Solvent effects on the infrared frequency of inorganic carbonyl and nitrosyl bands

The infrared spectra of a number of transition metal carbonyl and nitrosyl compounds have been recorded in a series of solvents. The behavior of the various bands is discussed in terms of the relative frequency shift plots of Bellamy and co-workers. The magnitude of the frequency shifts is correlated with the relative electron density around the metal atom in a series of compounds in which the ligands are varied. Solvent effects on NO and CO vibrators appear to be fairly similar in character, with the NO band shift greater in magnitude. This is discussed in terms of dative metal-ligand π-bonding in these complexes. It is found that in molecules with more than one CO band, the frequency shifts of these bands correlate fairly well among themselves but do not correlate with those of complexes having only one CO band. The bridging carbonyl band in Co₂(CO)₈ has a solvent shift behavior very similar to organic ketones. In certain solvents, however, there is evidence for the formation of the Co(CO)₄⁻ anion.