide of nickel and palladium with potassium in liquid ammonia the complexes K,Ni(CN) 4O and K,Pd(CN) 4l have been isolated.Both are yellow solids and are strong reducing agents which gradually decompose water with the evolution of hydrogen. The palladium compound is less stable than its nickel analogue and no corresponding platinum compound has been des- cribed. These compounds are electronically similar to Ni(CO), and since the metal atoms have completed 3d shells the filling of the next sp3 orbitals will presumably lead to the tetrahedral configuration. The red colour of reduced solutions of K,Ni(CN) is due to the presence of the intermediate compound K,Ni(CN), a derivative of univalent nickel. This substance absorbs carbon monoxide and nitric oxide to give respectively K,Ni(CN),CO and K,Ni(CN),NO in which the nickel is presumably 4-covalent. The red solution of K,Ni(CN) is diamagnetic,a2 which is unusual since univalent nickel contains an odd number of electrons.It has been suggested that an Ni-Ni bond may explain the diamagnetism but an explanation in terms of two bridging cyano-groups 81 is possible ; in any case the anion is almost certainly polymerised to the 4-covalent state. The simple cyanide NiCN is known but has been little studied. The Bivalent State.-The complexes of bivalent nickel and to a lesser extent bivalent cobalt have been the subject of much investigation because these elements also show the planar configuration. The tetrahedral con- figuration may be distinguished from the planar configuration by magnetic dipole-moment and X-ray diffraction measurements. Of these the first is the most widely used. With bivalent nickel there is as yet no certain method for predicting whether a group will give rise to the planar or the tetrahedral configuration.The shape may be correlated fairly closely with the electronegativity of the attached groups and with the colour or better still the absorption spectrum of the complex. The tendency to form the 37 H. M. Powell and A. F. Wells J. 1935 359. 38L. 0. Brockway and P. C. Cross J. Chem. Physics 1935 3 828. 39 C. R. Bailey and R. R. Gordon ibid. 1938 6 225. 40 J. W. Eastes and W. M. Burgess J. Amer. Chem. SOC. 1942 64 118. 41 J. T. Burbage and W. C. Fernelius ibid. 1943 65 1484. 42 D. P. Mellor and D. P. Craig unpublished. 332 QUARTERLY REVIEWS tetrahedral configuration increases as the difference in electronegativity between the nickel atom and the attached groups increases.Thus all com- plexes in which nickel is attached to four oxygen atoms are tetrahedralY4 whilst with four sulphur atoms the complexes show th6 diamagnetism characteristic of the planar arrangement. The correlation with colour is of wider applicability. It has been known for some time that the dia- magnetic complexes of nickel range from red through reddish-brown to a yellow colour whilst the paramagnetic tetrahedral complexes are usually blue or green.44 As the colour of the co-ordinating group itself may cause confusion Mellor and his co-workers 45 investigated the absorption spectra of a large number of internal nickel complexes and the chelate groups from which these were obtained. It was found that the absorption spec- trum of paramagnetic complexes differs only very little from that of the chelate itself.However diamagnetic complexes usually showed a new absorption band of considerable intensity near 4000 A. In a few cases this nickel band was partly obscured by bands due to the chelate group itself but it seems to be almost invariably the rule that planar nickel com- plexes show this strong absorption band near 4000 A. There are only two known exceptions to this rule namely the 8-mercaptoquinoline-nickel complex and the bisformylcamphor-ethylenediamine compound both of which are paramagnetic but show the characteristic absorption band at The many attempts to obtain tetrahedral complexes of other elements in Group VIII have met with little success for the elements Fe Ru Os Rh and Ir have a marked preference for the 6-covalent state.The absence of the tetrahedral configuration with these elements is a reflection of their tendency to form covalent bonds and of their complexes to have the minimum magnetic moment. Some apparently 4-covalent complexes of the formula R1,[RuC1,] (R = pyridinium or other organic bases) have been described by L. W. N. Godward and W. Ward1aw.de These compounds are diamagnetic and hence from Table IV are apparently not planar. The compounds H,0s14 47 and K,OsI 48 have not been investigated recently but may be similar. The diamagnetism may indicate d ? q ~ orbitals for which an irregular tetra- hedron is expected but no other physical measurements are available. Higher Va1encies.-The tetrahedral codguration for the oxide OsO 49 is indicated from electron-diffraction measurements and this structure is supported by the X-ray investigation of K+[OsO,N]-.F. M. Jaeger and J. E. Zanstra 50 claimed that this compound is tetrahedral. Ru04 is no 4000 A. 43 D. P. Mellor and D. P. Craig J . Proc. Roy. SOC. N.S.W. 1940 7'4 476. a4 L. Pading op. cit. p. 119. 4 5 H. A. Mckenzie D. P. Mellor J. E. Mills and L. N. Short J . Proc. ROY. SOC. 4 6 J . 1938 1422. 47 E. P. Alvarez Chem. News 1905 91 172. 48 N. A. Orloff Chem.-Ztg. 1907 31 1063. 40 L. 0. Brockwey Roy. Mod. Physics 1936 8 260. Proc. Acad. Sci. Arnst 1932 35 610 787. N.X.W. 1944 78 70. NYHOLM RECENT STEREOCHEMISTRY OF QROTP VITI ELEMENTS 333 doubt similar to OsO,. Osmium also shows a valency of eight in the fluoride OsF, which is one of the few cases where an element is 8-covalent. If the bonds used in this molecule are d5~p2 a face-centred prismatic structure has been predicted.3 Co-ordination Number Five Apart from the carbonyl Fe(CO), this co-ordination number had not been reported until recently in Group VIII.However several compounds have now been obtained for which this unusual co-ordination number is claimed. When investigating the complexes of bivalent nickel with ter- tiary phosphines Jensen 33 observed that oxidation of the trans-planar form of bistriethylphosphinedibromonickel (I) with bromine gave a com- pound NiBr3,2PEt3. This is a typical covalent complex being soluble in benzene in which it is monomeric and i t is therefore apparently not a binuclear derivative of octahedrally co-ordinated nickel. E’urther data on this compound are now available ; the magnetic moment of 1.72-1.90 B.M.indicates the presence of one unpaired electron presumably in the 3d shell and suggests the use of d q 3 bonds in the nickel atom. Jensen claims that this also excludes the bridged structure since he expects that such a bridged compound would be diamagnetic.52 Kimball predicted that the use of dsp3 bonds should lead to a trigonal bipyramid (111) but Daudel and Bucher have suggested that in addition the tetragonal pyramid (11) would arise if the d electron has a lower quantum number than the s and p . A choice between these is possible from a determina- tion of the dipole moment. Structure (11) should have the dipole of a G1 K. A. Jensen Acta Chem. Scnnd. in tlla press. 6% Idem unpublished. 334 QUARTERLY REVIEWS single Ni-Br bond about 2-3 Debye units; (IIIA) would be zero and (IIIB) and (IIIc) by comparison with cis-compounds like PtBr,,BPEt, should have a moment of the order of 7-10 D.The experimental value of 2.5 D. is consistent with the tetragonal pyramid (11) and with Daudel and Bucher’s prediction for the use of 3d4s4p3 bonds. Since the compound is very unstable X-ray confirmation of the structure would be difficult and the isolation of more stable com- pounds is desirable. A similar 5-covalent complex is obtained by the action of bromine on bisdimethylglyoximenickel. The preparation of corresponding 5-covalent complexes of other metals has been attempted. The planar bivalent complexes of palladium and platinum with arsines and phosphines when treated with halogens give rise to octahedral quadrivalent complexes only. Tervalent compounds of palladium and platinum are uncommon ; the compound PtC13,2NH3 is probably a dimer containing one bi- and one quadri-valent platinum atom.A 5-covalent complex of tervalent cobalt 52 has been obtained by oxidising CoC1,,2PEt3. The product CoC13,2PEt3 contains the two unpaired elec- trons required for dsp3 bonds. The original cobaltous complex contains three unpaired electrons and is tetrahedral; one of these unpaired 3d electrons becomes paired in the cobaltic compound to change the hybridisa- tion from 8p3 to &p3. The unusual character of this cobaltic compound is emphasised by the fact that octahedral CoIII complexes are invariably diamagnetic. It is of interest to note that the tetragonal pyramid has been sug- gested l o for the compound IF, but electron-diffraction studies of this compound were inconclusive.s6 Co-ordination Number Six This is the commonest co-ordination number which the Group VIII elements display and has been the subject of extensive investigation with these elements particularly in their higher valency states.The octa- hedral distribution arising from the use of d2sp3 bonds has been well established by studies of geometric isomerism optical resolution and X-ray diffraction ; recent investigations have been more concerned with establishing the relative positions of the six groups in certain geometric isomers and with elements showing 6-covalency in valenciea which were formerly rather doubtful. Univalency.-There is no known case in which a univalent metal is octahedrally co-ordinated. Biva1ency.-In this valency one might expect to find both 4- and 6-covalent complexes and for many of the elements this is so but usually there is a marked preference for one or the other.Bivalent E’e Ru and 0s are almost invariably octahedral in their complexes and the complexes of these elements with dipyridyl or o-phenanthroline are sufficiently stable to permit resolution into optically active isomers. In their octahedral complexes bivalent Fe Ru and 0 s are isoelectronic with tervalent Co Rh and Ir and with quadrivalent Ni Pd and Pt. With the exception of NYHOLM RECENT STEREOCHEMISTRY OF GROUP Vm ELEMENTS 335 NiIV and to a lesser extent PdIV this series gives rise to some of the most stable octahedral complexes in Group VIII all of which are diamagnetic when covalent. Of special interest is a comparison of the complexes of bivalent cobalt rhodium and iridium; stable complexes of the last two in this valency state have only recently been obtained.Bivalent cobalt gives octahedral complexes which are oxidised very readily to derivatives of tervalent cobalt but in addition it forms both tetrahedral and planar 4-covalent complexes. One might expect that RhII and IrII would form 4-covalent complexes from a consideration of the relative behaviour of bivalent nickel palladium and platinum. However it is found that bivalent rhodium and iridium form exclusively 6-covalent complexes and are thus more like ruthenium and osmium to the left rather than cobalt or the elements to the right. Complexes of bivalent rhodium and iridium have been obtained with a variety of ligands (ie. co-ordinating groups ; e.g.NH,) such as pyridine tertiary arsines and dimethyIglyo~ime,5~ but in every case the metal is 6-covalent. Even the most favourable conditions for their formation have failed to give any 4-covalent complexes. In all cases the bivalent rhodium complexes were obtained by reducing the corresponding tervalent complex. It can be shown from the potentials Rh/RhlIr (c + 0-8 v.) and RhI1/Rh1II (c + 0.1 v.) that the Rh++ ion is thermodynamically unstable like the cuprous ion in solution. A dark red solution of bivalent rhodium quickly forms a deposit of metallic rhodium and a solution of the lighter-coloured rhodic salt. Hence to keep rhodous complexes in contact with water they need either to be very insoluble or to dissociate only to a very small extent. The behaviour towards dimethylglyoxime illustrates the tenacity with which the 6-fold co-ordination is retained.This substance forms a strong acid (IV) with tervalent rhodium chloride but attempts to reduce this fail to give a 4-covalent complex. Unstable rhodous complexes containing three dimethylglyoxime groups are formed in the absence of chloride ion. As dimethylglyoxime usually tends to occupy only 4 positions around a metal atom the reluctance to form 4-covalent complexes is emphasised. r CL 63 F. P. J. Dwyer and R. S. Nyholm J . Proc. Roy. SOC. N.S.W. 1941 75,127 ; 1942 76 133; 1942 76 275; 1944 78 266. z 336 QUARTERLY REVIEWS The behaviour towards pyridine is shown below Reflux with Reflux with J aqueons HRr CRh PYS Br212 + r.Rl1 PY2 Br,Y alcoholic HBr Further refluxing with hydrobromic acid gives pyridiniixm salts containing rhodium in the anion.The complexes of bivalent iridium 54 are similar to those of bivalent rhodium but in general are less stable. An example of the ability of bivalent nickel to show both 4- and 6- covalency is given by the substance of empirical formula Ni(CN),,NH,,CBH,. The molecule of benzene is held very tenaciously and thus caused much speculation as to the type of supposed bond between the hydrocarbon and the nickel atoni. A complete X-ray diffraction study of this substance by H. M. Powell and J. R. Rayner 55 has shown that it contains equal numbers of octahedral and 4-covalent planar nickel atoms with the benzene imprisoned in the lattice. It belongs to a class of molecular compounds to which the general name of " clathrate " compounds has been given.It is noteworthy that the magnetic moment of this compound had been shown to be consistent with half of the nickel atoms being bound with square covalent bonds and half with ionic bonds.56 The structure of the compound is shown in Fig. 1 and consists of sheets of polymerised Ni(CN) with two FIU. 1 NH groups on alternate Ni atoms one of the NH from Nature 1949,183,566.) groups being above the other and below the plane. Undoubtedly many other structures in which the element appears to have an unusual co-ordination number will prove on X-ray examination to retain the usual co-ordination number by polymeri- sation. Although bivalent nickel forms quite stable octahedral complexes those of bivalent palladium and platinum are rare ; this is unusual for one might expect the tendency to form complexes with the higher co-ordina- tion number to increase with increasing atomic weight.All octahedral bivalent nickel complexes have a magnetic moment corresponding to two unpaired electrons but of the other two elements only the complex salt [Pt(NH3)a(CH3*CN),]C1 appears to have been measured and it is diamag- 0 Ni O=O CN 0 NH3 * '" (Reproduced by kind permission 64 F. P. J. Dwyer and R. S. Nyholm J . Proc. Roy. Xoc. N.S.W.,1943 77 116. 6b Nature 1949 168 566. 66 D. P. Mellor and D. P. Craig unpublished. NYHOLM RECENT STEREOCHEMISTRY OF GROUP VIII ELEMENTS 337 netic. Although the compound [Ni(NH3),]C12 is probably ionic the very stable red [Ni(dipy),]Cl complex which has been resolved seems to be covalent. If the usual 3d24s4p3 bonds are used for bonding in the latter it becomes necessary to promote .two electrons presumably to the 5s and 5p levels and the fact that these compounds are not readily oxidised to form quadrivalent nickel has been considered a drawback to Pauling’s theory.The difficulty of preparing octahedral complexes of bivalent pal- ladium and platinum seems to be due to the absence of available orbitals. A square complex of Ptn for example has only one unfilled 6p orbital and the formation of an octahedral bivalent complex using 5d26s6p3 bonds necessitates the promotion of two electrons from the 5d shell. The dia- magnetism of platinum compounds indicates that this is unlikely; R. F. Asmussen 57 has avoided this difficulty by the suggestion that two of the NH groups in the compound [Pt(NH3)4(CH3~CN),]Clz are bound by “ ion- dipole ” bonds.One would expect the stability of compounds formulated in this manner to be less than that of truly covalent complexes. The Tervalent State.-Complexes of the elements in this valency state are almost invariably octahedral and those of Fe Ru 0s ; Co Rh and Ir have been studied with many different co-ordinating groups. Although tervalent cobalt rhodium and iridium form ammines of marked stability those of the other three elements are less well defined. K. Gleu and his co-workers 67 have studied the ammines of ruthenium but those of osmium warrant further investigation. The elements Ru Os Rh and Ir form 6-covalent non-ionic complexes with tertiary arsines of the general formula MIII(Ha1) 3,3AsR3,58 which are soluble in organic solvents in which they are monomeric indicating that the metals are 6-covalent.Ferric chloride forms unstable complexes which are probably polynuclear. 59 Little is known of the tervalent compounds of palladium and platinum but the only complex of tervalent nickel investigated so far is 5-covalent. . As mentioned previously not many well-defined complexes of tervalent osmium have been described and its chemi$try needs further study. Several X-ray structure determinations have been carried out on these octahedral complexes. In the main these investigations have confirmed the structures previously assigned on chemical grounds. However the X-ray examination of the silver derivative of Erdmann’s salt has given a surprise. When this salt (NH4)[Co(NH3),(N02),] is treated with oxalic acid two cis-NO groups are replaced by the oxalate group forming (NHa)[Co(NH3),(N02),(C204)].If the two NH groups are originally trans only a single optically inactive derivative is expected. However if the two NH3 groups are originally cis two geometric isomers are expected one of which should be capable of optical activity. Since the latter was observed Erdmann’s salt was believed to be the cis-diammine. The X-ray “ Magnetokemiske Undersgelser Over Unorganiske Kompleksforbindelser” Copen- hagen 1944 (Doctorial Thesis) p. 243. 