Ligand field strengths of carbon monoxide and cyanide in octahedral coordinationy

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 M1⁄4Cr, 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 3d metal hexacarbonyl, hexaisocyano and hexacyano complexes: VðCOÞ 6 , Cr(CO)6, MnðCOÞ6 and FeðCOÞ 2þ 6 ; CoðCNÞ 3 6 and FeðCNÞ 4 6 ; and VðCNHÞ 6 , Cr(CNH)6, MnðCNHÞ þ 6 and FeðCNHÞ2þ 6 . 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.