Some Properties of Metal Complexes Containing One Metal-Carbon BondBY H. A. 0. HILL, J. M. PRATT AND R. J. P. WILLIAMSInorganic Chemistry Laboratory, South Parks Road, OxfordReceived 16th January, 1969The physical and chemical properties, including electronic absorption, infra-red, and 'H n.m.r.spectra of cobalt(1II) complexes of the type CoA4XY, where .A4 can be corrin, a Schiffs base, ordimethylglyoxime and X a carbon ligand, are discussed in terms of the change in electron densityin the complex. It is shown that alkyl ligands act as strong donors, often giving rise to five-co-ordinate complexes. The relationship of the properties of these five-co-ordinate cobalt(III) complexesto those of analogous cobalt(lI) nickel(I1) and palladium(II) complexes is discussed and the differencebetween the cobalt-carbon bond in the five- and six-co-ordinate complexes considered.We are interested in the properties and influence of the metal-carbon bond incomplexes of the type MA4XY, where A4 represents a ligand or ligands co-ordinatedto the central metal through atoms other than carbon and arranged in a plane asshown in fig.1. The axial ligands X and Y can be varied in each complex; X isco-ordinated through carbon, e.g., CN, CH3, C6H5, whereas Y can be co-ordinatedthrough N, 0, P or C .FIG. 1.-Structure of complexes of thetype M&XY.The metal-carbon bond can be affected by the trans-ligand Y or the cis-ligand(s)A, and conversely the properties of Y and A4 can be altered by a change in the carbonligand X. We shall use various physical properties of A,, X and Y to uncover thenature of the co-operative interaction between the groups.We have shown that in monocyano derivatives of cobdamins and cobinamideswhere M is formally Co(III), A4 is the corrin ljgand and X = CN, the CN stretchingfrequency depend on the ligand Y as shown in table 1.We note that when Y = C2H5,the stretching frequency is close to that of free cyanide. From these results wemight conclude that as Y becomes a " better donor " to the cobalt, the trans-metal-carbon bond becomes weaker until perhaps the six-co-ordinate complex does notform at all. Similarly in other cobalt(1I.I) complexes, the physical properties of onetrans-ligand approach those of the " free "-figand as the donor character of the other16166 COMPLEXES WITH ONE METAL-CARBON BONDtrans-ligand increases. For example, in a series of cobalt(III) dimethylglyoximates,2Co(DH), pyridine X, the ‘H n.m.r.of the co-ordinated pyridine is close to that offree pyridine when X=CH3. In a series of c~balamins,~ the ‘H n.m.r. of the co-ordinated 5,6-dimethylbenziminazole (Bz) is at higher field when X is a strong donor.Does the bond length to the group Y also depend on X? In the only two examplesavailable, this is indeed the case ; ~yanocobalamin,~ Co-Bz 2.07 A, Co-corrin(average) 1 -905 ; 5’-deo~yadenosylcobalamin,~ Co-Bz 2-23 A, Co-corrin(average) 1 -94 A.TABLE DE DEPENDENCE OF THE CN STRETCHING FREQUENCY IN CYANOCOBALAMINS ANDCYANOCOBINAMIDES ON THE AXIAL LIGAND TRANS TO CN,igand axial 5,6-dimethyl- benziminazo,e OH- CN- HCrC- CHz=CH- ~~~~~~~i CHZ CH3CH?’_ free CN-ICNstretchingfrequency(cm-l) 2132 2130 2119 2110 2093 2091 2088 2082 2079Taking this process further it has been possible to prepare complexes with thefollowing A, ligands which have a variety of carbon ligands X but no sixth ligand;bis(acety1acetone)ethylenediimjne (BAE), bis(salicy1aldehyde)ethylenediimine(SALEN), bis(trifluoromethylacety1acetone)ethylenediimine * (BTFAE), anddimethylgly~xime.~ We have also concluded O from the temperature-dependenceof the absorption spectra and lH n.m.r.of cobinamides that there are three classesof cobinamides: (a) Those with relatively “weak” ligands X and Y, e.g., H20,NH3, whose ‘H n.m.r. and electronic absorption spectra are insensitive to temperature.(b) Those with one very strong ligand X, e.g., i-C3H7 whose lH n.m.r.and electronicabsorption spectra are virtually temperature independent. (c) Those with ligandsof intermediate strength, e.g., -CH=CH2. With such ligands there is a markeddependence of the lH n.m.r. and electronic absorption spectra with temperaturesuggesting the presence of two complexes in equilibrium.For various reasons which have been given in detail elsewhere,lO we considerthe complexes in class (c) to be an equilibrium mixture of six-co-ordinate cobaltcomplex as in class (a) and a complex in which the co-ordination of the cobalt ap-proaches more closely five-co-ordination as in class (b). We observe that as thedonor power of X is increased there is a discontinuity in the properties of the cobaltcomplexes as shown schematically in fig.2.The lH n.m.r. of the carbon ligand X, e.g., the Co-CH3 is also sensitive to bothY and A4 as shown in table 2. These effects are more difficult to interpret in termsof the M-C bond due to the influence of the metal and the A, ligands directly,i.e., through-space rather than along-the-bonds, on the ‘H-chemical shift. However,we