Bonding properties of ligands: a Mössbauer study of carbonyl complexes of iron(II)

Abstract

The Mössbauer spectra of a large number of previously prepared and new iron(II) low-spin compounds are reported. We have derived partial quadrupole splittings (p.q.s.) and partial centre shifts (p.c.s.) for many of the common neutral ligands. The p.q.s. values and carbonyl i.r. stretching frequencies have been used to assign the stereochemistry of compounds of the type FeA₂B₂C₂ and FeAB₃C₂(A = neutral ligand, B = CO, C = halide).

P.c.s. and p.q.s. values are used to estimate qualitatively the σ-donor and π-acceptor abilities of the ligands in this study. The variable bonding properties of CO are discussed. Trimethyl phosphite is the strongest σ-donor and π-acceptorof the tertiary phosphorus ligands in this study, and is very similar to isocyanides in its bonding properties. All phosphites are found to be better π-acceptors than their corresponding phosphines.

The general correlation between p.c.s. and p.q.s. values suggest that σ bonding is usually dominant in determining the c.s. and q.s. Mössbauer parameters for CO are also correlated with theoretical calculations of carbonyl orbital populations and carbonyl stretching frequencies.