INFRARED AND C-13 AND N-15 NMR-STUDIES OF CYANO(LIGAND)COBALOXIMES - EVIDENCE FOR COBALT-TO-CYANIDE PI-BONDING

A series of 17 cyano(ligand)cobaloximes have been prepared, including 6 with 4-substituted pyridine ligands, 6 with primary amine ligands, 3 with 4-substituted aniline ligands, the ammonia complex, and the potassium salt of the thiocyanato complex. The analogous series of complexes enriched in C-13 and N-15 in the cyanide ligand have been used to locate the cyanide C-13 and N-15 NMR resonances and the cyanide stretching frequencies in the infrared spectra. As the trans axial ligand is varied, an inverse dependence of the N-15 chemical shifts on the C-13 chemical shifts is observed, and for 14 of the complexes the chemical shift data give an excellent linear correlation with a slope of -0.614. These data, together with trends in delta(N-15), delta(C-13), and nu(CN) with the basicity of the trans axial ligand, are interpreted to indicate that cobalt-to-cyanide pi-bonding is important in these cyanocobalt complexes.