Infra-red studies of amine, pyridine and phosphine derivatives of manganese pentacarbonyl bromide

Abstract

The CO stretching frequencies of a series of amine, pyridine and phosphine substituted cis-Mn(CO) 3(L) 2Br complexes (where L  o-ClC 6H 4NH 2, C 6H 5NH 2, p-CH 3OC 6H 4NH 2, (CH 3) 2CHNH 2, 3-BrC 5H 4N, C 5H 5N, 4-CH 3C 5H 4N, P(C 6H 5)(C 2H 5) 2 or P(C 4H 9) 3) have been studied as a function of the p K a of L. Using non-π-bonding amine ligands, there is a small, but significant, decrease in the CO stretching frequencies as the p K a of the amine is increased. The frequency shifts must be a result of changes in the MnL σ-bond. That the CO stretching frequencies of the pyridine and phosphine complexes show similar small shifts with analogous changes in the p K a of the pyridine or phosphine suggests that there is little, if any, π-bonding between these ligands and Mn either. In view of the well-documented π-bonding abilities of the phosphines, and to a lesser extent the pyridines, it appears that CO stretching frequency shifts may not be a reliable measure of the π-bonding abilities of ligands in substituted metal carbonyl complexes.