see that in the limited series available with BAE as the A,-ligand, the chemicalshift of the Co--CH, increases with increasing basicity of the nitrogen ligand Y .All these data on the influence of the carbon ligand X on the properties of the A4and Y ligands and the effect of A, and Y in X-ligands can be interpreted in terms of achange in electron density on the cobalt.In a general sense the cobalt atom acts asa relay of electron density from one part of the molecules, X, Y or A4, to any other.If change in electron density is the principal factor determining the properties of theligands, we might expect to observe close relationships between the properties ofA4, Y and the carbon ligands. Several examples have been observed. Thus, thH . A . 0. HILL, J . M. PRATT AND R . J . P . WILLIAMS 167CN stretching frequency correlates 1 * lo with the IH chemical shift of the C-10hydrogen in the corrin ligand (fig. 3), which in turn correlates lo with the energy of the0-1 vibrational component of the first electronic transition (fig. 4). The latter cannotbe a general relationship but is expected here as the first excited state l4 of corrin hasa node at C-10 and therefore the energy of the transition reflects changes in electrondensity at C-10 in the ground state.a correlation betweenthe CN stretching frequency and the equilibrium constant for replacement of oneaxial ligand by another, e.g., H20 by CN or benziminazole by CN in the vitaminB12 series, fig. 5. When A4 is BAE, and Y is a para-substituted phenyl derivativethere is a correlation between the lH n.m.r. of the methine hydrogens in the planarSimilarly, there is-.IBRIUM. -/ 6-COORD IN ATEdonor strength of ligandFJG. 2.Cchematic representation of the changes in physical properties, e.g., chemical shift, energyof electronic transitions, with the change in the donor strength of the ligand.At low donor strengththe complexes are six-coordinate ; at high donor strength five-coordinate. In the intermediateregions the two types of complex are in equilibrium and can give rise to distinct physical properties,absorption spectra, or to a single timeaveraged property, n.ni.r. In the figure the continuous linesshow the region of donor strength in which one or other of the two species predominates.TABLE 2.-INFLUENCE OF AXIAL AND PLANAR LIGANDS ON THE CHEMICAL SHIFT OF THE COBALT-METHYL HYDROGENSplanar ligandcorrinwrrindimethyfglyoximeBAE l1SALENBTFAEtetrasulphonatopht halocyanineaetioporphyrin laxial ligand Ybenziminazolewatertriphenylphosphinep yridinepiperidine4methylpyridinepyridine4-cyanop yridinedimethylsulphoxidepiperidine4-methylpiperidinepyridine4-cyanopyridinewater?--C 4 H 3 t10.1410.248.829.187.757.667-467.437.417.887-257-166.976-966.9416.115.1168550COMPLEXES WITH ONE METAL-CARBON BONDDicyanocobinumide ysc / y Ethylcobalamin"in $ CObdam;n/x/w Methylcobahmin-2 1302 120n e k 2110W2; 9210020902080 0 3.95 4.00 4-a 4.10 4.15chemical shift C-10 H (7)FIG.3.--Chemical shift 7 of the C-10 hydrogen in cobdamins against the stretching frequency (cm-I)of the cyanide ion in cyanocobalamins and cobinamides.ligand and the equilibrium constant for formation of the six-co-ordinate complexwhen Y is pyridine (table 3). Here both properties are also related to the electron-donor character of the para-substituent.A similar correlation has been observedin the cobalt(I1I) dimethylglyoximates in which both alkyl and cyanide ligandsaffect the chemical shifts of the in-plane hydrogens in proportion to their Hammettcr para-substituent constants as do other non-carbon ligandsH . A . 0. HILL, J . M. PRATT A N D R . J . P . WILLIAMS 169From a consideration of all these properties in several series of complexes, weconclude that alkyl ligands have and cause properties which are the natural extensionof those of weaker donor ligands such as NH3. The alkyl ligand is just a very strongdonor. However, in certain cases the donation may be so marked as to prevent theformation of a six co-ordinate species and simultaneously this donation causes abreak in the gradation of the properties of the planar ligand (fig.2). We then askI1 1 I 1 I2080 2 roo 2120 2140cm-lFIG. 5-Gxrelation between the stretching frequency of co-ordinated CN- (in cyanocobalamins)and the formation constants for the substitution by CN- of H20 in cobinamides and of Bz in cobal-amins (reproduced by permission of the Chemical Society).TABLE 3.-cORRELATION BETWEEN THE CHEMICAL SHIFT OF THE METHENE HYDROGENS IN~-SUBSTTTUTED COBALT(ZII) BAE AND THE FORMATION CONSTANT FOR FORMATION OF THESIX-CO-ORDINATED PYRIDINE COMPLEX.para-substituent r-methine Kpy (1. mole-1)OCHJ 4.59 6.1 AO-8CH3 4.59 5.0 f0.6H 4-58 5.3 f0-7I 4.58 19 f2.5Br 4-57 20 f2.5CN 4.52 54 f7NO2 4.49 85f13the critical question : does the nature of the M-C bond also differ markedly whenthe change from six- to five-co-ordination occurs? We can see a possible answerto this question by a comparison of the cobalt(III) Complexes with those of loweroxidation states.Cobalt(II1) complexes are normally considered to adopt fairlyregular geometries and we would expect that, e.g., when X and Y = H20 or Cl-,the cobalt would lie in the plane of the A4 ligand. However, as the ligand X donatesmore charge to the cobalt so the formally Co(1II) metal ion has an electron densit170 COMPLEXES WITH ONE METAL-CARBON BONDcloser to the real charge of a lower oxidation state. Thus, we might expect that thecobalt ion should have properties more like those of a low-spin Co(U)d7, or evenlow-spin Co(I)d8.