68 F. P. Dwyer and R. S. Nyholm J. Proc. Roy. SOC. N.S. W. 1946 80 217 ; 1947 81 272; 1942 76 140; 1945 79 121. 6B R. S. Nyholm ibid. 1944 78 229. 338 QUARTERLY REVIEWS study of the silver salt Ag[Co(NH,),(NO,),] by A. F. Wells has shown that this is definitely the tram-diammine.The reason for this difference is not clear but the migration of groups during chemical reactions seems the most likely explanation. This result is a reminder of the need for caution in interpreting the results of some chemical reactions. The Quadrivalent State.-Well-defined quadrivalent compounds of the last six elements exist but little is known of the quadrivalent state for iron cobalt and nickel which elements form very few compounds wherein the valency of the element .exceeds three. The best-known quadrivalent compounds are the double chlorides of formula M1,MIVC1,. The isolation of the compound Cs&hC1 61 completes the series for the palladium and the platinum triad. This compound obtained as EL green powder by oxidising Cs,RhCl with ceric nitrate is isomorphous with (NH4),PtCl, the Rh-Cl distance being 2.3 A.The corresponding osmium c o m p o u n d K,OsC1, is isomorphous with K,PtCl,. The complexes of quadrivalent platinum with ammonia and with many other co-ordinating agents have been studied in very great detail. Pal- ladium forms similar compounds but they are reduced very easily to the bivalent state and decompose on stand- - FIU. 2 ing. The octahedral ammines of quadri- valent iridium have been well studied but those of ruthenium and osmium need further investigation. Quadrivalent platinum forms some interesting organo-metal com- pounds in which the bonds were originally believed to be tetrahedral.ss Recent electron-diffraction studies of the compounds tetramethylplatinum [Pt(CH3),I4 and trimethylplatinum chloride [Pt(CH,),Cl] show that these substances are tetramers 62 in which' the platinum atom is octahedrally co-ordinated a result more consistent with the observed diamagnetism.Molecular-weight determinations on [ (CH,),PtCl], although not very accurate owing to low solubility also support the four-fold as~ociation.~~ The basic structure of [Pt(CH,),Cl] is shown in Fig. 2 ; the three CH groups which are attached to each metal atom are omitted from all but one Pt atom for the sake of clarity. The co-ordination number of 6 for the Plfv atom is attained by making use of the donor properties of two other chlorine atoms. In polymerising to the extent necessary to reach the stable co-ordination number this compound is similar to the tetramer [CuI,AsEt,], in which the copper atom attains a co-ordination number of 4 by making use of iodine bridges.s4 The Pt-C1-Pt angle has been estimated 6O 2.Krist. 1936 94 447. 61 F. P. Dwyer and R. S. Nyholm Nature 1947 160 502 ; J . Proc. Roy. SOC. 6 2 R. E. Rundle and J. H. Sturdivant J . Amer. Chem. SOC. 1947 69 1561. 6 8 R. C. Menzies and H. Overton J . 1933 1290. 64 F. G. Mann D. Purdie and A. F. Wells J. 1936 1603. N.S.W. 1947 81 267. NYHOLM RECENT STEREOCHEMISTRY OF GROUP VIII ELEMENTS 339 at 99’. Tetramethylplatinum is also found to be a tetramer and in this case the polymerisation to the octahedral configuration is reflected in the physical properties for whilst [Pt(CH,),] is a crystalline solid other com- pounds of the type M(CH,) are low-boiling liquids. The structure is basically the same as that of trimethylplatinum chloride except that each chlorine atom is replaced by a CH group.The conclusion that carbon violates the octet rule and becomes 6-covalent seems at first sight inescap- able but it has been suggested that the electron pair involved in bond formation from the CH group resonates between all three Pt atoms. Unfortunately interatomic distances involving carbon atoms had to be assumed but it is significant that the Pt-C distance is at least greater than the sum of the PtIV and CIV bond radii. The problem involved in formulating this compound is similar to that encountered with the alkyl- aluminium dimers e.g. [Al(CH,),],. For these compounds structures involving “protonated bonds” have been suggested by L. 0. Brockway and N. R. Davidson 6 5 similar to those of K. S. Pitzer 66 for the boron hydrides but as the determination of the position of a proton is still very difficult these formulations are still largely speculative.Polynuclear Complexes Complexes in which many bridging groups connect two similar metal atoms have been known for a long time Werner having described com- plexes of cobalt and chromium in which such diverse groups as -OH -NO, -02 -0- -0*S02- (sulphato) and -O*C(CH,)=O- (acetato) acted as bridges between two metal atoms. Examples of such complexes are (V) and (VI). No special problems are involved in formulating these 1 (en = Ethylenediamine.) compounds since in all cases the ability of the groups to donate electrons is well established. More recently much attention has been paid to the O 5 ,J. Amer. Chenz. SOC. 1941 63 3287. 67 2. anorg. Chem. 1936 227 237 ; 1938 235 201 ; 1938 237 187 197 326 335 68E.G. Cox and K. Webster 2. K&L 1935 90 561. IIbid. 1945 67 1126. 360. 340 QUARTERLY REVIEWS complexes in which two halogen atoms act as a bridge between two metal atoms. These are of special interest since the formation of co-ordinate links by halogen atoms other than in bridged compounds is very rare; examples include the ions Br,- and I,-. A comprehensive review of the occurrence of these bridges is given by P. G. Mann.69 Since that report further work on their chemical behaviour and structure has been done. The halogen bridged complexes of the Group VIII elements are especially suitable for investigation because the stability of the co-ordination com- pound in various solvents enables the use of reactions which are not so convenient with other bridge-forming elements like Al Hg Cd ctc.Examples of halogen bridges in simple halides of Group VIII elements include PdC1 and Fe,Cl,. Palladous chloride consists of chains of the type (VII) of indefinite length.'* The shape of the ferric chloride molecule c1 c1 c1 c1 \ / \ / \ / c1 c1 c1 (VII.) has not been settled. In the solid state the a layer lattice 71 in which each iron atom is six chlorine atoms. However the compound c1 \ anhydrous compound forms surrounded octahedrally by sublimes readily and below 700" R. exists in the vapour state as double molecules. It is-also dimeric in solvents which do not co-ordinate with the iron atom; in pyridine however the molecular weight indicates that the monomer is present presumably as the result of the reaction E'e,CI + 2py- 2py+FeC1,.Magnetic measurements 72 show that the vapour is strongly paramagnetic. It has been customary to regard the bonds in such a molecule as ionic but L. Pauling 73 has suggested that magnetic measurements may distin- guish not necessarily between ionic and covalent bonds but often between weak covalent and strong covalent bonds the former using unstable orbitals a t higher levels leaving singly occupied orbitals at lower levels. The physical properties of the ferric chloride such as volatility and solubility in organic solvents are more consistent with the idea of essentially covalent bonds and the formula generally accepted involves a halogen bridge between two 4-covalent ferric atoms. If the bonds used are 4s4p3 a slightly dis- torted tetrahedral configuration for each iron atom as with Al,CI is expected.Compounds of every element in Group VIII except cobalt and nickel have been reported in which halogen bridges have been postulated to retain the usual 4- or 6-co-ordination. The structures of very few of these have been established with certainty but the palladium compounds have been Bg Ann. Reports 1938 35 148. 70 A. F. Wells 2. Krist. 1938 100 189. 7 2 A. Lallemand Ann. Physilc 1935 3 97. 'l N. Wooster ibid. 1932 83 36. 7 3 J . 1948 1461. NYHOLM RECENT STEREOCHEMISTRY OF GROUP vm ELE~MENTS 341 very thoroughly investigated by Mann and his co-workers.74~ 7 5 Their study was confined chiefly to the arsine and phosphine complexes of the general formula [PdX,,A], where X = C1 Br I CNS NO, and A = PR or AsR ; these are more suitable than the ammine complexes for investi- gation because the latter are insoluble in suitable organic solvents and furthermore the ammines have a great tendency to form salt-like sub- stances.These palladium compounds are formed readily and allow of easy replacement of groups but this behaviour means that care is neces- sary in interpreting results since rearrangements may occur ; some earlier work gave apparently contradictory results for this reason. The complex [PdCl,,AsR,] (or its phosphine analogue) might be expected to occur as one or all of three geometric isomers (VIII) (IX) and (X) since the compound is known to be dimeric in solution. (1) The dipole moment expected for these three structures are 12-14 7-8 and 0 D. respectively. The value found experimentally is 2.34 for the tributylphosphine compound and 2-52 for the tributylarsine analogue.These figures whilst suggesting a preponderance of (X) in solution indicate that (IX) and possibly even (VIII) may be present also. (2) Treatment with a suitable amine (NH,R) e.g. p-toluidine produces two moles of the monomer PdCl,,AsBu,,NH,R only; this suggests that diagonal splitting along the dotted line of (IX) or (X) occurs since both of these would give the same product. Failure to obtain any of the com- pound PtC1,,2NH2R which is only very slightly soluble and thus could not escape isolation appears to eliminate (VIII) entirely unless rearrange- ment occurred during the reaction. (3) When the tributylphosphine compound reacts wit)h 2 2'-dipyridyl two products are isolated PdCl,,ZAsBu and dipyPdC1,. This was origin- ally believed to support structure (VIII) which was presumed to react by vertical splitting along the dotted line.Similar evidence was provided by ethylenediamine. However it was found that the palladium complex (XI) in which the oxalato-group bridges the metal atoms gave a similar reaction PdCl,,ZPBu and dipy Pd( COO) being formed. Since X-ray diffraction studies had shown that the bridged oxalato-compound has a centre of symmetry and thus definitely has one chlorine atom on each pal- ladium atom one concludes that migration of PBu molecules had occurred. This reaction emphasises t h t in the case of palladium reactions based on 7 4 F. G. Mann and D. Purdie J . 1936 873. 75J. Chatt and F. G. Mann J. 1939 1622; J . 1938 1649. 342 QUARTERLY REVIEWS the use of cis-chelating groups need to be interpreted with care ; a chelate group such as dipyridyl must of necessity occupy two cis-positions because of its dimensions and when such a strong co-ordinating bidentate compound is used eviction of a cis-PBu group and rearrangement of the two C1 atoms necessary consequence.is a (4) Further evidence against structure (VIII) has been provided by attempts to isolate a complex (XII) by using a cis-chelating diarsine as the co-ordinating group. 76 This chelate co-ordinates very strongly with biva- lent palladium forming a completely strainless ring but no bridged complex could be obtained. Although negative evidence the fact that no bridged complex can be obtained in which both arsine groups are attached to the same palladium atom must be considered as good evidence against their existence.The complete X-ray analysis of the compound [PdBr,,As(CH,),] shows that in the solid state the trans-symmetric structure (X) is present." All Pd-Br distances in the bridge are equal and the angles between the bonds are almost 90". Bivalent platinum gives bridged complexes similar to those of palla- dium; compounds of the formula [PtCI,,A] have been reporteds' where A = SR, PPr, PCI, P(OCH,), CO C2H4 etc. They are not as easy to obtain as the corresponding palladium complexes but once formed they appear to be quite stable. 78 Since platinous complexes exhibit geometric isomerism more often than any other planar element it might be possible to isolate isomers of types (IX) and (X) from the platinum complexes. Bivalent rhodium forms some bridged complexes of the general formula [RhX2,3AsR,], where X = C1 Br I.These provide examples of bridging bet ween 6- covalent metals at oms. Molecular-weight determinations show that they are dimeri~,'~ but their structure has not yet been investigated in detail Iridium forms similar complexes to those of rhodium but they are less ~table.5~ A few bridged complexes between dissimilar metal atoms are known. They are much less stable than complexes between two metal atoms of the same kind and highly specific factors appear to be involved in their formation. The palladium-mercury complex with tripropylarsine (XIII) is an example of this type of complex.80 Although it has been 7 6 J. Chatt and F. C. Mann J . 1939 2086. 77 F. G. M a n and A. F. Wells ibid. p. 702. 79 F. P. Dwyer and R. S. Nyholm J. Proc. Roy.SOC. N.S.W. 1941 75 127. *OF. G. Mann and D. Purdie J. 1940 1230. J. Chatt private communication. NYHOLM RECENT STEREOCHEMISTRY OF GROUP VIII ELEMENTS 343 customary to formulate these bridges in such a way as to involve one covalent and one so-called co-ordinate link from each halogen atom in the Pr,As Br Br \ / 4 / Pd' H i / \ / 7 AsPr Br Br (XIII.) bridge K. A. Jensen and R. W. Asmussen*l have recently suggested resonance among the structures (X1V)-(XVII). They have pointed out c1 c1 c1 c1 M M M \ M M / / M M \ M C1 \ / c1 c1 \ / c1 (XIV.) (XV.) (XVI.) (XVII.) the reluctance of the halogen atom in alkyl halides for example to form co-ordinate links and have directed attention to the deep colour of most of these bridged complexes ; furthermore the diamagnetism of certain compounds like Fe,(CO) can be explained on their hypothesis without the need for postulating metal-metal bonds.Many other observations will need to be taken into account in it more complete picture of these com- pounds. They have been reported chiefly in lower valency states although a few of metals of higher valency states exist e.g. [Pt(CH,) py I]2.s2 They occur only where the metal-halogen bond is essentially covalent. Pacts relating to the specificity of the metal and the relative strength of the bridges when different halogens are involved do not yet permit of reliable generalisa t ions. Summaxy and Outlook The approach to inorganic chemistry has been much influenced during the past few years by certain factors which indicate the probable direction of future development. The orientation of four and six bonds around a metal atom was correctly predicted more than 50 years ago and for a long time workers were concerned mainly with establishing which particular configurations occur for a given element.Inorganic chemistry however received for many decades much less interest than its importance warrants. The development of the X-ray diffraction method of examining sub- stances has led to an entirely new structural chemistry of the solid state.s3 Since most of these structure determinations take some time it is natural that so far attention has been directed very largely to key substances ; compounds of the elements in less common valcncy st,at,es have been 81 2. anorg. Chenz. 1944 252 234. 8 2 C. S. Gibson R. V. G. Ewens and M. E. FOSS Nature 1948 162 692. 8 3 A.F. Wells " Structural Inorganic Chemistry ', Oxford 1945. 344 QUARTERLY REVIEWS generally ignored. In Group VIII for example bond lengths for the bivalent and tervalent states of many elements have never been measured Many other radii have been obtained by extrapolation procedures which are open to doubt. More determinations in this group are needed par- ticularly with complexes of unusual co-ordination numbers. The application of quantum mechanics has enabled us to understand why molecules take up certain shapes but the complexity of the molecules limits us to qualitative predictions a t present. One of the most promising of recent developments is the attempt by Y. K. Syrkins4 to correlate reactivity with bond type. Starting with the square complexes of bivalent platinum an explanation of the relative reactivity of the chlorine atoms in trans-diamminoplatinous chloride as compared with the cis-compound has been advanced.Knowledge is gradually being accumulated on the stability of com- plexes as a function of the metal and the valency state and the nature of the ligand attached. Further work is necessary to enable us to cor- relate the strength of attachment of a group and the metal its valency and the stereochemical type. For this purpose a more detailed investiga- tion of some of the less common valency states is required. It is hoped that this review whilst indicating the developments which have taken place recently will emphasise the scope for further investigation. The Reviewer wishes to acknowledge helpful criticism by Professor C. K. IngoId F.R.S. and valuable discussion with Dr. D. P. Craig Dr. F. P. Dwyer Dr. P. B. D. de la Mare and Dr. J. Chatt. He also wishes to acknowledge the award by the University of London of an Imperial Chemical Industries Fellowship during the tenure of which this Review was written. 84 Bull. Acad. Sci. U.R.S.S. Classe Sci. chim. 1948 69. 85 P. W. Selwood " Magnetochemistry " 1943 Interscience N.Y. 86 H. Braune and P. Pinnow 2. physikal. Chem. 1937 B 35 239. J. W. Mellor " Comprehensive Treatise of Inorganic Chemistry " Vol. XVI p. 361.

