A Mössbauer study of structure and bonding in iron(II) low-spin compounds

Abstract

The Mössbauer spectra of a large number of iron(II) low-spin compounds are reported. Together with the Mössbauer spectra of other low-spin compounds reported in the literature, we have derived partial centre shifts (p.c.s.) for 37 ligands and partial quadrupole splittings (p.q.s.) for 16 ligands. There is an inverse correlation between the p.c.s. values and spectrochemical ranking of ligands. With this correlation the position of hydride in the spectrochemical series has been re-examined. The p.c.s. values should also be helpful in the preparation of compounds in which the iron(II) exhibits intermediate spin or a high-spin–low-spin equilibrium.

The p.c.s. values are related to the total σ-donor plus π-acceptor properties of the ligands, while the differences in c.s. between the cis–trans pairs may be used to separate the σ- and π-effects. Our results indicate that SnCl₃^– is a good π-acceptor (comparable to ArNC) but a rather poor σ-donor.

The 2 : 1 trans-cis quadrupole splitting ratio holds for such ligands as Cl^–, SnCl₃^–, RNC, and CN^–, and the q.s. values are used, in conjunction with other spectroscopic data, to assign the stereochemistry of many of the isomers in this study. The 2 : 1 q.s. ratio is explained in terms of a simple molecular orbital model. The p.q.s. values can be useful for examining the bonding properties of many ligands, and can provide complementary bonding information to that obtained from the p.c.s. values.

Many of the spectra in this study were distinctly asymmetric. With this Goldanskii asymmetry, a method for obtaining the sign of the field gradient in these compounds is tentatively proposed.