ESR investigation of paramagnetic carbonyl-metal clusters of high nuclearity

An ESR study has been made of the high nuclearity paramagnetic metal cluster anions [Rh₁₂(CO)₁₃(μ₂-CO)₁₀(C)₂]^3-, [Co₁₃(CO)₁₂(μ₂-CO)₁₂(C)₂]^4- and [Co₆(CO)₈(μ₂-CO)₆C]^-. The assignment of the HOMO is based on a mixed valence model which relates the g tensor components of cluster systems to those of an appropriate conventional paramagnetic center. With this model the HOMOs of [Rh₁₂(CO)₁₃(μ₂-CO)₁₀(C)₂]^3- and of [Co₁₃(CO)₁₂(μ₂-CO)₁₂(C)₂]^4- are found to be mainly comprised of metal d_z² atomic orbitals, while for [Co₆(CO)₈(μ₂-CO)₆C]^- a large overlap between d atomic orbitals and ligand orbitals is suggested. The occupation of the valence molecular orbitals deduced from the ESR data is consistent with the variations in MM bond distance observed by X-ray analysis.