New insights concerning the trans influence based upon the linkage isomeric ratios of thiocyanate in cobaloxime complexes

Article abstract of DOI:10.1021/ic50208a019

The linkage isomeric ratios for thiocyanate trans to various donors have been determined for a series of cobaloxime complexes[trans-bis(dimethylglyoximato)(ligand)(thiocyanato)cobalt(III)] by utilizing proton NMR measurements. The chemical shift of the methyl protons of the dimethylglyoximate group is sensitive to the nature of the axial ligands, the τ values of the S-bound isomers being ca. 0.1 ppm less than those of the N-bound isomers in an aromatic solvent such as nitrobenzene. It was found that the presence of a small amount of Co(II) is necessary in order to facilitate isomerizatiori and the attainment of equilibrium. The ratio of sulfur- to nitrogen-bound species, K(S/N), is related to the trans influence and the basicity of the trans ligands. The observed order of decreasing trans influence is roughly the following: alkyls ? phosphites ≈ pyridines > anilines > phosphines > thioureas, K(S/N) decreasing in the reverse order. Within the pyridine, aniline, and phosphine series, K(S/N) decreases as the pK_a of the trans ligand increases. Greater through-space P-H coupling is present for phosphorus donors trans to S-bound thiocyanates than to N-bound thiocyanates. The bridging ability in inner-sphere electron-transfer reactions for the three ions studied decreases in the order NCS^- > CN^- > NO₂^-. The influence of solvent upon K(S/N) and the clarification of some discrepancies concerning previously reported cobaloximes are also discussed.