Ligand field strengths of carbon monoxide and cyanide in octahedral coordination

Abstract

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, 2922Soc., 85, (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 ¼ 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 3d 6 metal hexacarbonyl, hexaisocyano and hexacyano complexes: VðCOÞ . We agree with Baerends and colleagues that CO exerts a much stronger ligand field than was originally thought.

Fe L-Edge XAS Studies of K4[Fe(CN)6] and K3[Fe(CN)6]: A Direct Probe of Back-Bonding

“…These values are in the same range as those predicted by calculations reported both here and elsewhere. 16,17,27,29,30 They also agree well with the more qualitative results of other experimental techniques, including …”

Chemical bonding in aqueous hexacyano cobaltate from photon- and electron-detection perspectives

“…This is due to the strong π-acceptor nature of the CO ligand. Also note that the CO ligand is charge-free whereas the cyanide carries a negative charge, and this could partially hinder the charge transfer to the ligand sites45. We conclude this section by noting that the strength of back bonding decreases in the following order of metal complexes: Fe(CO) 5  > [Fe(CN) 6 ] 4−  > [Co(CN) 6 ] 3− .…”

Is Cyanide Really a Strong‐Field Ligand?

“…[9] More recently, Scheidt et al reported a complex containing five-coordinate iron that strongly demonstrates the weaker-field nature of cyanide. [10] The complex in question is [K(222)][Fe(tpp)(CN)], which is the first monocyano complex of an iron(II) porphyrin.…”

“…Prior to this work, some calculations on the ligand field strengths of strong-field ligands had been reported. [8,9] Scheme 1. The spectrochemical series developed by Tsuchida.…”