ISSN:0009-2681    

DOI:10.1039/QR9490300321

出版商:RSC    

年代:1949

数据来源: RSC

摘要:

By B. TOPLEY M.A. F.R.I.C. (ALBRIGHT AND WILSON LTD.) IN this review it has seemed better to consider mainly the facts about the condensed phosphates of sodium because these are on the whole by far the best authenticated; and to select the facts which bear upon the covalent structures of the condensed phosphate anions. Another selection has been made in favour of properties with important practical applications. There is a substantial body of published work on the condensed phosphates of metals other than sodium and much of it especially where concerned with the heavy-metal metaphosphates repays renewed study as a stimulus to further investigation ; but as it stands it is very indefinite. Contro- versial matter in the literature concerns for the most part experimental technique rather than interpretation of facts in terms of general ideas.Other reviews present a more critical account of the discrepancies in pub- lished work. No attempt has here been made to give a balanced account of contradictory reports and where the weight of evidence is not clear the point in question has usually been omitted. A few references are made to early literature for their historical interest or because they have remained unnoted by many subsequent authors. Phosphoric anhydride is known in three polymeric crystalline forms and as a glass. X-Ray and electron-diffraction measurements have shown that a tetrahedral unit of four oxygen atoms attached to every phosphorus atom as in (I) is a structural feature common to the oxide polymers and to crystalline ortho- pyro- and meta-phosphates.Acids 0- salts and esters in which two or more PO units are linked by sharing oxygen atoms form a numerous set of compounds 0-P-0- 0- (1.) " acid " side of the sequence through the metaphosphates and polyphosphates as far as the pyrophosphates (diphosphates) on the " alkaline " side. Poten- tially this field even though it excludes the silicophosphates borophos- phates and other heteropolyphosphates is an extensive one but little has yet been done to provide some of even the simplest physicochemical data. However because of the scale of usage (in total probably more than 300,000 tons a year) of sodium pyrophosphate the meta- and poly-phos- phate glasses and especially sodium triphosphate interest in the systematic and preparative chemistry of the condensed phosphates is being stimulated and the more highly polymerised phosphates are beginning to receive attention in their r61e of colloidal electrolytes.Literature.-The discovery of the condensed phosphates is credited to two Scotsmen Thomas Clark (1827) and Thomas Graham (1833). As a / designated as condensed phosphates. The set includes in \ principle compounds for which the atomic ratio P/O is in the range 2/5 < P/O > 2/7 extending from (P20& on the 345 346 QUARTERLY REVIEWS matter of historical interest J. Berzelius (1816) described the preparation of a pure specimen of disodium orthophosphate showing that it formed a dodecahydrate and his analysis of the product obtained by dehydrating this at red heat proved that it was 2Na20,P20,. It remained however for Clark to demonstrate that the substance prepared by strongly heating disodium phosphate hydrate is a chemical individual not the same in its properties as disodium orthophosphate from which merely the water of crystallisation has been removed and to recognise a difference of kind between the easily removable water and the last and much more tenaciously held water.Graham ‘‘ friend and fellow townsman ” of Clark published a notable memoir entitled “ Researches on the Arseniates Phosphates and Modifications of Phosphoric Acid ” which deservedly has been read by generations of his successors ; in this he established the constitutional relationship of ortho- pyro- and meta-phosphate and demonstrated the existence of a crystalline soluble variety and an insoluble variety as well as the soluble glassy variety of sodium metaphosphate.Again on a point of historical accuracy it should be recorded that there are earlier references to the preparation of sodium metaphosphate. Berzelius 1 described also the preparation of monosodium orthophosphate and mentions heating it to redness before analysing it for Na20 and P20,. Glassy sodium meta- phosphate was described in some detail by J. L. Proust 4 who obtained it by heating NaNH,HPO and stated that the resulting transparent deli- quescent and nearly neutral glass was a new salt equivalent to a union of ordinary sodium phosphate with the phosphoric acid left after the ammonia had been driven off. An early paper interesting because of its insight into the polymeric aspect of the structure of condensed phosphates and their hydrolytic instability in the presence of acids is that of Th.Fleitmann and W. Henne- berg who first suggested the generalised concept of condensed phosphates as including compounds intermediate between meta- and pyro-phosphates (Le. polyphosphates) and attempted to prepare Na,P40, and Na,H,P40,,. In a century of work following Graham’s memoir there grew up an unfortunate state of confusion apparent in the contradictory systems of nomenclature adopted for the polymetaphosphates. Most of the trouble arose from the inadequacy of the methods used to establish chemical individuality and to assign degrees of polymerisation. In recent years several helpful reviews of parts of the earlier literature have been made H. Terrey,6 “ The Metaphosphates and Polyphosphates of Sodium ” ; K. Kqbe and G. Jander,7 ‘‘ Die Metaphosphate ” ; 0.T. Quimby,8 “ The Chemistry of Sodium Phosphates ” ; E. P. Partridge,g “ The Peculiar Phosphates ”. Mention may also be made of two modern experimental Ann. PhysiE 1S16 54 31. Phil. Trans. 1833 123 253. Annalen 1848 65 304. 6 Ann. Reports 1937 34 115. KolI. Beihefte 1942 54 1. Chem. Reviews 1947 40 41. Edinburgh J . Sci. 1827 7 298. Ann. Chim. Phys. 1820 14 281. * Chem. Eng. News 1949 27 214. TOPLEY THE CONDENSED PHOSPHATES 347 papers 10 l1 which have done much to clarify the phase equilibria of the system Na,O-P,O in the range from NaPO to Na4P20, encountered when the melts are cooled. Brief mention must be made of the discovery that certain organic derivatives of condensed phosphoric acids play an important part in the storage and release of energy in metabolic processes and in muscle action which has brought into prominence the biochemical interest of the magni- tude of the free-energy decrease accompanying hydrolysis of the condensed phosphate (pyrophosphate and triphosphate) portion of these substances.The so-called " high energy phosphate bonds " have been reviewed by F . Lipmann .I2 Structural Formulae.-In this Review it is explicitly assumed that when a 4-co-ordinated P atom is directly linked to H C N or 0 or a combina- tion of these elements the P atom possesses an octet of shared electrons; and as part of the same assumption that in molecules or ions such that a transfer of electrical charge from the P atom is required to make the octet assumption possible (" formal positive charge " on the P atom) then to a substantial extent the charge distribution actually exists.Free use of this simplifying assumption is made in discussing structures from a unified point of view. In an overwhelming proportion of all phosphorus compounds the phos- phorus atom is 4-co-ordinated. Phosphorus atoms that are 3-co-ordinated and possess an unshared pair of valency electrons are very reactive when conditions are provided such that the unshared electrons can become shared as part of a tetrahedral 4-co-ordinated structure. For example pure PCl rapidly absorbs oxygen in the cold,* forming POCl, and trialkyl phosphites readily oxidise to trialkyl phosphates. Also the unshared pair of electrons of PCl and P(OR), with R = alkyl easily attack water so that the phosphorus atoms form a fourth bond :PCl + 3H20 = PHO(OH) + 3HCl :P(OR) + H20 = PHO(OR) + R*OH The reactions of P40 (11) with oxygen to form P,O, (111) or with In the acids and anions PO(OH), water to form PHO(OH) are similar.lo E. P. Partridge V. Hicks and G . W. Smith J . Arner. Chem. Soc. 1941 63 454. l1 G. W. Morey and E. Ingerson Amer. J. Sci. 1944 242 1. l2 Advances in Enzymology 1941 1 9 9 ; 1946 6 231. * Trialkylphosphines oxidise to the stable phosphine oxides. An interesting question is the nature of the white crystalline product of the reaction PH + 0 = H + P02H which is described by H. J. van der Stadt (2. physikal. Chem. 1893 12 322) as occurring when the gases are mixed at low pressures. Van der Stadt regarded PO,H as metaphosphorous acid because it reacts with water t o form phosphorous acid. A monomeric structure seems impossible and the crystalline character suggests a substance of low molecular weight such as a cyclic trimer 0 0 0 + + + o-P-o-P-o-P-, 348 QUARTERLY REVIEWS PHO(OH), PQ(OH),*PO(OH) * and PH,O(OH) the 4-co-ordinate struc- ture is certainly present despite the internal charge separation which in the last three would in part be avoided if odp\o 0ep‘o unshared electron pairs were the actual 1 j l(n) (m)J ,lo I structures.:;QIP ?<:yo Compounds in which some of the oxygen 0 0 atoms in PO(OH) are replaced by the iso- electric groups CH and NH as in the alkane phosphonates and the amido- and imido-phosphates and compounds in which the P-H of alkyl phosphites is replaced by P-Hal. further illus- trate the stability of compounds containing 4-co-ordinated phosphorus atoms. The stability of 4-co-ordinated phosphorus exemplified is relative to tervalent phosphorus Le.3-co-ordinated phosphorus with an unshared electron pair but there is also a wider body of chemical experience con- cerning the tendency of phosphorus to gain and retain the 4-co-ordinate condition especially when oxy-phosphorus compounds stable a t elevated temperatures are in question. Against this background we may consider the hypothetical monomeric metaphosphate ion PO,’ and the relation of PO,’ and PO4‘” to NO,‘ and NO4”’. The high dissociation constant of nitric acid is attributable to the double positive formal charge on the nitrogen atom when it binds three oxygen atoms by six electrons. It is possible that the reason for the non- existence of NO4’” is that steric interference of the four oxygen atoms would raise the internal energy of the structure too much.A simple calculation from the van der Waals radius of the oxygen atom with com- pleted octet (1-32-1.40 A+) together with the (3-0 N-0 P-0 S-0 and C1-0 atomic distances in crystals certainly makes it appear that there must be some mutual lateral repulsion of the oxygen atoms even in the larger tetrahedral ions SiO,’”’ PO4”’ SO4” and ClO,’ ; the de-stabilising action of lateral repulsion must be greater in NO4”’ than in PO4’” and 0 alternative structures involving octets with * The case for the P-P formula for hypophosphoric acid is now very strong. The “ single ” formula H,PO has an odd number of electrons which is inconsistent with the diamagnetism of hypophosphates (F. Bell and S. Sugden J . 1933 48). A “ double ” formula H,P,O is consistent with the Raman spectrum (J.Gupta and A. K. Majumdar J . Indian Chem. SOC. 1942 19 286) and the freezing-point lowering of aqueous solutions of hypophosphates (P. Nylh and 0. Stelling 2. anorg. Chem. 1933 212 16). Neither of the unsymmetrical formulae PO(OH),*O-PHO(0H) and PO(OH),.O.P(OH) is easily reconcilable with the comparatively large fourth dissociation constant pK rn 10 since from both would be derived the same quadri- valent mion ”P0,-O*P02” in which the right-hand portion is comparable with a hypothetical bivalent ion from monoethylphosphorous acid ; but there seems to be no tendency for monoethylphosphorous acid to form a disodium salt (cf P. Nylh “ Studien uber organischen Phosphorverbindungen ” Thesis Uppsala 1930 pp. 37 151). On the other hand the tetrabasicity of hypophosphoric acid and the smgle- ness of the X-ray absorption edge (Nyl6n and Stelling Zoc.cit.) are consistent with the symmetrical formula PO(OH),*PO(OH),. Recently (B. Raistrick and E. Hobbs Nature 1949 164 113) confirmation of the symmetry of the hypophosphate anion has been obtained by X-ray structure work with diammonium hypophosphate. TOPLEY THE CONDENSED PHOSPHATES 349 greater in NOp"' than in NO,'. Certainly simple analogy with NO,' is not a valid reason for expecting the existence of monomeric PO,'. If a monomeric variety of soluble sodium metaphosphate could by some means be prepared the phosphorus atom would have a formal charge of + 2 and only six electrons in its valency shell-at least reasoning from the high acid strength of nitric acid such a structure would contribute considerably to the mesomeric structure.This would favour the addition of H' and OH' during the process of dissolution 0' 0' .. O ~ ; + H O H -+ HO:P:OH .. .. 0 0 so that the monomeric metaphosphate would .be rapidly hydrated to acidic orthophosphate without the appearance of pyrophosphate intermediately. If the hydration of the hypothetical monomer were slow enough for observa- tions to be made on its properties before hydration it should be the salt of a monobasic acid about as strong as nitric acid. All the known varieties of sodium metaphosphate dissolve either in water or in dilute sulphuric acid and in no case is the hydration or hydrolysis to orthophosphate immediate or very rapid nor does the solution in any case have the pro- perties expected for the monomer.So far as the dissolved state in water is concerned it seems improbable that monomeric metaphosphate will ever be found. By the same reasoning it is unlikely that an open polyrneta- p h s p h t e chain structure terminating in a 3-co-ordinate phosphorus atom will be found as an entity in aqueous solution ; if a structure such as (IV) exists in the solid state on dissolution it would pass from the metaphosphate (PO,) to the polyphosphate (PO,),-,(PO,) composition (V) 0 0 0 I" 0 0 0 "'/' OPOPOP +HzO -+ OPOPOPO +2H+ [ O O O I [ 0 0 0 1 (IV.) (V.1 Cyclic metaphosphate structures with single rings of alternating phos- phorus and oxygen atoms e.g. (V1)-(VIII) are true metaphosphates in I I 0 '/' OPO \ o / 0-P /o;p\Ol (VII.) 0 - 0 0 P-0-P 0 / o o \ \ 0 o / 0 P-0-P (VIII. ) - 0 0 '"1 the sense of having exactly the composition (PO,),"-.The trimetaphos- phatea (VII) and tetrametaphosphates (VIII) are known compounds dis- cussed later in this review. Several claims that dimetaphosphoric acid or its sodium salt has been obtained appear in the literature of the past twenty years but the evidence is very incomplete. 350 QUARTERLY REVIEWS -0 0- -OP+-0 0-+PO- P -0 \ / 0- /+\ 0 -0 I P 0- 0- ! -op+-o-p+ +p-o-+p-o-+po- 8- (IX.) phosphorus atom is 4-co-ordinated with a formal charge of + 1 and every oxygen atom except those which join two phosphorus atoms has a formal charge of - 1. The formal charge on one of the phosphorus atoms of the ring is not balanced by a formal negative charge on an adjoining oxygen atom. This is a slight infringement of the microscopic neutrality principle which however may not create a situation electrostatically too unfavour- able for such an element of structure to exist.It is included here because of its interest in connection with the products of hydrolysis of metaphos- phate glass discussed later. In any case the corresponding formulation for the ion obtained by omitting one of the (PO,)" groups at the top of (IX) does not have this possible disability. The point which it is desired to make here is that chain-wise polymerisation provided the chain starts or terminates on a closed ring can be visualised as producing true metaphosphates in the sense of having the composition (PO,),n- but which also have the terminal doubly charged groups characteristic of polyphos- phates and pyrophosphates. The point is not merely formal because some of the most interesting properties of condensed phosphates depend upon the distribution of (net) negative charge on the anion; for example the important property of forming stable soluble complexes with calcium is most probably to be interpreted in terms of ion association for which the distribution of the anionic charge plays a dominant part.Polymorphism of Condensed Sodium Phosphates.-Polymorphism is fie- quent both the kind which arises from different lattice arrangements of the same ions and the kind which arises from different molecular structures of the condensed phosphate ions that form sub-units of the crystal struc- ture. Typical of the first kind are the five forms lo of Na,P,O which appear in succession as the molten salt cools the three forms l3 of anhydrous sodium trimetaphosphate and the two forms of anhydrous sodium tri- phosphate.10 Polymorphism which arises from structural often polymeric differences 18 R. W. Liddell J . Amer. Chem. SOC. 1949 71 207. TOPLEY THE CONDENSED PHOSPHATES 351 in the anions is accompanied by notable differences in chemical properties which are retained in solution. Nomenclature of Condensed Sodium Phosphates.-Seven anhydrous crystalline forms of sodium metaphosphate are known with certainty and various names have been given to them. The most recent system is that of Partridge,g who suggests that Roman numerals should be used as labels starting with sodium trimetaphosphate as NaP0,-I because this is stable at its m.p. (628") ; the polymorphic forms of this metaphosphate are labelled 1' and I".Varieties of (NaPO,) prepared at lower temperatures by dehydrating NaH,PO are assigned numerals in sequence with the decreas- ing temperature of preparation. It is not claimed that this system is really less arbitrary than its several predecessors. Thus a highly polymerised fibrous variety (Kurrol salt) which can only be prepared by crystallisation from the supercooled melt is labelled NaP0,-IVY and the cyclic sodium tetrametaphosphate has not been accommodated in the scheme at all. In this review a Greek prefix will be used to indicate the degree of poly- merisation where this is well established i.e. for sodium trimetaphosphate and tetrametaphosphate and to these the Roman numeral designation will be added in parentheses ; otherwise Partridge's notation will be followed where he has assigned numerals and the name most common in the litera- ture will be added in parentheses.Knowledge of the polyphosphates has not yet developed to a point where the need for a systematic nomenclature is pressing; apart from pyrophosphate which might with advantage be called &phosphate the only individual in this class actually isolated is the triphosphate. It is regrettable that this compound is so often burdened with the unnecessary name " tripolyphosphate ". Sodium Trimetaphosphate Na,P,O (NaP0,-I) .