Abstract
The equilibrium structures and vibrational frequencies of the iron complexes [Fe^0^(CN)~n~(CO)~5−n~]^n^^−^ and [Fe^II^(CN)~n~(CO)~5−n~]^2−n^ (n = 0–5) have been calculated at the BP86 level of theory. The Fe^0^ complexes adopt trigonal bipyramidal structures with the cyano ligands occupying the axial positions, whereas corresponding Fe^2+^ complexes adopt square pyramidal structures with the cyano ligands in the equatorial positions. The calculated geometries and vibrational frequencies of the mixed iron Fe^0^ carbonyl cyanide complexes are in a very good agreement with the available experimental data. The nature of the FeCN and FeCO bonds has been analyzed with both charge decomposition and energy partitioning analysis. The results of energy partitioning analysis of the FeCO bonds shows that the binding interactions in Fe^0^ complexes have 50–55% electrostatic and 45–50% covalent character, whereas in Fe^2+^ 45–50% electrostatic and 50–55% covalent character. There is a significant contribution of the π‐ orbital interaction to the FeCO covalent bonding which increases as the number of the cyano groups increases, and the complexes become more negatively charged. This contribution decreases in going from Fe^0^ to Fe^2+^ complexes. Also, this contribution correlates very well with the CO stretching frequencies. The FeCN bonds have much less π‐character (12–30%) than the FeCO bonds. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010