Electronic structure and bonding characteristics of cyclopentadienyl d⁸ metal-ligand complexes. core and valence ionization study of CpM(CO)₂ where M = Co and Rh and Cp = ƞ⁵-C₅H₅ and ƞ⁵-C₅(CH₃)₅¹

A combined investigation of the core and valence ionizations of d⁸ cobalt and rhodium complexes of the form CpM(CO)₂, where Cp is either ƞ⁵-cyclopentadienyl or ƞ⁵-pentamethylcyclopentadienyl, is used to reveal the important electronic interactions in these complexes. It is found that, unlike the usual similarity of ionization features of analogous first-and second-row d⁶ metal complexes, there are major differences between the valence ionizations of these cobalt and rhodium species. The primary orbital characters associated with each ionization feature are determined by comparing the relative He I/He II intensities, the relative core and valence ionization shifts, and the consistent shifts with ring methylation. Evidence for strong filled-metal with filled-ring orbital interaction is found in the first ionization band and in a band in the region of the cyclopentadienyl e₁” ionizations. Vibrational fine structure is also observed in the first ionization band, revealing substantial back-bonding into the carbonyls. This band shifts only slightly from cobalt to rhodium. The other predominantly metal ionizations increase substantially in ionization energy from cobalt to rhodium, and the splitting of the ionizations originating from the cyclopentadienyl e₁” pair of orbitals more than doubles. In contrast to these large changes in the valence ionizations from cobalt to rhodium, there are essentially no changes in the core ionizations, the ground-state geometrical structures, the carbonyl stretching frequencies, and other physical properties. The combined data strongly indicate that the changes in valence ionizations are largely dependent upon excited-state effects in the positive ions. These results have implications for the comparative stability and chemical behavior of cobalt and rhodium complexes.