-This is obtained almost exclusively when NaH,PO is heated to between 550" and 628" (m.p.). A considerable but very variable proportion of the metaphosphate obtained by dehydrating NaH,PO or Na,H,P,O at any temperature between about 250" and 500" usually consists of trimetaphosphate.All other varieties including metaphosphate glass are converted into the trimetaphosphate in the last 50" or so below the m.p. The conversion is so rapid near the m.p. that all varieties including sodium Kurrol salt (NaP0,-IV) appear to melt a t the same temperature. The two polymorphic forms of sodium trimetaphosphate NaP0,-I' and NaPO,-I" are obtained l3 by controlled cooling of molten sodium meta- phosphate; they change into the stable form of Na,P,O (NaPO,-I) if cooled too slowly after crystallisation. The stable form is obtained as an opaque crystalline mass when prepared by direct solidification of the melt. All three forms of Na,P,O are very soluble in water and can be recrystal- lised as Na3P,0,,6H,O or above about 40" as Na,P,O,,H,O. The latter recrystallises as the anhydrous salt [identical with Na,P,O (NaP0,-I)] under its saturated solution at 60" in the course of a few days.Isothermal dehydration of the hexahydrate in vacuum at room temperature removes over 90% of the water of crystallisation without destroying the meta- phosphate but at higher temperatures or under conditions less favourable AA 352 QUARTERLY REVIEWS to the rapid removal of water vapour the dehydration is accompanied by extensive conversion into acidic ortho- and pyro-phosphate.14 The cyclic trimeric formula VII was first proposed by C. G . Lindbom.16 The evidence now available in its support is cogent. (1) The physicochemical properties and the reproducible metathetical changes are those of a molecularly homogeneous salt of a moderately strong acid.The solutions show none of the irreversible solubility and the viscous effects characteristic of the crystalline varieties NaP0,-I1 (Maddrell salt) NaP0,-111 and NaP0,-IV (Kurrol salt). (2) Molecular-weight determinations by freezing-point lowering in sodium sulphate decahydrate gave 307-313 (Na,P,O = 306). I n water as solvent allowing for the effect of ionic strength upon the f.p. lowering the molecular weight was found l7 to agree better with the supposition of a tervalent anion than with any alternative. (3) Recent measurements l8 of the conductance of dilute solutions of sodium trimetaphosphate (NaP0,-I) show that the salt is uni-tervalent. Older work (4) The titration curve of sodium trimetaphosphate 2o with hydrochloric acid and sodium hydroxide is practically indistinguishable from the titration curve of sodium chloride at the same equivalent concentration.It follows that the structure of H,P,O does not include the feature responsible for the small value of the ratio K,/K (w 2 x lo+) in pyrophosphoric acid. This feature is the dissociation of a proton from an O(P0,)OH’ group already carrying a single negative charge. Thus in trimetaphosphoric acid all three protons come from different OPO,H. groups. This is consistent with the cyclic structure (VII) but two other trimeric structures would also have one hydrogen atom associated with each phosphorus atom. Written in terms of the formal charges involved these are (X) and (XI). using the Ostwald empirical rule gave the same result. ro 111 - - - 0 0 0 -OP+OPfOP++ [.?!l4 This review is written in the light not only of published work but also of a decade of experimental work by the research department of Albright and Wilson Ltd.Unpublished work previously reported verbally at meetings of the Society of Chemical Industry includes experiments by A. G. Taylor J. E. Such D. R. Peck R. H. Todinson and F. J. Harris and is acknowledged by ref. (14) in the text. Ber. 1875 8 122. 16P. Bonneman-BQmia Ann. Chim. 1941 16 395. P. NylQn 2. anorg. Chem. 1937 229 30. lac. W. Davies and C. B. Monk J. 1949 413. l9 A. Wiesler 2. anorg. Chem. 1901 28 187. 2O H. Rudy and H. Schloesser Ber. 1940 73 484 ; W. D. Treadwell and F. Leiit- wyler HeEv. Chim. Acta 1938 21 1450. TOPLEY "HE CONDENSED PHOSPHATES 353 Structure (X) is unacceptable as a description of the trimetaphosphate anion in aqueous solution for reasons discussed in connection with the non-existence of monomeric metaphosphate in solution.In addition there would be difficulty in understanding the absolute magnitude of K ; Davies and Monk l8 calculate an approximate value K m loA2 at 25" by taking account of the deviation from Onsager's formula of the slope of a plot of conductance against dconcn. for H,P@g. Thus K for H,P,og is almost as large as K for H4P20,. But considered as a base the "O,PO group in struc- ture (X) would be comparable with the pyrophosphate ion "PO,*O*PO,(OH) so that the first proton bound by (X) would dissociate as weakly as the second proton bound by the pyrophosphate ion (K m 2 x 10-7). The objections to structure (XI) are (a) the rarity of stable compounds in which there is reason to assume the existence of a 5-co-ordinate phosphorus atom ( b ) the circumstance that the first proton to be bound would attach itself to the central PO,- group and because of the absence of formal charge on the phosphorus atom would be almost as weakly acidic as silicic acid.(5) X-Ray crystallographic evidence is consistent with a cyclic trimeric anion in anhydrous sodium trimetaphosphate. An alternative possibility that the crystal contains monomeric NaPO is not yet quite excluded but is extremely improbable even on crystallographic grounds alone. 21 (6) Sodium trimetaphosphate (NaP0,-I) forms a series of well-defined salts containing one atom of Na* and one of M" of which NaBaP,O9,4H2O is typical. It was the discovery of these double salts that originally led Fleitmann and Henneberg to the conclusion that the soluble crystalline sodium metaphosphate which they obtained by slow cooling of a melt was trimeric.Sodium trimetaphosphate has so far found no application; it con- spicuously lacks the properties which underlie the industrial use of some other condensed phosphates. A reaction of some interest is its hydrolytic conversion into triphosphate described later. The trimetaphosphates of metals other than sodium are in general fairly soluble. Sodium Tetrametaph0sphate.-This salt is thermally less stable but in resistance to hydrolysis by alkali to polyphosphate somewhat more stable than the trimetaphosphate which in several respects it resembles. It is not and perhaps cannot be made by dehydration of NaH2P04. A. Boull6 22 described as irreversible the transformations 440' 550" Na4P401 4 NaP0,-I1 (Maddrell salt) -+ Na,P,09 Sodium tetrametaphosphate forms two hydrates Na4P40,2,4H,014 and a hydrate Na4P40,, 10H,0,14 which have high and reproducible solubility between 0" and at least 80".The decahydrate and the stable tetrahydrate establish definite equilibrium vapour-pressure relationships and can be fully dehydrated without appreciable hydrolytic alteration of the metaphosphate anion since recrystallisation of the dehydrated substance reconverts it into one or other of the hydrates. 21 B. Raistrick Communication to Canadian Institute of Chemistry Halifax 1949. 2 2 Ann. Chirn. 1942 17 213. 354 QUARTERLY REVIEWS Until very recently all preparations of sodium tetrametaphosphate have been by the procedure adopted by Fleitmann and Henneberg of forming the metaphosphate of bivalent copper or lead by heating the oxide with excess of phosphoric acid to 350400” then treating the insoluble heavy- metal phosphate with sodium sulphide solution and crystallising the sodium salt.Confirmatory and extensive investigations of this preparative method employing heavy-metal metaphosphates made a t various temperatures have been published by Th. Fleitmann,23 A. G l a t ~ e l ~ ~ G. Ta1nmann,~5 and F. Warschauer.26 (There is no doubt that these authors obtained other poly- metaphosphates besides the tetrametaphosphates but it is questionable whether any other pure individuals were isolated.). A more convenient preparation of tetrametaphosphate is directly from phosphoric anhydride. Such and Tomlinson l4 have shown that yields of Na4P401,,4H20 exceeding 50% of the theoretically possible can be crystal- lised from the reaction product obtained by hydrating the volatile form i .e . P4OlO with Na,CO,,lOH,O or a cold suspension of NaHCO,. This result is easily visualised by considering the symmetrical tetrahedral struc- ture 27 of the P401 molecule. All the anhydride P-0-P linkages in (111) are equivalent so that the initial step in the hydration can have only one result. It appears that of the three possibilities for the reaction of a second molecule of water the attack is preferentially upon the anhydride bridge in (XII) giving the cyclic tetrametaphosphate (VIII). [In (111) (XII) and (VIII) as written here the formal charges are shown.] I I -OP+-0-+PO- -0i;. ‘ I {,+jO- “0 I I 0- 0-+P-0 lo- I I O- +P -0 I P+ - - 0 0 0-P-0 - - 0 0 0-,P-0 0- (111.) 0 0 - - (XII.) 0 0 (VIIT.) - - The anion of sodium tetrametaphosphate has not been structurally determined by X-ray crystallography,* but in all probability it has the tetrameric and cyclic structure (VIII) first suggested by GlatzeLlg The ‘evidence now available is similar to that listed for sodium trimetaphosphate (NaP0,-I). (1) The same general statement can be made as for the trimetaphosphate. 23 Ann. Phys. Chem. 1849 78 233 ; 338. 25 J . pr. Chem. 1892 45 417. 27 G. C. Hampson and A. J. Stosick J . Amer. Chem. SOC. 1938 60 1814. * In the crystalline state the only metaphosphate whose structure has been worked out is Al(PO,) (L. Pauling and J. Sherman 2. Krist. 1937 98 481). These authors interpret the results in terms of a lattice built up from Al”’ ions and closed ring anions P0Oll”” consisting of four tetrahedral PO groups joined through 0 atoms held in common between consecutive P atoms.2 4 Thesis Wiirzburg 1880 26 2. anorg. Chem. 1903 36 137. TOPLEY THE CONDEXSED PHOSPHATES 355 (2) Molecular-weight determinations cryoscopically in sodium sulphate decahydrate 16 gave 414 and 417 (Na,P,O, = 408). In water allowing for the ionic strength the molecular weight was found l7 to be in better agreement with the supposition of a quadrivalent anion than any other. (3) The slope of the plot of conductance against dconcn. for sodium tetrametaphosphate is somewhat greater than corresponds to Onsager’s limiting slope for a uni-quadrivalent electrolyte but the excess is explained by assuming some ion association to NaP,O,,”’.Davies and Monk l8 summarise their analysis of their data “The value of the dissociation constant (i.e.y of NaP4012”’) the anion’s ( i - e . P4012””) mobility and the slope of the conductivity curve all support the quadrivalency assumed for the anion.” Older work l9 using the Ostwald empirical rule gave the same result. (4) The titration curve l4 of repeatedly recrystallised sodium tetrameta- phosphate is practically identical with that of a salt of a strong acid. The deduction that the tetrametaphosphate anion is cyclic is the same as that already stated for sodium trimetaphosphate (NaP0,-I). Mild alkaline hydrolysis of Na,P,O, gives mixtures containing pyro- phosphate and orthophosphate but also polyphosphates doubtless both tetra- and tri-phosphate. At certain stages the hydrolysate possesses in a high degree the power to combine with bi- and multi-valent cations forming soluble complexes.So far the isolation of pure sodium tetra- phosphate Na6P,01 has not been achieved. The tetrametaphosphates of several metals besides the alkali metals and ammonium are fairly soluble. Insoluble Sodium Metaphosphates NaPO,-111 and NaP0,-I1 (Maddrell Salt).-The first identifiable product obtained by heating NaH,PO or its hydrates is the acid pyrophosphate Na2H2P207; in an open vessel the dehydration of NaH2P0 proceeds slowly at temperatures (in the solid) not much above 150” and rapidly above 200”. Further dehydration of the acid pyrophosphate which for practical purposes becomes measurable at about 240” is a complicated process. No known product intervenes between pyrophosphate and metaphosphate.Fleitmann and Henneberg 5 believed they had evidence that an acid polyphosphate Na,H,P,O, is formed when Na,H2P207 is slowly heated at 220” but this result is certainly not easy to confirm. Continued heating a t temperatures below the fusion point (628’) gives rise to three distinct varieties of sodium metaphosphate of which one is Na,P,O (NaP0,-I) which as already described becomes the only product between about 550” and 628”. Below 550” the proportions of the three forms vary widely according to the time and temperature of heating the partial pressure of water vapour round the solid and the physical nature of the solid (hard cake obtained by partial fusion of the NaH,PO in the water initially liberated quickly or thin layer of finely divided solid).The two varieties NaP0,-I11 and NaPO,-11 (Maddrell salt) are practically insoluble in water. Both have well-defined X-ray powder diagrams and are obviously crystalline when viewed under the polarising microscope. 356 QUARTERLY REVIEWS Insoluble sodium (and potassium) metaphosphate was obtained by R. Maddrell 28 by heating phosphoric acid with sodium nitrate (or potassium chloride). Insoluble sodium metaphosphate is very often referred to in the literature as Maddrell salt but with little reason." It is convenient to refer to '' insoluble sodium metaphosphates " distinguishing between the low- and the high-temperature variety [NaPO,-I11 and NaP0,-I1 (Maddrell salt)]. The two compounds are physically and chemically very similar. In the dehydration of NazHaP20, the two insoluble varieties can be isolated as follows.At the lowest temperatures (250-280") the low-tem- perature variety (NaPO,-111) is obtained mixed with trimetaphosphate (NaPO,-I) and unchanged pyrophosphate and the last two are removed by washing. In the range 350400" the product consists of the high- temperature insoluble variety NaP0,-I1 (Maddrell salt) together with trimetaphosphate (NaP0,-I) from which the latter can be removed by washing. Very careful adjustment of the conditions of heating can give directly a product containing over 96% of NaP0,-I1 (Maddrell salt). NaP0,-I11 is itself slowly but completely transformed into NaP0,-I1 (Mad- drell salt) at about 350" and NaP0,-I1 (Maddrell salt) is transformed into NaP0,-I with barely measurable velocity just above 410° rapidly at 500".Neither change is reversed at lower temperatures. Nothing is known about the molecular structure of either NaP0,-I11 or NaP0,-I1 (Maddrell salt). The following observation l4 indicates that they are salts of highly polymerised metaphosphoric acids. When the finely ground solids are warmed (NaPO,-111) or boiled [NaPO,-I1 (Maddrell salt)] in a decimolar solution of an ammonium or any alkali-metal salt other than sodium the metaphosphate readily dissolves. Alcohol precipitates a highly hydrated metaphosphate which after being washed to remove most of the added salt redissolves in water to form a decimolar solution with viscosity about ten times greater than that of water. The excess viscosity is almost eliminated by addition of electrolytes. The metaphosphate in these col- loidal solutions gives a titration curve with a single inflexion curve near pH 7.Taken in conjunction with the viscosity this suggests that long- chain or large-ring anion structures are present in the solution. The average size of the units of the dissolved metaphosphate is probably much less than that of the solids because in the process of dissolving some hydrolytic degradation is unavoidable ; continued heating of the solutions reduces their viscosity to that of water. In other respects (formation of stable complexes with bivalent cations ; precipitation from acid solutions as the benzidine salt 29) they behave like solutions of metaphosphate glass. The peculiar mechanism of dissolving in which alkali-metal ions other than 28 Annalen 1847 61 63; Phil. Mag. 1847 30 322. 29 C. J. Munter Communication to American Chemical Society 1936 Pittsburgh meeting.* Maddrell only contributed two brief but discordant descriptions of a single experi- ment ; the substance obtainable in very small yield by repeating Maddrell's preparation has the same X-ray powder diagram as NaPOs-II prepared by dehydrating NaH2P04 at 300° as had already been done by Graharne3 TOPLEY THE CONDENSED PHOSPHATES 357 sodium are active is presumably in some way connected with the highly polymerised state of the metaphosphate and related to the base-exchange which is observed when NaP0,-I1 (Maddrell salt) is acted on by concentrated solutions of non-sodium salts. Insoluble Sodium Metaphosphate (NaP0,-IV ; Kurrol salt) .