[This ambiguity when X is a covalent ligand is reflected in therelative weight of the resonance forms : CH; Co(III)++CH,. Co (II)-CH; Co(1).The relative weights could be very different in the five- and six-co-ordinate forms.]The absorption spectra of the five-co-ordinate forms of the cobalt(II1) corrins,cobalt(II), nickel(I1) and palladium(I1) corrins, have two main absorptionbands -450-470 mp and 308-320 mp, whereas in the six-co-ordinate forms thefirst two intense bands are 500-600 and 350-370mp. This similarity is alsoobserved l6 in the circular dichroism of the cobalt(II1) and cobalt(I1) complexes.The similar values of the equilibrium constants for substitution of the benziminazoleby H 2 0 on acidification of the cobalt(I1) complexes (pK,-2-5) and of the methyl-cobalt complex (pKa = 2.5) suggest that the electron density on the metal ion isapproximately the same in both cases.In other words, the effect of the carbon ligand bound to the cobalt(II1) has beentwo-fold: it has induced a lowering of the cobalt co-ordination number to thattypical of a lower oxidation state and it has also caused the ligand A4 to have thesame electronic characteristics and perhaps even structure, as judged by the absorp-tion spectra and the circular dichroism, as found in the lower oxidation state com-plexes.Clearly, this could markedly alter the polarity of the cobalt-carbon bond.We are studying the reactivity of these complexes to see if there is such aneffect.We now turn to the exact character of the change in geometry on going from thecobalt(II1) to the cobalt(I1) state.The marked reluctance of some cobalt(I1) com-plexes to co-ordinate two axial ligands, e.g., pyridine suggests that the cobalt(I1)ion in the monopyridine derivative does not lie in the plane of the A4 ligand(s).5-COORDINATEP I G n U r2d I s t o r ted-16-COORDI NATE-----distorted4twZWE!planar3FIG. 6.--Schematic representation of the energy of the four possible forms of five- and six-co-ordinatecomplexes.By inference, the cobalt atom in the five-co-ordinate alkyl cobalt(II1) complexes maybe similarly displaced. It is interesting that in an attempt l7 to calculate the effectof the ring current in phenylcobalt(II1) BAE on the chemical shifts of hydrogensforming an A2B2 system in the plane of the BAE ligand, results consistent with thoseobserved could be obtained only if the molecule was considerably distorted fromplanarity.The physical and chemical properties of these different series of cobalH . A . 0. HILL, J . M. PRATT AND R . 3 . P . WILLIAMS 171complexes of a given co-ordination number suggest that we should now considertwo possible structures for both five- and six-co-ordination, planar and distorted.The degree of distortion in A4 will depend in either case on the A4 and X ligands.Presumably, in the corrin and BAE complexes discussed above, the equilibriumbetween five- and six-co-ordinate species is that represented by 1 +3 in fig.6. Thoseligands which have a stronger tendency to remain planar, e.g., phthalocyanine anddimethylglyoxime, may be more reluctant to form five-co-ordinate species presumablybecause the distorted form is of higher energy, and so in such a case the equilibriummay be better represented by 2+3, in which case we would not expect the discontinuityillustrated in fig. 2.The above discussion is relevant to the reactivity at the cobalt atom. The transi-tion state for the replacement of the sixth ligand in CoA4XY must resemble the five-co-ordinate species which lies close to the ground state in the alkyl cobalt complexes.It is not surprising therefore that all their replacement reactions are fast.The unusual condition of the metal in these cobalt complexes should also beobserved in nickel(1V) l8 and iron(I1I) l3 alkyl complexes.The change in stereo-chemistry with change in oxidation state is related to that which occurs on uptakeof hydrogen or oxygen by d8 metal complexes which are known l9 to have interestingcatalytic properties.We thank the Medical Research Council and N.A.T.O. for financial support.' R. A. Firth, H. A. 0. Hill, J. M. Pratt, R. G. Thorp and R. J. P. Williams, J. Chem. SOC. A ,' H. A. 0. Hill and K. G. Morallee, J. Chem. SOC. A , 1969.1968, 2428.H. A. 0. Hill, B. E. Mann, J. M. Pratt and R. J. P. Williams, J. Chem. SUC. A, 1968, 564.D. C. Hodgkin, J. Linsey, R. A. Sparks, K. N. Trueblood and J. G. 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