Ligand Field Strengths of Carbon Monoxide and Cyanide in Octahedral Coordination

Ligand field splittings were extracted from absorption spectra of metal hexacarbonyl and hexacyano complexes over 35 years ago by Gray and coworkers J. Am. Chem. Soc., 85, 2922 1963 ibid. 90, 4260, 5713 1968. Recent time-dependent density functional theory calculations by Baerends and coworkers J. Am. Chem. Soc., 121, 10356 1999 on MCO6 with MCr, Mo, W raised questions about the magnitudes of these ligand field splittings. In order to reexamine such effects systematically, we report here the splittings for a series of 3d6 metal hexacarbonyl, hexaisocyano and hexacyano complexes VCO6- , CrCO6, MnCO6 and FeCO62 CoCN63 and FeCN64 and VCNH-6, CrCNH6, MnCNH6 and FeCNH62 . This work demonstrates that the 3d splittings induced by the ligands in question generally fall in the order COCNHCN- . We agree with Baerends and colleagues that CO exerts a much stronger ligand field than was originally thought.