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The rate of substitution of the H2O β‐ligand in aquacobalamin (vitamin B12a) is surprisingly fast for CoIII. This may arise as a result of transfer of electron density to CoIII by the corrin, conferring on the metal atom a softer and more labile character, and would require electronic communication between the equatorial ligand and the axial coordination sites of the metal atom. We have substituted H2O in B12a with the ambidentate nucleophiles SCN−, SeCN−, NO2− and S2O32−, examined the UV/Vis spectra of the resultant complexes, crystallised them, and determined their structures by X‐ray diffraction methods. The UV/Vis spectra of these complexes, as well as those of H2OCbl+, SO3Cbl− and MeCbl, were fitted by Gaussian functions. The principal bands move to lower energy in response to an increase in donation of electron density from the axial ligand, demonstrating direct electronic communication between the axial and the equatorial ligands. In the solid state, the thiocyanato ligand in SCNCbl is coordinated through N, although 13C NMR spectroscopy shows that the complex exists as a mixture of two linkage isomers in solution. SeCN−, NO2− and S2O32− are coordinated through the softer available donor in each case (Se, N and S, respectively). There is a regular ground‐state trans effect such that as the donor power of the β‐ligand increases, the trans Co−Nax bond length increases. A survey of the structures available in the CSD shows that this preference for the softer donor atom in ambidentate nucleophiles is not unusual in CoIII complexes. The Co−N bond length in SCNCbl is marginally long for coordination to CoIII, but the axial bond lengths of the other complexes are as expected. Hence, whereas the corrin macrocycle clearly imparts lability on CoIII, presumably by transfer of electron density onto the metal atom, this is insufficient to have a major effect on the ground‐state structures of its complexes. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Time-dependent density-functional theory and density-functional theory are applied to study the cis influence of the equatorial macrocycle in vitamin B(12) derivatives. A series of dicyanocobalt corrinoids, CN-[Co(III)-corrin]-CN, where the C(10)H of the corrin ring is replaced by different substituents, X, is considered. The calculated UV-visible absorption spectra, the charge distribution obtained from a Bader QTAIM analysis of the electron density, the CN stretch frequencies of the axial cyano ligands and the electron densities at some bond critical points are compared. The main absorption bands in the UV-visible spectra depend on the electron donating or withdrawing power of X, as assessed from its Hammett σ(p) constants. For X with a stronger electron donating power than H, the other properties do not change appreciably. However, when σ(p)(X) > σ(p)(H), these properties vary linearly with the electron withdrawing power of the substituent. This helps explain the experimental observation that substitution of the axial ligand is more difficult and proceeds more slowly with the increase of the electron withdrawing power of the C(10) substituent.

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