Molecular Orbital Analysis of Iron-Group Cyanides

In the iron-group cyanides the low-lying antibonding orbitals of the cyanide ligands do not contain electrons. These π* orbitals play an important role in the covalency of the complexes. A molecular orbital analysis of their role is presented in order to correlate the orbital reduction factors derived from the ESR experiments of Baker, Bleaney, and Bowers with the isomer shifts of the Mössbauer experiments. By fitting the wavefunctions to the ESR experiment it is shown that the Fe2+ complex donates about one more electron to the π* orbitals than does the Fe3+. This shows that the effective charges of Fe2+ and Fe3+ are approximately the same and explains their similar isomer shifts.