Interactions of carbonyl groups in compounds containing metal-metal bonds

Infra-red spectra of compounds of the type {(CO)3. Fe[X. C6Y5]}2, (X = S, Se or Te; Y = H or F), have been studied in the carbonyl stretching region. Four bands were observed. The interactions between the carbonyl groups were assumed to take place via iron 3d orbitals. A model to describe the spectra was developed in which a three-fold axis was retained at each Fe(CO)3 group and between the two iron atoms. It was concluded that the carbonyl-carbonyl interaction route between the iron atoms was via direct 3d–3d overlap and not via the d or p orbitals of X. Interaction force constants were calculated and their variation with X and Y discussed. The model was extended to explain the spectra of related compounds.