Density functional investigation and bonding analysis of pentacoordinated iron complexes with mixed cyano and carbonyl ligands.

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 Fe-CN and Fe-CO bonds has been analyzed with both charge decomposition and energy partitioning analysis. The results of energy partitioning analysis of the Fe-CO 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 pi- orbital interaction to the Fe-CO 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 C-O stretching frequencies. The Fe-CN bonds have much less pi-character (12-30%) than the Fe-CO bonds.