-Rapid cool- ing of molten sodium metaphosphate results in formation of a glass very slow cooling gives sodium trimetaphosphate (NaP0,-I).If the cooling is only moderately slow the result is sensitive to such factors as the rate of cooling at different stages between 630" and 500" (and almost certainly also the thermal history of the melt above 630") ; the purity of the meta- phosphate ; the nucleation deliberate or accidental of the supercooled melt. Whatever crystalline phases are formed during moderate cooling substantial quantities of glass often accompany them. Occasionally along with glass and trimetaphosphate a small proportion of a brittle fibrous crystalline variety (NaP0,-IV ; Kurrol salt) grows spontaneously and escapes complete transformation into trimetaphosphate. By nucleating the surface of the melt between 600" and 550" crystallisation of this variety is promoted.The most effective method is to seed with fragments of pre- formed NaP0,-IV (Kurrol salt) which must be well washed to remove tri- metaphosphate since the latter has the higher crystallisation rate. Other finely divided solids can act as nuclei 14 for growth of NaP0,-N (Kurrol salt) including silica carborundum insoluble potassium metaphosphate and NaPO,-II (Maddrell salt). P. Pascal 30 described a preparation of NaP0,-IV (Kurrol salt) by heating sodium methyl (and ethyl) hydrogen phosphate to low red heat but attempts l4 to repeat this have over a wide temperature range given only mixtures of sodium trimetaphosphate (NaP0,-I) and NaP0,-I1 (Maddrell salt). A. G. Taylor 14 and H. Huber and K. Klumpner 31 noted that NaP0,-IV (Kurrol salt) is more readily obtained if the ratio Na20/P,0 is a little higher (A.G.T.) or lower (H.and K.) than unity. The explanation in both cases might be that the crystallisation of Na,P,O is preferentially restrained. The ratio Na,O/P,O in the fibrous crystals has not been examined. The description of NaP0,-IV (Kurrol salt) as insoluble needs qualifica- tion. In cold distilled water after some hours much more quickly in hot water the fibrous crystals swell and eventually give a viscous solution. A 1% solution may be compared with glycerol but the solution is really a very weak gel. The viscosity and structure of the solution is sometimes retained for months sometimes lost in the course of a week or so. The reason for this variable behaviour may be the presence of traces of impuri- ties such as multivalent cations which catalyse (see p.365) the hydrolytic fission of the macro-ions or there may be size differences in the anions themselves. Different preparations of NaP0,-IV (Kurrol salt) at least from a melt of the correct Na,O/P,O ratio give the same well-characterised X-ray powder diagram. The gel structure and viscosity of the colloidal solution is destroyed in a few minutes by boiling. The swelling and dis- solving i s very much accelerated by the presence of alkali-metal cations ao Bull. SOC. chim. 1924 35 1119. *l 2. anorg. Chem. 1943 $351 213. 358 QUARTERLY REVIEWS other than sodium whilst sodium salts greatly reduce the extent of swell- ing. The effect of foreign alkali-metal ions which is evidently to be com- pared with their effect upon the other two insoluble varieties of sodium metaphosphate was described by A.G. Taylor,S2 who showed also that insoluble potassium metaphosphate swells and dissolves in the presence of sodium or ammonium ions. It is remarkable that a h e l y ground mechanical mixture of any variety of " insoluble " potassium metaphosphate with any variety of '' insoluble " sodium metaphosphate will go completely into solution on boiling for a short time by reason of the reciprocal action of the initially very small concentrations of the dissolved salts. A rough comparison of the time required at 60" for swelling and solu- tion of NaP0,-IV (Kurrol salt) in the presence Of M/200 chlorides of univalent cations14 puts the efficiency of the cations in the order K > Li' > NH,' > Rb' = Morpholine > Triethanolamine > Benzylamine The action is to a first approximation independent of the anion of the added salt.The mechanism is not obvious but presumably it is related to a kind of base exchange which occurs when " insoluble " alkali-metal metaphosphates are immersed in more concentrated salt solutions. The effect of morpholine and triethanolamine salts was first observed by Huber and K l ~ m p n e r . ~ ~ The discovery of NaP0,-IV (Kurrol salt) was attributed by G. Tam- mann 33 to his collaborator Kurrol. It is referred to by him 34 as having been obtained by the slow cooling of molten metaphosphate. Tammann also mentions other experiments attributed to Kurrol whereby the yield of NaP0,-IV (Kurrol salt) was improved by slow dehydration of NaH,P04 apparently without fusion and crystallisation. The occasional spontaneous formation of a little of this fibrous variety from the melt has often been confirmed but not the other procedure mentioned by Tammann.The term " Kurrol salt " has been much used (in France and Germany especially) not only for NaP0,-IV (Kurrol salt) but also for all the insoluble varieties of potassium metaphosphate and for other alkali-metal meta- phosphates. No criterion has been proposed by which a class of " Kurrol salts " can be characterised except by implication the property of yield- ing viscous solutions. Since the other two '' insoluble " varieties of sodium metaphosphate share this property as also does sodium metaphosphate glass when prepared in a certain way it would seem better to reserve the name (if used at all) for the fibrous sodium metaphosphate discovered by Kurrol.What has already been said about the probable high molecular weight of NaP0,-I11 and NaP0,-I1 (Maddrell salt) applies also to NaP0,-IV (Kurrol salt). The remarkable fibrous appearance of the latter is also found with KPO under some conditions of crystallisation from the melt. Sodium Metaphosphate Glass.-Molten sodium metaphosphate remains fluid a t least down to 500" if cooled fairly quickly ; by rapid cooling con- a' B.P. 643,218 (1941). s r Z . physikd. Chrn. 1890 6 140. 33 J . pr. Chern. 1892 45 417. TOPLEY THE CONDENSED PHOSPHATES 369 tinued down to below 200" devitrification is avoided and a brittle trans- parent colourless glass is obtained which softens and tends to devitrify if reheated above about 300". This glass is sometimes called Graham's salt and often but only by custom and no longer implying a view about the degree of polymerisation " sodium hexametaphosphate ".It is hygro- scopic and easily soluble without definite limit in water but not alcohol. Molten sodium metaphosphate has a strong affinity for water. When any anhydrous crystalline sodium metaphosphate is fused in undried air the melt absorbs some water vapour (1% by weight is common). Much or all of this is retained on cooling if a glass is formed but is suddenly and vigorously evolved if and when a large proportion of the supercooled melt crystallises to sodium trimetaphosphate (NaPO,-I). The presence of this water affects the properties of the glass and certainly some if not all of it is chemically combined. When pure Na,P,O or NaNH,HPO is fused in air of ordinary humidity the glass obtained by quick cooling is slightly acid (pH w 5.7 at 1% weight concentration in a solution freshly prepared in water of pH 7).The solution is not appreciably different in viscosity from water. If a shallow layer of the same melt is kept for a day at 700" in a stream of dry air before chilling the solution has pH 66-7. The metaphosphate glass now dissolves by a process of swelling and the solution is a very weak gel. Solutions of metaphosphate glass (or the free acid obtained by ion exchange) titrate with strong acids and bases as the salt of a fairly strong acid to a first appr~ximation.~~ 0. Sam~elson,~~ working with undried metaphosphate glass determined the small titration values to the phenol- phthalein change point and interpreted them in terms of end-group titra- tions of long unbranched-chain polyphosphate anions.The calculated average molecular weights increased with the temperature from which the melt had been chilled varying from 10,800 for 650" to 17,200 for 950". Earlier authors also considered the diagnostic use that might be made of titration values between pH 4.5 and 9-5 but it may be remarked that interpretation of the titration values in terms of unbranched chains or with the aid of the assumption that a true metaphosphate (NaPO,) neces- sarily corresponds to an acid with no hydrogen titratable between pH 4 and pH 10 is open to the consideration that metaphosphate structures like (IX) which have a side chain attached to a closed ring would behave like a linear polyphosphate in respect of the titration of the terminal groups [excepting probably the two a t the top of (IX)].The presence of acid polyphosphates in the glass could account for the steam evolved as sodium trimetaphosphate (NaP0,-I) crystallises as well as the pH observations already mentioned and the conspicuous increase in colloidal character of the chilled metaphosphate from the dried melt Measurements of the amount of steam evolved during crystallisation have not been made but it would be interesting to compare it with the end- s6 E. P. Partridge Dual Service News Hall Laboratories Pittsburgh 1937 ; Glatzel ref. (24) ; W. Teichert and K. Rinman Acta Chem. Scad. 1948 2 226. 8eS~enak Kern. Tick. 1944 56 343. 360 QUARTERLY REVIEWS group titration and the initial pH of solutions of glass chilled from the same melt. Estimates by dialysis measurements of the average molecular weight of dissolved sodium metaphosphate glass (not from dried molten metaphos- phate) have been made by Karbe and Jander and by Teichert and Rin- man.35 The results are in reasonable agreement and indicate values in the range 1000-8000 increasing with the temperature from which the melt is chilled.Karbe and Jander's experiments are particularly interesting because of the wide temperature range (645" to 1280") and the drastic chilling of the melts by pouring into partially frozen carbon tetrachloride. The ultracentrifuge has been applied 37 to study sedimentation in solutions of sodium metaphosphate glass affording apparent molecular-weight values up to 13,000. Quimby points out that the kinetic particle effective in sedimentation and dialysis might be an aggregate (micelle) of smaller poly- merised units.Davies and Monk determined conductivities of sodium metaphosphate glass extending their measurements down to a concentra- tion of 5 x ~ O + M . ; the rapid decrease of equivalent conductivity between 5 x 1 0 - 6 ~ . and 5 x 1 0 - 3 ~ . is attributed to the anion having colloidal dimensions but the authors discussing Quimby's suggestion point out that there is no evidence of micelle dissociation such as is found with typical micellar electrolytes. It is difficult to avoid the conclusion that true macro-ions are present in some circumstances in view of the gel structure possessed by dilute solutions of NaP0,-IV (Kurrol salt) and with dehydrated sodium metaphosphate glass. Po1yphosphates.-Only one series of polyphosphates has so far been isolated the salts of triphosphoric acid with the anion structure (V).There is no reason to doubt that an indefinitely large number are capable of existence and that in the future others will be isolated as chemical individuals. For example a linear sodium tetraphosphate could no doubt be isolated by cautious hydrolysis of Na,P,O, in alkaline solution. Sodium triphosphate Na,P,O1, has become an important commercial chemical because of excellent detergent effects when it is used as an auxiliary washing agent with soap or sulphonates. F. Schwarz 3* observed that when a melt of composition 5Na20 3P20 is cooled slowly small crystals of Na,P,07 form in the liquid; neverthe- less the compound Na5P,0, can be prepared in the form of a hydrate by dissolving and crystallising the cold mass.K. R. Andress and K. Wust 39 confirmed this and from X-ray powder diagrams deduced the existence of two crystal forms of sodium triphosphate. Partridge Hicks and Smith lo and Morey and Ingerson l1 made a thorough investigation of the prepara- tion of sodium triphosphate by the thermal method. No evidence was found of the formation as a pure phase of any other polyphosphate from melts covering the whole range Na20 P20 from 1 1 to 2 1. 37 0. L a m and H. Mrtlmgren 2. anorg. Chein. 1940 245 103 ; 0. Lamm Chem a8 2. anorg. Chem. 1895 9 249. Abs. 1945 39 3716. aB Ibid. 1938 237 113. TOPLEY THE CONDENSED PHOSPHATES 361 The behaviour of a melt of composition 5Na20:3P205 on cooling is interesting. At about 860" crystals of Na4P207 are present in the fluid system and with further cooling there is formed a thick suspension of small crystals of Na4P207 in a liquid increasingly rich in P205 relative to Na20 until a t 622" the two-phase system contains approximately 44% of solid Na4P20,.At this temperature Na,P,O, appears as a third phase. Further removal of heat if slow enough to maintain equilibrium would result in isothermal conversion of the Na,P,O into Na5P301 with oom- plete disappearance of the liquid phase. The reaction between the Na4P20 crystals and the molten phase is slow ; a considerable amount of Na,P,O, is formed and the system becomes rigid in the form of an opaque vitreous mass containing besides the high-temperature form usually denoted by Na5P,010-I Na,P20 and glassy material rich in P,O,. If further cooling to about 450" is slow enough conversion into Na5P,010-I continues almost to completion.With still further cooling the solid mass breaks up into a fine powder usually between 250" and 150". It is probable that this spontaneous disintegration is caused by partial transformation into Na5P,0,,-11 the form stable at room temperature. Partridge Hicks and Smith l o observed that a change to phase I1 in the X-ray powder diagram accompanies the disintegration but it must be remarked that sometimes the disintegration occurs whilst the X-ray diagram remains predominantly that of Na5P,010-I.14 Reversal of the polymorphic change occurs at 515" on the evidence of heating curves but the temperature of reversible trans- formation is unknown. Na,P,Ol,-I when obtained by heating the disinte- grated form of Na5P,010-II does not readily revert to phase I1 on cooling again.This suggests that the polymorphic change is of the type in which the rate of propagation of the phase interface is high but the probability of spon- taneous nucleus formation for phase I1 is low a conclusion supported by observation of the transformation I -+ I1 as it occurs in a large block of slowly cooled melt. When the phase I is in a finely divided form growth of the second phase from each nucleus as it forms may be limited by the dimensions of the particle. Sodium triphosphate is not usually manufactured by a process involving fusion and slow cooling but by the reaction 2Na,HPO + NaH,PO = Na,P,OI + 2H20 The two orthophosphates are intimately mixed (e.g. by rapid drying of the appropriate solution in a spray drier or on a rotating drum drier or by heating the hydrated salts together so that they first fuse in their water of crystallisation) and the mixture is heated usually in a rotary calciner.The polyphosphate product containing well over 90% of Na5P,OlO is formed even a t as low a temperature as 300" and provided a temperature of about 400" is not exceeded it is chiefly in the form Na5P,0,,-11. At 500-550" there is a considerable proportion of phase I. The hydrate Na,P30,,,6H,O obtained when either form of Na,P,O, is exposed to the atmosphere or dissolved and crystallised has a remarkably low dissociation pressure. If much of the water is pumped off in vacuum, 362 QUBRTERLY REVIEWS even below 100" anhydrous triphosphate is not the product but a mixture containing ortho- and pyro-phosphate.At higher temperatures reconver- sion occurs into Na,P,O,,-I or -11 according to the temperature. There is some evidencels that an octahydrate which readily passes into the hexa- hydrate can be obtained by vacuum evaporation below 20". The solu- bility of Na,P3Olo,6H2O is approximately 15% of Na,P,O, a t 20° and 16% at 40". The phase I material is converted into hydrate almost immediately on contact with water but the phase I1 can form a solution almost twofold supersaturated with respect to the stable hydrate from which the latter crystallises comparatively slowly. The difference is practically important phase I being unsuitable for use in some chemical plant because of separation of granular slowly dissolving hexahydrate. Since phase I must potentially have a higher solubility than phase I1 with respect to which it is metastable the explanation may be that local supersaturation with respect to the hydrate immediately round the particles of phase I is high enough to force spontaneous nucleation for crystallisation of the hydrate ; or it may be that the lattice of phase I is itself able to initiate the growth of hydrate crystals.There is no evidence to support the suggestion that either phase I or phase I1 is a polymer of Na,P,O,,. The most convenient method for the preparation of a pure specimen of Na,P,0,,,6H20 is hydrolysis of Na,P,O,. If a concentrated solution of the latter is mixed with an excess of sodium hydroxide solution a large crop of Na,P30,,,6H,0 crystallises after some hours.40 Acids hydrolyse trimetaphosphate more rapidly than bases but steps must then be taken to protect the triphosphate anion from further hydrolysis-e.g.by having excess of Zn" present in the acid solution whereby the well-crystallised salt NaZn2P,01,,9H,0 is precipitated. The structural formula (XIII) for the anion of sodium triphosphate represents the substance well within the limits of any single representation for the following reasons (XIII.) O; 0- i5- [-i:+-*-P OS -0-P 0- 0- (1) It provides a natural explanation of the stepwise alkaline hydrolysis of cyclic trimetaphosphate through triphosphate to an equimolecular mixture of pyrophosphate and orthophosphate Na3P30 + Na,P30, + Na,P,O + Na3P0 (2) Neither the anhydrous compound nor the hexahydrate is a " mole- cular compound " (Na,PO + $Na,P,O,) or (Na,P20 + &Na,P,O,) this possibility being excluded 16 by comparison with the behaviour on crystal- lisation of solutions containing the relevant proportions of Na,P,O with ortho- or pyro-phosphate and by absence of precipitation reactions charac- teristic of the last two.