Complexes between Planar Boron Clusters and Transition Metals: A Photoelectron Spectroscopy and Ab Initio Study of CoB₁₂^– and RhB₁₂^–

Small boron clusters are known to be planar, and may be used as ligands to form novel coordination complexes with transition metals. Here we report a combined photoelectron spectroscopy and ab initio study of CoB₁₂^– and RhB₁₂^–. Photoelectron spectra of the two doped-B₁₂ clusters show similar spectral patterns, suggesting they have similar structures. Global minimum searches reveal that both CoB₁₂^– and RhB₁₂^– possess half-sandwich-type structures with the quasi-planar B₁₂ moiety coordinating to the metal atom. The B₁₂ ligand is found to have similar structure as the bare B₁₂ cluster with <i>C</i>_3<i>v</i> symmetry. Structures with Co or Rh inserted into the quasi-planar boron framework are found to be much higher in energy. Chemical bonding analyses of the two B₁₂ half sandwiches reveal two sets of σ bonds on the boron unit: nine classical two-center–two-electron (2c–2e) σ bonds on the periphery of the B₁₂ unit and four 3c–2e σ bonds within the boron unit. Both σ and π bonds are found between the metal and the B₁₂ ligand: three M–B single σ bonds and one delocalized 4c–2e π bond. The exposed metal sites in these complexes can be further coordinated by other ligands or become reaction centers as model catalysts