Abstract
Gas-phase He(I) and He(II) photoelectron spectra of the complexes (η)-C 5H 5)M(PMe 3)X (M=Co, Rh; X=CO, CS) and (η-C 5Me 5Rh(PMe 3)CO are reported and discussed in terms of energy sequence and composition of the valence molecular orbitals. Differences between cobalt and rhodium in bonding to the ligands are observed, rhodium interacting more strongly with the orbitals of the cyclopentadienyl ring, but being less involved in π-back donation to the empty π ★ levels of the PMe 3 and X ligands. The commonly accepted opinion that CS is a better σ-donor and π-acceptor than CO is consistent with the spectroscopic data. Differences in reactivity between the “half sandwich” complexes (η-C 5R 5)ML 2 (L = CO, PR 3) are related to the observed trends in ionization energies of the HOMO.
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