(3) Cryoscopic measurements in sodium sulphate decahydrate ** gave 372 for the molecular weight (Na,P,O, = 368). 40 W. Faber D.R.-P. 734,611 (1943) ; G. B. Hatch U.S.P. 2,366,910 (1944). TOPLEY THE CONDENSED PHOSPHATES 363 (4) The electrometric titration curve 4 1 of sodium triphosphate shows that the corresponding acid has for every three phosphorus atoms three rather strongly acidic protons and two much less strongly acidic; this is consistent with the formal charge distribution of the anion structure (XIII). (5) Alternative formulation to (V) is not possible while retaining 4- co-ordinate phosphorus.Complex-ion Formation.-Bi- and ter-valent metal ions are precipitated by alkali-metal polyphosphates and pyrophosphates and in most instances the precipitates dissolve in excess of the reagent even when the system is very dilute. This manifestation of complex-ion formation is found also with the colloidal solutions obtained from NaP0,-IV (Kurrol salt) NaP0,-I1 (Maddrell salt) and NaP0,-I11 by the action of non-sodium alkali-metal ions and with analogous solutions obtained from other " insoluble " alkali- metal and ammonium polymetaphosphates and with solutions of meta- phosphate glasses and glasses of polyphosphate composition. The cyclic tri- and tetra-metaphosphate anions particularly the former form much less stable complexes with metal cations.This marked tendency to form complexes has been recognised (especially with the pyrophosphate ion) for more than a century but the composition of the complexes and the nature of the forces responsible for their stability in solution have been little studied. L. B. Rogers and C. A. Reynolds 42 demonstrated by conductimetric electrometric and polarographic measure- ments the formation in dilute solution (10-3-10-4~.) of the complex ions (M"P,O,)" by Cd Co Cu Pb Mg Ni Zn and of (M"P207)' by A1 and Fe. Calcium and strontium ions usually less prone to complex formation than the metals of variable valency nevertheless show qualitatively the same behaviour as the latter when treated with a solution of sodium meta- phosphate. This was observed by Tammann 25 for a polymetaphosphate thought to be hexametaphosphate but actually of unknown molecular weight.Tammann likened the complexes between metal ions and meta- phosphate to such complex anions as Fe(CN),"" and PtC1,". A significant feature however of the complex-anion formation by condensed phosphates is the generality of the phenomenon-most if not all bi- and ter-valent cations form rather stable complexes with all condensed phosphate anions except the cyclic tri- and tetra-metaphosphates (and with the last the interaction is considerable). The practical usefulness of the power to com- bine with Ca" and Mg*' was not realised until R. E. Hall,43 some forty years after Tammann's observations saw in glassy sodium metaphosphate a means of softening natural waters without producing precipitates.From the time of this patent the calcium complexes with condensed phosphates have been studied mostly for their applications. Much of the work is only briefly described in patents or journals of applied science although some experimental work has been more fully reported by H. Rudy H. Schloesser and R. Watzel 44 and by Rudy 45 and 0. Stelling and G. F ~ - a n g . ~ ~ 41 Rudy and Schloesser ref. (20). 4 3 U.S.P. 1,966,515 (1934). 44 Angew. Chem. 1940 58 625. 42 J . Amer. Chem. SOC. 1949 71 2081. 46 Svemk Kern. Ti&. 1941 68 270. Ibi&. 1941 54 447. 364 QUARTERLY REVIEWS The calcium complexes are sufficiently stable for rather insoluble salts such as CaCO, CaC,04,H,0 and calcium soaps to be dissolved by solutions of condensed phosphates of alkali metals.For example,32 a solution con- taining 0.01634 mol./l. of Na5P3OlO (held near to pH 9-5 by addition of a little sodium hydroxide) is in equilibrium with CaC,O,,H,O at 25" when it has dissolved 0.00732 mol./l. The solubility product of CaC,O,,H,O is 2.3 x 10-9 so the concentration of free Ca ion is about 3-1 x 10-7 mol./l. in the triphosphate solution and only a fraction 4.3 x 10-5 of the total calcium is uncombined. It being assumed that the complex is CaP3Ol0"' the ionic equilibrium concentration product ~Ca"][P30105-]/[CaP,010'"] equals 3.9 x lo-' mol./l. When the highly condensed glassy metaphos- phates or solutions of NaP0,-IV (Kurrol salt) or of NaP0,-I1 (Maddrell salt) or NaP03-111 or analogous salts of other alkali metals are used instead of Na5P,010 anionic complexes are produced in which one Ca" ion is on the average associated with 5-6 PO,' groups.The stability of the ion association involved is comparable with that of CaP3Ol0"'. When Ca" is replaced by Mg" or Zn" (the latter is conveniently studied electro- metrically as well as by solubility of ZnC,O,) the results are qualitatively similar. The complex formation is always manifested except when some extremely insoluble phase intervenes as for example when the condensed phosphate employed itself forms a particularly insoluble salt with the bivalent metal e.g. barium pyrophosphate. For the calcium complex with the cyclic tetrametaphosphate ion assumed to be CaP4010" Taylor,l4 using calcium oxalate and Davies and Monk,ls using calcium iodate found K w 3 x 10-5 and 1.3 x 10-5 respec- tively; the latter is probably the better value.For the cyclic trimeta- phosphate complex assumed to be CaP30,' these authors found 8 x (by oxalate solubility) and 3.3 x lo- (by iodate solubility) and again Davies and Monk's value is probably the better. It seems very probable that ion association as interpreted by the Bjerrurn theory is a sufficient explanation of these complexes. The polymetaphos- phates (other than the cyclic compounds) and the polyphosphates are flexible structures carrying a number of negative charges and it may be supposed that the flexibility of the -0-P-O-P- chain allows a cation M " to draw on to itself in an energetically favourable way the total negative charge of five or six -OP03' groups. Terminal PO," groups so far as they are present would be even more effective.Recalling that even for bi- valent sulphates 47 such as Ca"S0," and Zn"S0," the dissociation con- stant R M 5 x 10-3 mol./l. it is qualitatively clear that when the effective negative charge is increased several fold (the -0-PO3' or PO," groups being not seriously different in size from SO,") the appearance of values of K as low as 10-7 mol./l. is to be expected. Similarly consideration of the dissociation constant 48 for the complex ion NaP20,"' K = 4.5 x makes the stability of the complex pyrophosphate ions M"P2O," seem natural. 47 R. W. Money and C. W. Davies Trans. Faraday SOC. 1932 28 609 ; C. W Davies J . 1938 2093. C. B. Monk J . 1949 423. TOPLEY THE CONDENSED PHOSPHATES 365 When a polymetaphosphate or a polyphosphate in which the proportion of terminal PO," groups is small binds a cation it is not to be expected that the pH of the solution (as long as it is above about 5 ) will have much effect upon the extent of formation of the complex since competition by protons for the negative charge of the -0P0,' groups will be ineffective.With pyrophosphate short-chain polyphosphates or structures such as (IX) competition for the charge of the PO," groups is to be looked for below about pH 10. In practice a large effect of pH is found with pyrophos- phate triphosphate and the commercial glasses in which Na,O/P,O > 1. On the other hand using a metaphosphate glass prepared in dry air and having Na20/P,05 = 1.00 Taylor 1 has found electrometrically for the Zn" complex that there is practically no effect on the ion association of pH in the range 4-9-7.7 and with the same metaphosphate dissolving calcium oxalate to equilibrium only a very slight effect of pH was found in the range 6.0-7.8.A solution of this glass gave a titration curve with a single inflexion at pH 7. Consideration of the values of pK and pK4 for pyrophosphoric acid and pK and pK for triphosphoric acid relative to K for the calcium complexes shows that in the pH range 7-10 partial ejection of protons by M** ions is to be expected and in fact the pH of a sodium polyphosphate solution in this range falls when bivalent cations are added. Hydrolytic Instability.-In aqueous solution all condensed phosphates acids salts and esters hydrolyse at the P-0-P linkages complete stability being attained only with ultimate conversion into orthophosphate. The hydrolysis is acid-base catalysed.There is a large variation in the ratio of the velocity constants k,/koH as the intramolecular environment of the P-0-P linkage is varied. For example,l4 with tetraethyl pyrophosphate hydrolysing to diethyl orthophosphate k H / k o H < 10-6 at 25" whereas for sodium metaphosphate glass (Na,0/P205 = 1) hydrolysing to a series of pro- ducts including pyrophosphate and orthophosphate k,/ko > at 100". Several papers have appeared in the past fifty years which provide frag- mentary information on the rates of hydrolysis of condensed phosphoric acids and their salts but scarcely a beginning has yet been made with quantitative measurements. The problem is complicated by the number of ionic species which have to be taken into account in the consecutive reactions even with a relatively simple substance such as Na,P,O,, or with a definite metaphosphate such as Na,P,O,, and analytical difficulties have so far prevented an effective study.The papers of R. N. Bell 49 and R. Watzel 50 contain rates of hydrolysis which are useful for practical applications. In alkaline solutions (pH > 11) the pyrophosphate and triphosphate anions particularly the former are very much more resistant to hydro- lysis than P4012"" P309"' and the anions of more complex metaphosphates and polyphosphates. This may be due in part at any rate to the strength of the electric field in the proximity of the terminal double charge which 4u I d . Eng. Ckem. 1947 39 136. Die Chemie 1942 55 366. 366 QUARTERLY REVIEWS repels the OH' ion. It is not unexpected to f h d that the reduction of the local negative field of a portion of the anion resulting from association with M" to form an ion pair considerably accelerates hydrolysis by OH'.For example,14 addition of 5 x lo-* mol. of MgSO to 1 litre of a solution containing 7.8 x lo- mol. of potassium polymetaphosphate (calculated as KPO,) dissolved in dilute sodium carbonate increased the rate of hydro- lysis at 100" nearly ten-fold. Presumably so long as there is sufficient metaphosphate left to re-associate bivalent cation liberated by the hydro- lysis the de-stabilising action continues. On the other hand small additions of Mg" or Ca" would not be expected to accelerate hydrolysis in acid solution when the hydrolytic catalyst is H,O'. In confirmation addition of 5 x mol. of KPO dissolved in dilute NaH,PO did not alter the rate of hydrolysis.The nature and the molecular-weight distribution of the intermediate species produced during hydrolysis of the more highly condensed polymeta- phosphates e.g. NaP0,-IV (Kurrol salt) is an interesting matter await- ing investigation. Bell 49 found that when a metaphosphate glass with Na,O/P,O = 1 is allowed to hydrolyse in dilute solution at 70" or 100" the products are orthophosphate and trimetaphosphate in the molecular proportions 6 to 1 and concluded that part of the polymetaphosphate is hydrolysed part depolymerised. It is difficult to see a possible mechan- ism for depolymerisation in aqueous solution except hydrolytic fission of P-0-P linkages and difficult to see how the cyclic trimetaphosphate structure can arise during hydrolysis unless pre-existent as part of a more complex structure.The structure (XIV) represents a hypothetical anion mol. of CaCl to 1 litre of a solution containing 7.8 x 1 9 -0 0- r -op+o- -op+o- (XIV.) (Po,)gg- which might be expected to hydrolyse comparatively readily to 6NaH,PO + Na,P@g. The metaphosphate glass used by Bell presumably had a titration curve with a single inflexion at pH 7. In structure (XIV) each P+ atom in the ring is linked to two OPO," groups ; if this structure is to represent the dissolved metaphosphate it is necessary to suppose that the formal positive charges on the three Pf atoms involved is suffi- cient to raise to a pK of about 4 the acidity of the first hydrogen atom TOPLEY THE CONDENSED PHOSPHATES 367 bound. In the undissociated acid one of each pair of phosphate groups attached to the ring becomes O(PO)(OH) and the other becomes O(P0,)OH.The infringement of the rule of microscopic neutrality has already been referred to in connection with structure (IX) ; there does not seem to be any reason to reject (XIV) as impossible. Surface Eff ects.-It is not surprising that condensed phosphates are strongly adsorbed on many surfaces in view of the multiple negative charges carried by a single kinetic unit. Some resulting phenomena are both interesting and practically important. At concentrations in the range O . l - l . O ~ o by weight polymetaphos- phates (other than the cyclic tetra- and tri-metaphosphates) and poly- phosphates and to some extent pyrophosphates exert a strong dispersive effect upon suspensions of finely divided solids such as clay.The rapidly growing use of condensed phosphates especially sodium triphosphate in conjunction with synthetic detergents for washing is partly attributable to this property. The growth of crystalline precipitates from some supersaturated salt solutions is delayed or indefinitely prevented by very low concentrations of condensed phosphates. Calcite is a notable example much studied because of its application to the treatment of raw water t o reduce scale deposition. The effect was apparently first noticed by R. F. Reitemeier and A. D. Ayers 51 in 1935 and first described by L. R~senstein.~~ Con- centrations of glassy metaphosphate as low as 1-5 x 10-5~. (in terms of NaPO,) will inhibit precipitation of calcite in solutions very supersaturated with respect to CaCO, initially containing low3 mol.of Ca(HCO,) per litre to which excess of ammonia is added or from which carbon dioxide is withdrawn by boiling or by bubbling air. All soluble condensed phos- phates except the cyclic tri- and tetra-metaphosphates but including pyro- phosphates are effective inhibitors for calcite precipitation. As pointed out by G. B. Hatch and 0. Rice,53 it is probable that nuclei of calcite become covered by adsorbed metaphosphate and further growth is pre- vented. Other suggestions have been disproved by appropriate measure- ments on the solutions the calcium carbonate does not go into a colloidal state stabilised by the metaphosphate ; 54 the activity of Ca" in the super- saturated solution is not appreciably reduced 51 by the minute concentration of metaphosphate ; the phenomenon does not depend 5 5 upon any effect of the metaphosphate upon the rate of the conversion of HCO,- --+ GO3=.A study of the special case of calcium carbonate precipitation as affected by metaphosphate and pyrophosphate has been made.5*9 55 It was found that the inhibitory effect at about l O W 5 ~ . is " a highly specific characteristic of the inorganic salts containing phosphorus in the pentavalent form ". The authors suggested as a possible explanation " Since complex formation 51 J . Arner. Chem. Soc. 1947 69 2759. 5 2 U.S.P. 2,038,316 (1936) re-issue 1937 1938. 53 Ind. Eng. Chem. 1939 31 51. 6 4 T. F. Buehrer and R. F. Reitemeier J . Physical Chem. 1940 44 552. 65 R. F. Reitemeier and T. F. Buehrer ibid. p. 535. BB 368 QUARTERLY REVUEWS occurs to a negligible extent we must conclude that the action is largely an indirect one involving either a stable electrostatic attraction between calcium and metaphosphate ion or a marked decrease in the activity of calcium ion due to the presence of the metaphosphate." The second alternative was later eliminated by Reitemeier and A y e r ~ .~ ~ There has been some discussion as to the stage in the precipitation most affected by the condensed phosphate whether it is the " nucleus " stage or the sub- sequent growth of crystals of calcite. B. Raistrick 56 has drawn attention to the fact that the size of the repeating unit in a polymetaphosphate chain relative to the spacing of the Ca" ions in a layer of the calcite lattice is such that the chain can lie without strain on top of the latter so that consecutive centres of negative charge occupy positions centrally above adjoining triangles of doubly charged Ca** ions; it is suggested that this favourable coincidence causes very strong adsorption.Buehrer and Reitemeier 64 found only 1 atom of P to 300 atoms of Ca in calcite pre- cipitated in the presence of glassy sodium metaphosphate at a concentration just below the " threshold " value of 1-5 x 10-5~-NaP0, above which precipitation is virtually stopped ; the crystals are retarded in growth and distorted but give the characteristic X-ray powder diagram of calcite. In connection with this absence of appreciable solid-solution formation Raistrick56 points out that when a layer of singly charged metaphosphate groups covers a layer of Ca" ions there will be a rough approximation to electrical neutrality just outside the adsorbed metaphosphate since the double positive charge of the Ca" ions is offset by their greater distance.This also makes it easier to understand the growth inhibition since there will be no strong electrostatic force to retain a new layer of Ca" outside the adsorbed metaphosphate. It must be remarked that Reitemeier and Ayers 61 have shown that glassy metaphosphate a t 1-5 x 10-6~. concentration (with respect to NaPO,) will also stabilise a supersaturated solution of calcium sulphate against crystallisation as dihydrate ; this and also the fact that pyro- phosphate in alkaline solution is no less effective than glassy metaphosphate (although the adsorbable anion is then carrying a double charge at each end) are points requiring further explanation. 56 Faraday Society Bristol Discussion on Crystal Growth 1949.

