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

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 ΔE_π may even be larger than ΔE_σ. 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.