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-B-12 dusters 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 B-12 moiety coordinating to the metal atom. The B-12 ligand is found to have similar structure as the bare B-12 cluster with C-3v 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 B-12 half sandwiches reveal two sets of sigma bonds on the boron unit: nine classical two-center two-electron (2c-2e) sigma bonds on the periphery of the B-12 unit and four 3c-2e sigma bonds within the boron unit. Both sigma and pi bonds are found between the metal and the B-12 ligand: three M-B single sigma bonds and one delocalized 4c-2e pi bond. The exposed metal sites in these complexes can be further coordinated by other ligands or become reaction centers as model catalysts.