Quantum Chemical Investigations and Bonding Analysis of Iron Complexes with Mixed Cyano and Carbonyl Ligands^†

Abstract

The equilibrium structures and vibrational frequencies of the iron complexes [Fe(CN)x(CO)y]q (x = 0−6 and y = 0−5) have been calculated at the BP86 level of theory. The nature of the Fe−CN and Fe−CO has been analyzed with an energy partitioning method. The calculated Fe−CO bond lengths are in good agreement with the results of X-ray structure analysis whereas the Fe−CN bonds are calculated somewhat longer than the experimental values. The theoretically predicted vibrational frequencies of the C−O stretching mode are always lower and the calculated CN- frequencies are higher than the observed fundamental modes. The results of the bonding analysis suggest that the Fe−CO binding interactions have ∼55% electrostatic character and ∼45% covalent character. There is a significant contribution of the π orbital interaction to the Fe−CO covalent bonding which increases when the complexes become negatively charged. The strength of ΔEπ may even be larger than ΔEσ. The Fe−CN- bonds have much less π character. The calculated binding energy of the Fe−CO π-interactions correlates very well with the C−O stretching frequencies