ISSN:0009-2681    

DOI:10.1039/QR9490300345

出版商:RSC    

年代:1949

数据来源: RSC

摘要:

CARBOHYDRATE SULPHATES By E. G. V. PERCIVAL D.Sc. PH.D. F.R.I.C. (READER IN CHEMISTRY UNIVERSITY OF EDINBURGH) IT is not always realised how widespread is the occurrence of carbohydrate sulphates in Nature. In the plant kingdom many algal polysaccharides belong to this group the red seaweeds Chondrus crispus and Gigartina steZhta contain the carragheen polysaccharides Dilsea edulis and Iridez Zaminarioides contain galactan sulphates and the important polysaccharide agar-agar which occurs in many species of Bhdophycea is either a member or a close relative. In the still more common brown seaweeds (Pkophycea?) the polysaccharide sulphate fucoidin is found. Still greater interest is attached to the animal carbohydrate sulphates which include the chondroitin sulphate of cartilaginous tissue the mucoitin sulphate of the gastric mucosa which is probably identical with the hyalu- ronic acid sulphate of the cornea and the blood-anticoagulant heparin.The jelly coat of sea-urchin eggs contains polysaccharide sulphates,l and galactose sulphates have been reported in brain lipids. The functions of the compounds under review are not always well understood ; in the red algB the polygalactose sulphates usually make up the major part of the plant but this is not so with the fucoidin of the brown seaweeds where the principal polysaccharide is alginic acid and considerable amounts of laminarin and mannitol occur at the proper seasons. In the animal group the main purpose of heparin is clear and chon- droitin sulphate is believed to act as a cementing material in connective tissue probably as a gel-like network in association with protein.2 The animal carbohydrate sulphates in loose combination as salts with protein^,^ as in the mucins derived from mucoitin sulphate form viscous slimy layers with lubricating and protective properties ; furthermore by an inhibiting action on the action of trypsin and pepsin autodigestion of the gastric mucosa is prevented.Quite apart from their complexity as carbohydrates it is not surprising that our knowledge of the constitution of the natural carbohydrate sul- phates is scanty when one considers their distinctive physical properties such as solubility in water to form viscous solutions in some cases and gels in others. They also occur as mixtures of salts of several different metals and especially in the animal group are difficult to separate from proteins.The only means of purification are usually dialysis or electro- dialysis and fractional precipitation. Progress has been delayed too by an inadequate knowledge of the properties and reactions of simple mono- saccharide sulphat es. E. Vasseur Acta Chem. Scad. 1948 2 900. a G. Blix Biochem. J . 1946 40 vi. =Karl Meyer Cold Spring8 H w b w Symp. &w;cnt. BWl. 1938 6 91. 369 370 QUARTERLY REVIEWS Monosaccharide Sulphates Monosaccharide sulphates are usually prepared by treating the sugar derivative dissolved in pyridine with chlorosulphonic acid,4 sulphuryl ~hloride,~ or preferably,s with the pyridine-sulphur trioxide complex.' By choosing appropriate derivatives it is possible to prepare sulphates usually as the barium or brucine salts with this group in a definite posi- tion in a particular sugar molecule and to compare the properties of such products with those of the sulphates prepared directly from the parent sugar.Thus H. Ohle by the partial hydrolysis of 1 2-5 6-diisopropylidene glucose-3 sulphate obtained 1 2-isopropylidene glucofuranose-3 sulphate which differed from the monosulphate obtained by the direct sulphation of 1 2-isopropylidene glucofuranose. Removal of the isopropylidene group from the last -mentioned substance gave the glucose monosulphate which is obtained directly from glucose ; it is inferred therefore that the hydroxyl group on C(s) was sulphated in the last two preparations. On the other hand galactose-6 sulphate prepared through 1 2-3 4-diisopropylidene galactopyranose was shown to differ from the galactose sulphate prepared directly from galactose.Where more than one free hydroxyl group exists owing to the vigorous nature of the sulphating agents generally used and the difficulties of isolating crystalline derivatives the possibilities of the formation of mixtures of monosulphates and of polysulphates must be borne in mind so that it is necessary to use the direct method with caution. In any event the synthetic method of attack is only of limited application and other methods have been sought in attempts to answer the outstanding question of the location of the sulphate groups. It has been claimed lo that hydrolysis by acids may be used to distin- guish poly- from mono-sulphates since the former are hydrolysed more rapidly but differences in the rate of hydrolysis of selected monosaccharide sulphates with acid reagents 59 9 9 10 have been shown to be too small t o act as a guide More useful although not by any means always decisive is a study of the behaviour of the unknown sulphate under alkaline conditions.The Eydrolysis of Carbohydrate Sulphates with AlkaU.-A salt such as sodium methyl sulphate is relatively stable to hot aqueous alkali so that the observation 9 that diisopropylidene galactose 6-sulphate was not attacked by sodium hydroxide solution ( 2 ~ . ) during 6 hours at 100' is hardly sur- prising. On the other hand the glucose and galactose sulphates prepared directly from the free sugars yielded all the sulphate in ionisable form within five minutes at 100" in N/lO-sodium hydroxide but since disruption of the monosaccharide residues occurred this observation is of no diagnostic value.When a-methylglucopyranoside sulphate was examined heating 4C. Neuberg and L. Liebermarm Biochem. Z . 1921 121 326. 5 P. A. Levene and G. M. Meyer J . BioZ. Chem. 1922 53 437. 6 R. B. Duff J. 1949 1597. 8 Biochem. Z . 1922 131 601 ; 1923 136 428. * E. G. V. Percival and T. H. Soutar J. 1940 1475. S O T . Soda and W. Nagai J . Chem. SOC. Japan 1936 56 1268. P. Baumgarten Ber. 1926 59 1166 1977. PERCNAL CARBOHYDRATE SULPHATES 371 with alkali caused rapid hydrolysis of the sulphate group with the produc- tion of an anhydromethylhexoside and on further investigation l1 it was established that the sulphates of a- and p-methyl-glucopyranosides and -galactopyranosides and a-methylmannopyranoside gave the corresponding 3 6-anhydromethylhexosides.It was concluded therefore that unless the removal of the sulphate group can lead to the production of an anhydro- ring for which a suitably placed hydroxyl group is necessary hydrolysis of the sulphate group with alkali proceeds only slowly. E'urther evidence in support of this view was collected in the glucofuranose series.12 The hydrolysis of 1 2-isopropylidene glucofuranose-3 sulphate (I) with alkali was found to be exceedingly slow-in fact just as slow as for 1 2-5 6- diisopropylidene glucose-3 sulphate (12% hydrolysis in 47 hours at 100" in 2*8~-sodium hydroxide). On the other hand the corresponding 6-sulphate (11) was readily hydrolysed to give almost equal amounts of 1 2-isopropyl- H O-CMe 1 1 O-CMe (I.) m.) (II 1 idene 3 6-anhydroglucofuranose (111) and 1 2-isopropyfidene gluco- furanose (IV).Leaving aside for the moment the fact that the primary alcoholic group in (I) is apparently suitably placed for 3 6-anhydride formation (3 6- anhydro-derivatives have been isolated from methylglucofuranoside-3 sul- phates l2) it is necessary here to draw attention to the close parallel between the behaviour of sulphates and of such sulphonic esters as toluene-p-sul- phonates and methanesulphonates. Thus diisopropylidene galactopyranose 6-toluene-p-sulphonate diisopropylidene glucofuranose 3-toluene-p-sulphon- ate,l3 and 1 2-isopropylidene glucofuranose 3-toluene-p-sulphonate 1 4 are hydrolysed with comparative difficulty by alkali and no 3 6-anhydride is produced from the last substance. On the other hand methylhexoside 6-toluene-p-sulphonates are readily converted into the 3 6-anhydromethyl- hexosides,15 and a knowledge of the properties of these compounds was of the first importance in developing the subject under discussion.l1 Duff and Percival J . 1941 830. l3 J. W. H. Oldham and G. J. Robertson J . 1935 885. l* Ohle and H. Wilcke Ber. 1938 71 2316. l5 See S. Peat " Advances in Carbohydrate Chemistry " Academic Press New l2 Percival J . 1945 119. York 1946 vol. 2 p. 37. 372 QUARTERLY REVIEWS There is however a property of the sulphonic esters of even greater fundamental importance which is observed when the sulphonic ester group is adjacent to a free hydroxyl group and in trans-relationship to it. Treat- ment of such compounds with an alkaline reagent usually sodium methoxide causes an ethylene oxide ring to appear between the carbon atoms concerned with Walden inversion on the atom which carried the sulphonic ester residue.To give but one of many examples 3 4 6-triacetyl /&methyl- glucoside 2-toluene-p-sulphonate (V) on treatment with sodium methoxide CH,*OAc CH,*OH C H,-OMe {-'FMeNaOMe OAc H '4TgMe A920 Me1 $kqMe AcO H HO H Me0 6 .I H H OTs H H H H (n) (md (rn) JNaOMe C H,.O Me I C H . 0 Me Me0 OMe H + gives 2 3-anhydro-~-methylmannopyranoside (VI) 17 The dimethyl derivative (VII) of (VI) by the further action of sodium methoxide gives 2 4 6-trimethyl /?-methylglucoside (VIII) and 3 4 6-trimethyl ,&methyl- altroside (IX) in practically equal am0unts.l' At the point of entry of the new methyl groups Walden inversion takes place and the formation of two products shows that the ethylene oxide ring may be broken in either of the two ways indicated by the dotted line in (VII).It should CH,-OH $H,*OH 4-F. H OH (X) ( X I ) CUI) be pointed out however that two products cannot invariably be recog- nised in such a reaction the chances of the ethylene oxide ring breaking in a particular way depending presumably on steric factors. If sodium hydroxide instead of sodium methoxide acts on (VII) a mixture of 4 6- dimethyl p-methyl-glucoside and -altroside is produced. When the sul- phonic ester group is cis- to the neighbouring hydroxyl group no anhydride l6 W. N. Raworth E. L. Hirst and L. Panizzon J . 1934 154. 17 W. H. G. Lake and Peat J. 1938 1417. PERCIVAL CARBOHYDRATE SULPHATES 373 formation takes place and hydrolysis of the ester is comparatively difficult.Thus 4-methanesulphonyl b-methylgalactoside (X) is unaffected by sodium methoxide under conditions where the corresponding glucoside (XI) is readily converted into 3 4-anhydro-~-methylgalactoside (XII).18 With such examples in mind it became of obvious interest to determine whether carbohydrate sulphates could also take part in transformations cm cxm (xp) involving the intermediate formation of ethylene oxide rings especially as the interconversion of sugars in Nature might in aome cases depend on such processes. A superficial examination of the examples of the hydrolysis of sulphates already cited would appear to indicate that 3 6-anhydro-rings but not ethylene oxide rings were formed. There is evidence however that the formation of a 3 6-anhydride is in some cases preceded by the production N a0 Me 9 0' of an ethylene oxide derivative.Thus E. Seebeck A. Meyer and T. Reich- stein l9 showed that 1 2-isopropylidene 5 6-anhydroglucofuranose (XIV) prepared from the corresponding 6-toluene-p-sulphonate (XIII) under- went a transformation into the more stable 3 6-anhydride (XV) even on storage in a desiccator and that contrary to an earlier claim,20 no L-idose derivatives could be obtained on the fission of (XIV) with alkaline reagents. Clearly therefore the observation that alkaline hydrolysis of the corresponding sulphate gave (XV) could be interpreted in a similar fashion. In order to arrive at a decision barium 3-methyl 1 2-isopropyl- idene glucofuranose-6 sulphate (XVI) was prepared. The inclusion of a methyl residue a t C(3) not only prevented the formation of a 3 6-anhydride but also conferred sufficient solubility in methanol to permit the use of l8 A.Miiller M. Moricz and. G. Verner Ber. 1939 72 746. ID Helv. C h h . Acta 1944 27 1142. 2o Ohle and L. von Vaxgb Ber. 1929 62 2435. 374 QUARTERLY REVIEWS sodium methoxide as a hydrolytic agent; normally the barium salts of sulphate esters are soluble only in aqueous media. Treatment with sodium methoxide at 40" gave 3-methyl 1 2-isopropylidene 5 6-anhydrogluco- furanose (XVII) (50% yield) and by more vigorous treatment 3 6-dimethyl 1 2-isopropylidene glucofuranose (XVIII) was obtained. The production of (XVIII) is explained by the fact that the entering methoxyl anion attaches itself apparently exclusively to the primary carbon atom in an ethylene oxide derivative of this type.22 Although the experiment outlined proves beyond doubt that the hydro- lysis of a sulphate ester can give an ethylene oxide derivative because the sulphate group was not attached to an asymmetric centre and the fission of the ethylene oxide ring takes place in only one direction the Walden inversion so characteristic of the hydrolysis of sulphonic esters could not be observed.An attempt to settle this point by the hydrolysis of 6-methyl @-rnethylgalactopyranoside-Z sulphate 21 failed because of extensive decom- position with the production of reducing substances on treatment with sodium methoxide the glycosidic methoxyl group apparently becoming labile to alkali when a sulphate group is adjacent. By taking advantage of the well-known stability to alkali of 1 6-anhydrides this difficulty has been overcome and the analogy between the sulphate and sulphonic esters is now complete.1 6-Anhydro-~-~-galactopyranose-2 sulphate (XIX) was converted by means of sodium methoxide into the known 1 6-2 3-dianhydro-p-~-talopyranose (XX). CH,-O H *so (xnc.) (XXS The failure to isolate an anhydro-derivative from 1 2-isopropylidene glucofuranose-3 sulphate (I) mentioned on page 370 can now be explained to some extent. There is no hydroxyl group available for the production of an ethylene oxide derivative but one occurs in the most nearly compar- able case of the methylglucofuranoside-3 sulphate (ap-mixture) which was shown l2 to give 3 6-anhydromethylglucofuranosides together with methyl- glucofuranosides on hydrolysis with alkali.The toluene-p-sulphonates furnish a similar pair of examples,*4 for 3-toluene-p-sulphonyl 1 2-480- propylidene glucofuranose gives no anhydro-derivative with alkali whereas 5 6-dibenzoyl 3-toluene-p-sulphonyl 2-acetyl ,!I-methylglucofuranoside (XXI) gives 5 6-dibenzoyl 2 3-anhydro-~-methylallofuranoside (XXII) on treatment with alkali in aqueous acetone and this with boiling sodium hydroxide ( 2 ~ . ) produces 3 6-anhydro-~-methylgalactofuranoside (XXIII). 2 1 Duff and Percival J. 1947 1675. 22 Ohle and K. Tsssmar Ber. 1938 71 1843. PERCIVAL CARBOHYDRATE SULPHATES 375 It will be clear from the above that the isolation of a particular 3 6- anhydro-derivative by the hydrolysis of an ethereal sulphate does not of itself establish at once the position of the original sulphate group and it ___ H OAc H OH (Xxf.1 c!cxII) is not always an easy task to identify small quantities of the isomers which may have been formed by the fission of any ethylene oxide rings produced a t an intermediate stage.It is possible to say however that if the sul- phate group is readily hydrolysed with alkali then an adjacent trans- hydroxyl group is present or that the sulphate group is on (&) (in a hexose) and that a free hydroxyl group is present at C(5) or C(,) or both. If the sulphate group is stable to alkali then none of these conditions applies and by taking into account also the results of methylation experiments etc. it is sometimes possible to deduce the position of the sulphate group in a polysaccharide ethereal sulphate. Natural Carbohydrate Sulphates The representation of an unknown substance as a sulphuric ester may be deduced from the fact that while no sulphate ions can be detected in solution until after hydrolysis cations such as calcium are detectable e.g.by precipitation with ammonium oxalate. Another important check 23 is the fact that the amount of sulphate estimated in the whole polysaccharide either by hydrolysis or by fusion with sodium peroxide is double the amount found in the ash e.g. 2RO*SO,K -+ K,SO + SO + products of combus- tion. In practice this 2 1 ratio is seldom realised because of the reduction of sulphate to sulphide during the ashing process. Although the complete structure of a natural polysaccharide sulphate has in no case been worked out fully some progress has been recorded in a few instances.Some of the difficulties involved in the isolation and purification of these sulphates have been mentioned already. For experi- mental purposes they are only stable as salts since the free acids are rela- tively strong acids. Since unlike alginic acid the acids are soluble in water there is no simple method of isolating the free acid apart from dialysis against mineral acids or of converting them from the naturally- occurring mixtures of salts into the salts of a single cation. None of the salts is insoluble so the facile double decomposition reactions like the conversion of sodium into calcium alginate cannot be applied as a means of isolation and purification. A further handicap in structural investiga- tions lies in the difficulty of acetylation and of methylation-the former because of the presence of an inorganic ion attached to the sulphate group 23 P.Haas Biochem. J. 1921 15 469. 376 QUARTERLY REVIEWS making the resulting salt difficult to disperse in pyridine and the latter for two reasons first the methylated derivatives are soluble in water and insoluble in organic solvents so that dialysis is necessary for purifica- tion at each stage and secondly the presence of the sulphate groups hinders the methylation process itself. If it were possible to remove the sulphate groups without affecting the glycosidic links an examination of the products of hydrolysis of the methylated desulphated polysaccharide in comparison with those from the original methylated substance would fix the positions of the sulphate groups. Unfortunately the sulphate groups cannot be removed by alkaline hydro- lysis under ordinary conditions and acid hydrolysis splits the polymeric links.The ideal process would be the use of enzymes but apart from T. Soda’s alleged glucosulphatase 24 isolated from Charonia lampas little work has been done in this field; preliminary experiments in Edinburgh with certain molluscs show however that some hydrolysis of sulphate groups can be induced. Polysaccharide Sulphates of Marine Algae.-(a) Carragheenin.-The material extracted by water from the red seaweeds Chondrus crispus and Gigartim stelkta (Irish moss carragheen) of some importance as a thicken- ing and emulsifying agent and as an article of diet has been the subject of chemical studies for at least eighty years. F. A. Fluckiger 25 obtained mucic acid by oxidation with nitric acid and B.Tollens et aZ.26 confirmed the pres- ence of galactose. At various times the presence of fru~tose,~’ pentoses (or methylpentoses 28) and 2-ketogluconic acid has been rep0rted.2~ The work of Haas 3% 32 and of B. Russell-Wells 31 showed that two polysaccharides appeared to be present the product extracted from Chondrus crispus by cold water containing less calcium but more sodium and potassium than the hot extract. The significant observation was also made that the sul- phate residues were very stable to alkali. Nova Scotia chondrus extracts 33 were found to have a much higher potassium content and a 3 1 ratio of total to ash sulphate attributed to the presence of ammonium salts but by dialysis against appropriate solutions pure calcium and potassium salts were obtained which gave the correct 2 1 ratio.In none of the above investigations was any progress made towards deciding the constitution of the polysaccharides concerned but a step in this direction was made by T. Dillon and P. O’Colla 34 who by treating carragheenin with acetic anhydride and sulphuryl chloride isolated an acetylated galactan devoid of sulphate residues although considerable There is also another difficulty. 24 Soda et al. Bull. Chem. Soc. Japan 1931 6 258 ; 1933 8 148; 1934 9 83. 