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 M(CO)₆ with M = Cr, 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: V(CO)₆^-, Cr(CO)₆ Mn(CO) ₆⁺ and Fe(CO)₆²⁺; Co(CN) ₆^3- and Fe(CN)₆^4-; and V(CNH) ₆^-, Cr(CNH)₆, Mn(CNH)₆⁺ and Fe(CNH)₆²⁺. This work demonstrates that the 3d splittings induced by the ligands in question generally fall in the order CO > CNH > CN^-. We agree with Baerends and colleagues that CO exerts a much stronger ligand field than was originally thought.