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 similar to55% electrostatic character and similar to45% covalent character. There is a significant contribution of the 7 orbital interaction to the Fe-CO covalent bonding which increases when the complexes become negatively charged. The strength of DeltaE(pi) may even be larger than DeltaE(sigma). The Fe-CN- bonds have much less 7 character. The calculated binding energy of the Fe-CO T-interactions correlates very well with the C-O stretching frequencies.