25 L c Repertorium of Pharmacie ” 1868 p. 350. 26 J. Hadecke R. W. Bauer and B. Tollens AnnuZen 1887 238 302. 27 F. Bente Bey. 1875 8 416. 28 A. Muther and B. Tollens ibid. 1904 37 302. 29 E. G. Young and F. A. H. Rice J . Biol. Chem. 1946 164 35. 30 Biochem. J . 1921 15 469. 31 B. Russell-Wells ibid.1922 16 578. asM. R. Butler ibid. 1934 28 759. Haas and Russell-Wells ibid. 1929 23 426. 3 4 Nature 1940 145 749. PERCIVAL CARBOHYDRATE SULPHATES 377 degradation occurred. Workers in Edinburgh 35 studying the hot- and cold-water extracts of ChoncErus crispw demonstrated that the polysac- charides were essentially identical although differing in their mineral con- stituents and that the principal sugar unit was D-galactose.* Both extracts could be methylated with sodium hydroxide and methyl sulphate without this loss of sulphate residues and 2 6-dimethyl and 2-methyl galactose were recognised as the principal products of hydrolysis of the methylated polysaccharides. The red alga Qigartiruz stellata which closely resembles Chndrus crisps waa also in~estigated.~~ The material extracted with hot water (ash 17.5 ; Cay 3.7 ; Mg 1.0 ; SO4 23.9%) gave on methyla- tion a product (ash 18.2; OMe 18.6; Ca 3-8; Mg 0-9; SO, 2407%) from which on hydrolysis a good yield of crystalline 2 6-dimethyl galac- tose was obtained.Assuming the sulphate groups to be linked directly to the galactose residues it is possible to decide how these are arranged in the polysaccharide which is considered to be fundamentally the same from both sources. Clearly the hydroxyl groups on C(2) and C(s) are free which leaves four possible structures Quite apart from the improbability that the units occur in the furanose form in the polysaccharide a conclusion based on the rate of hydrolysis by acids ( A ) is the only possible formulation (B) on treatment with alkali would lose the sulphate group to form a 2 3- and probably also a 3 6-anhydride (C) would give a 5 6- and (D) a 2 3- and 3 6-anhydride.In all these cases the sulphate group would be easily removed in sharp distinction to the facts. On the other hand ( A ) could not give rise either to a 3 6-anhydride or to an ethylene oxide ring and would be expected to resist alkaline hydrolysis. For these reasons it was concluded 3 5 9 36 that s5 J. Buchanan E. E. Percival and E. G. V. Percival J . 1943 51. 36 E. T. Dswar and Percival J . 1947 1622. * Recently by chromatographic separation on a cellulose column methyla,ted L-galactose derivatives have been isolated from the hydrolysis products of certain methylated carragheenin fractions. 37 378 QUARTERLY REVIEWS the galactose units in carragheenin are galactopyranose residues linked through the 1 and 3 positions and carrying the sulphate group on C(4).Confirmation of this view has been obtained 37 by the isolation of 2 4 6- trimethyl galactose from the hydrolysis of methylated partly-degraded specimens of carragheenin from which the sulphate residues had been removed and Dillon 3* has also supported the allocation of the 1 3-linkages. Much remains to be done before the full constitution of carragheenin is settled for the yield of galactose obtained on hydrolysis represents only about two-thirds of the organic matter present. Fructose has been stated to be a c o n s t i t ~ e n t ~ ~ ~ 28 and certainly ca. 20% of a constituent giving the colorimetric tests of a ketose can be detected.35* 36 The isolation of a crystalline derivative of 2-keto-~-gluconic acid has been recorded,29 but it is by no means certain that this product estimated to make up about 3% of the polysaccharide is present as such in the original material.Two possibilities exist one that carragheenin is a mixture of a poly- galactan sulphate and a labile polysaccharide or mixture of polysaccharides which has eluded characterisation so far and the other that the unidentified residues are present with galactose in the same polymeric structure. (b) The galactan sulphate of Dilsea edu2lis.-7. C. Barry and Dillon 39 isolated a galactan sulphuric ester from the red seaweed Dikea edulis by extraction with dilute acids. The sulphate content is much lower than for carragheenin and corresponds to one sulphate group in four or five galactose residues.A tentative formula (XXIV) has been advanced for the repeating unit from oxidation experiments with periodic acid only one galactose unit in five being attacked. The sulphate residue was found to be stable to alkali ..- I I CH,. OH I ' CH,. OH 3 - 1 L.. n and is therefore assigned to C(41 instead of to C(s) and 1 3-linkages pre- dominate in accordance with the evidence that most of the galactose residues are untouched by periodate. (c) Agar-agar.-There is some doubt whether agar-agar the important polysaccharide extracted from Gelidiam spp. and related algz can be regarded strictly as a sulphuric ester. Various estimates of the sulphur 37 R. Johnston and Percival unpublished. 38 Proc. Chern. Soc. 1949 34. 39 Proc. Roy. Irish Acad. 1945 50 349. 379 content of commercial agar give values approaching 2yo but specimens 4 0 prepared from Gracibria confervoides (S 0.43y0) Gelidium crimle ( S 0*47%) and Gelidium kctifolium 41 ( S 0.36y0) contain much less.There is a strong suspicion however that because agar contains 3 6-anhydro-~-galactose units in its molecule sulphate residues were present a t an earlier stage. The main structural feature of agar is a chain of /?-D-galactopyranose units linked through the 1 3-positions (XXV) since the chief hydrolysis PERCIVAL CARBOHYDRATE SULPHATES product of methylated agar was shown to be 2 4 6-trimethyl D-galac- tose. Subsequent investigations 43 showed that 3 6-anhydro-~-galactose residues were present also and this was thought a t the time to be related to the discovery that hepta-acetyl DL-galactose could be isolated on the acetolysis of agar.44 It was shown subseq~ently,~~ however that the acetolysis of 3 6-anhydro-/3-methyl-~-galactoyyranoside gave the same DL-galactose derivative and that the racemisation was due to a rearrange- ment made possible by the special symmetry of the galactose series.W. G. M. Jones and S. Peat 46 isolated 2 5-dimethyl 3 6-anhydro-~- galactonic acid from methylated agar which had been dialysed in acid solution remethylated and hydrolysed the production of the free acid being attributed to the formation of a free aldehyde by the opening of the pyranose ring in the 3 6-anhydro-~-galactose residue (a typical feature of 3 6-anhydrogalactopyranosides 4 7 followed by atmospheric oxidation. 2 5-Dimethyl 3 6-anhydro-~-galactonic acid was also isolated 48 by the acetolysis of methylated agar fdllowed by oxidation remethylation and hydrolysis of the mixture of disaccharide esters produced.It is certain therefore that the 3 6-anhydro-~-galactose residues are joined to the main chain through C(4). Jones and Peat 46 interpreted their results as showing that agar is made up of repeating units which are composed of a chain of nine D-galacto- pyranose units linked through 1 3-positions terminated by an L-galacto- pyranose residue linked through C(4) carrying a sulphate group on c(6) 40 Percival Nature 1944 154 673. 41V. C. Barry and T. Dillon Chem. and Ind. 1944 63 167. 42 Percival and 5. C. Somerville J . 1937 1615. 43 S. Hands and Peat Chem. and Ind. 1938 57 937 ; Nature 1938 142 797 ; Percival Somerville and I. A. Forbes ibid.p. 797 ; Percival and Forbes J . 1939 1844. 44 N. W. Pirie Biochem. J. 1936 30 369. 4 5 T. L. Cottrell and Percival J . 1942 749. 47 W. N. Haworth Jackson and F. Smith J . 1940 625. 48 Percival and T. G. H. Thomson J . 1942 750. Ibid. p. 225. 380 QUARTIoRLY REVIEWS (XXVI) from which the 3 6-anhydride is produced during methylation. This view has been contested 419 429 48 as an over-simplification of the problem and on the ground that the sulphur content of natural agar is too low to account for the yields of 3 6-anhydro-~-galactose derivatives actually isolated. It is possible though that the 3 6-anhydro-ring was formed at some stage in the elaboration of the polysaccharide by the alga. A speculation is made by Peat 46 as to the possible origin of the L-galactose residue linked through C(4).He points out that a D-galactopyranose residue (XXVII) linked through C(3) with a sulphate group on C(l) could by fol- lowing the sequence of events depicted below involving an oxidation- reduction process become transformed into an L-galactose residue (XXVIII) OH OH kH-OH C H.H SO ti H n ti OH lo OH fo H OH 1 (XxPcI) carrying the sulphate on It is also suggested that the sulphate group plays a part in the synthesis of agar in the same way as the phosphate group does for starch but evidence in support of either suggestion is likely to be difficult to obtain. H. Kylin 4B isolated from various common brown sea- weeds a soluble polysaccharide called fucoidin which gave a methylpentose on hydrolysis. This substance was shown to be a sulphuric ester by M. Bird and P.Haas 6* since the sulphate contained in the ash (15.1%) was half the total sulphate (30.3%). A similar polysaccharide was isolated from and linked through C(4). (a) Fucoidin. 2. physiol. Chem. 1913 83 171. 6o Biochem. J. 1931 25 403. PERCfVAL CARBOHYDRATE SULPHATES 381 Macrocystis pyrifera 51 and the principal building unit was identified as ~-fucose.6~ G. Lunde and his co-workers 53 prepared fucoidin from Lamin- aria digitah and estimated that 33-37% of the polysaccharide was L- fucose and that 35*5-37*7% of the sulphate was present in combination with sodium together with smaller quantities of calcium and magnesium. Since 20% of the molecule could not be accounted for the formula R,R'O*SO,*ONa was suggested where R = L-fucose R' = unknown. Several different sources of fucoidin have now been examined and it is now possible to account for 99% of the components of the poly~accharide.~~ One of the difficulties in the analysis of fucoidin is that the substance stubbornly retains water and ethanol but a specimen from Himanthalea Zorea after correction for these components gave fucose 56-7 ; galactose 4.1 ; uronic acid 3.3 ; xylose 1.5 ; sulphate 38.3 ; and metals (chiefly calcium) 8.2%.A calcium polyfucosan monosulphate (C,H,O,SCa,.,) would give fucose 66.9 ; sulphate 39.2 ; and Ca 8.2%. It is not possible to say at present whether the small quantities of carbohydrate material other than L-fucose are combined in a single polysaccharide but the broad pic- ture of fucoidin as a polyfucosan ethereal sulphate seems to be a good working hypothesis for future studies.The sulphate residues in fucoidin are very stable 1 4-linkage since with a sulphate group on 6CH3 to alkali; this excludes the possibility of a either C(a) or C(3) in a 1-substituted L-fucose HO (XXIX) there would be a tram-hydroxyl group in the reciprocal position. The polymeric link must therefore be on or C(3). Animal Carbohydrate Sulphates.-(a) Chondroitin sulphate. Nasal septa trachea aorta tendons sclera etc. contain chondroitin sulphate which is the only member of the animal group in which progress has been made towards determination of structure. Said to comprise some 40% of dried cartilage it is best extracted with calcium chloride,55 a method which gives less-degraded products than that used for the original isolation in 1861.56 P. A. Levene and F. B.LaForge 57 showed that equimolecular quantities of chondrosamine (2-amino-2-deoxy-~-ga~actose) acetic acid D-glucuronic acid and sulphuric acid were produced on hydrolysis and these ratios have been confirmed recently. 58 A degraded chondroitin sulphate of small mole- cular weight and devoid of sulphate has been examined by H. G. Bray J. E. Gregory and M. Stacey. 59 Evidence is presented based on methylation and hydrolysis that this product contains a terminal group of D-gluco- 44??... (-I OH H 61 D. R. Hoegland and L. L. Lieb J. Biol. Chem. 1915 23 287. 5 2 W. L. Nelson and L. H. Cretcher ibid. 1931 94 147. 53 G. Lunde E. Heen and E. Oy 2. physiob. Chem. 1937 247 189. 54 A. G. Ross Thesis Edinburgh 1949. 56Kar1 Meyer and E. M. Srnyth J. Biol. Chem. 1937 119 507. 66 C. Fischer and C.Boedeker Annulen 1861 117 111. ST J. Biol. Chem. 1914 18 237. 68 M. L. Wolfrom D. J. Weisblat J. V. Karsbinos W. H. McNeely and J. McLean J . rimer. Cltern. Soc. 1943 65 2077. 50 Biockm. J . 1944 88 142. 382 QUARTERLY REVIEWS pyruronic acid (isolated as the amide of 2 3 4-trimethyl a-methyl-D- glucuronoside) associated with doubly-linked glucuronic acid and acetyl chondrosamine residues (appearing as dimethyl derivatives). A branched- chain structure is suggested for chondroitin sulphate but no conclusions are drawn as to the position of the sulphate groups or the linkages involved. Kurt H. Meyer and his associatesYGo however propound a straight- chain structure. The relation between viscosity and molecular weight (27-33 x 103) as estimated by methods dependent on the presence of a free reducing group is held to indicate a straight-chain molecule containing about 120 monosaccharide residues and the specific rotation ([a]% - 31") is taken as evidence of @-linkages between them as in (XXX).The evidence for the 1 3-linkages and the allocation of the sulphate group to c@] in the acetyl galactosamine residue is briefly as follows. One macro-molecule (120 units) reduces only four periodic acid molecules so that large numbers of adjacent hydroxyl groups are excluded. By cautious hydrolysis it is claimed that half the sulphate groups can be re- moved without attacking more than 3% of the glycosidic links and the product obtained still requires only four molecules of periodate for complete oxidation. The sulphate group is therefore not adjacent to a free hydroxyl group and the possibility that a high proportion of the glucuronic acid residues are present as sulphated terminal groups is rendered remote.After methylation and hydrolysis followed by periodate oxidation ammonia is evolved almost quantitatively ; therefore a free hydroxyl is adjacent to the amino-group in the partly methylated chondrosamine which means that C(3! is either a main linking point or is blocked by a sulphate group in the mtrogenous unit. Furthermore after the same treatment one mole- cule of formaldehyde is liberated per disaccharide period which shows that c(6) is not methylated and that the methylated chondrosamine is the 4-methyl derivative. The consumption of periodate by the hydrolysed methylated chondroitin sulphate-three molecules per disaccharide period -is accounted for by the 4-methyl galactosamine so that the dimethyl glucuronic acid fragment uses no periodate from which it is concluded that it is 2 4-dimethyl glucuronic acid.The glycosides produced on methanolysis consumed one molecule of periodate per disaccharide period with a quantitative elimination of ammonia in agreement with this result. (XXX) requires that the sulphate residue should be stable to alkali since there is no possibility of anhydride formation ; this fits in with the observa- Kurt H. Meyer M. E. Odier and A E. Siegrist Helv. Chim. Acta 1948 31 1400. PERCIVAL CARBOHYDRATE SULPHATES 383 tion that repeated methylations with sodium hydroxide and methyl sulphate do not eliminate the grouping. Mucoitin sulphate occurs in gastric mucosa 61 and has also been isolated from ox cornea.62 The constituents are N-acetyl- glucosamine sulphuric acid and D-glucuronic acid in molecular propor - ti on^,^^ although it was only recently 63 that D-saccharic acid was obtained by the oxidative degradation of mucoitin sulphate to confirm the latter constituent.The arrangements of the building stones of the molecule are quite unknown a t present. It is of interest that the substance hyaluronic acid,g4 isolated from vitreous humour and present in umbilical cord synovial fluid etc. is thought to be mucoitin sulphate devoid of the sulphate residues. (c) Heparin. The blood-anticoagulant heparin first isolated in 1918 by W. H. Howell and E. Holt 6 5 from dog liver was early recognised to be carbohydrate in nature arid to contain nitrogen shown later 66 to reside in D-glucosamine.The presence of a uronic acid was inferred by the usual method and this was eventually proved to be D-glucuronic acid.67 That heparin was a polysulphate was suggested by E. Jorpes 68 in 1935 and A. F. Charles and D. A. Scott's crystalline barium salt 69 was shown to be a salt of a sulphuric ester. Charles and A. R. Todd 70 sub- mitted evidence that heparins from different sources e.g. lung and liver appeared to be analytically identical and also that conversion into the ammonium salt followed by reconversion into the barium derivative did not impair the physiological activity. It was also observed that periodic acid did not attack heparin and the substance was represented as a mucoitin sulphate containing five sulphuric ester residues in a tetra- saccharide unit (C2,H3,03,N2S5),Ba,,24H,0 with two carboxyl and two ncetamido-residues.Charles and Todd also observed a diminution of physiological activity with the progressive removal of sulphate groups. tJorpes,71 on the other hand considers heparin to he a mixture of poly- saccharides containing variable amounts of sulphate residues such as di- and tri-sulphuric esters of a glucuronic-glucosamine disaccharide unit of which the crystalline component is a relatively small fragment. In con- riexion with this view two components have been detected on electro- ph~resis.'~ According to &I L. Wolfrom 58 the crystalline barium acid heparinate is the same both analytically and biologically whether isolated from such varied sources as beef lung or dog pork or beef liver. The sulphate residues are all combined with barium but the carboxyl groups (b) Mucoitin sulphate.61 Levene and J. Lbpez-Suarez J . BioE. Chem. 1916 25 511. R 2 Karl Meyer and E. Chaffee ibid. 1941 138 491. 6 3 Wolfrom and Rice J . Amer. Chem. SOC. 1946 68 532. B4 Karl Meyer and J. W. Palmer Amer. .7. Ophthalmol. 1936 19 859. 65 Amer. J . Physiol. 1918 47 328. 66 E. Jorpes and 5. Bergstrom 2. physiol. Ghem. 1936 244 253. R7 Wolfrom and Rice J . Amer. Chem ~ o c . 1946 68 532. 68 Biochem. J . 1935 29 1817. Ibid. 1940 34 112. 71 Ibid. 1942 36 203. 7 2 Wolfrom and Rice J . Amer. Chena. SOC. 1947 69 2918. 69 Ibid. 1936 30 1927. c c 384 QUARTERLY REVIEWS are free. The claim is made that heparin contains no acetyl residues and that the glucosamine residues are joined to the glucuronic acid units of the polymer by >CH*NH*CH< links to the potential reducing groups of the latter.There is no direct evidence to support this unique type of linkage or to indicate any other structural features of the heparin molecule. The apparent connexion between '' heparin activity '' arid sulphate content 70 has led to a search for more accessible materials for physio- logical studies. An extract from Chondrzis crispus 73 has been shown to be about 40% as active as heparin but fucoidin is ina~tive.7~ Polysac- charide sulphates have been synthesised and heparin activity demonstrated in many of them although no product of practical value has been reported upon until recently since most of the synthetic products have the dis- advantage of high toxicity not possessed by heparin. Thus sulphated agar,75 cellulose starch and glycogen sulphates 76 have been prepared.A new clot-inhibitor " Paritol," has been reported 77 which is presumably an alginic acid sulphate and resembles heparin in containing carboxyl and sulphate groups but differs from it in the absence of glucosamine residues. This product is claimed to be no more toxic than heparin and since its physiological action lasts twice as long may be substituted for heparin for short -term administration to human beings although clinical test!s are not complete. 7 3 ,4. R. Todd private communication. 7 4 A. F. Charles private communication. 75 C. Neuberg and C. H. Schweitzer Monatsh. 1937 71 46. 7 6 E. Chargaff F. W. Bancroft and M. Stanley-Brown J. Bid. Chem. 1936 115 155; Bergstrom 2. physiol. Chem. 1936 238 163; P. Karrer H. Koenig and E. Usteri Helv. Chiin. Acta 1943 ; H. Gebauer-Fulnegg and 0. Dingler J. Amer. Chena. SOC. 1930 52 2849 ; W. Traube. B. Blaser and E. Lindemann Ber. 1932 65 603. 77 Chem. and Eng. News 2949 27 2162.

ISSN:0009-2681    

DOI:10.1039/QR9490300369

出版商:RSC    

年代:1949

数据来源: RSC