ELECTRONIC-STRUCTURE OF METAL DIMERS - PHOTOELECTRON-SPECTRA AND MOLECULAR-ORBITAL CALCULATIONS OF DICARBONYL-BRIDGED AND DINITROSYL-BRIDGED COBALT, RHODIUM, AND IRIDIUM CYCLOPENTADIENYL DIMERS
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Gas-phase, ultraviolet photoelectron spectra, and molecular orbital calculations are reported for Cp*2M2(-CO)2 (Cp* = C5(CH3)5; M = Co, Rh, Ir) and Cp2M2(-NO)2 (Cp = C5H5; M = Co, Rh). Comparison between series of calculations and series of spectra enable us to reassign the frontier region of the spectra as well as more thoroughly assign the metal and cyclopentadienyl regions of the spectra. Our results suggest that the two lowest energy ionizations, which occur from frontier orbitals of bg and bu symmetry, reverse their order in the dicarbonyl-bridged Co dimer relative to the dicarbonyl-bridged Rh and Ir dimers. Although others have suggested that Cp2Co2(NO)2 has a triplet ground state, our spectra rule out this possibility for both Cp2Co2(NO)2 and Cp2Rh2(NO)2. Also discussed is the correlation of molecular orbital theory and valence bond theory in the description of the interfragment bonding in these dimers. Only by counting 5 carbonyl to metal donations and metal to 2 carbonyl donations as contributing to metal-metal bonds can we arrive at the formal single and double metal-metal bonds required by the 18-electron rule for these dimers. Overlap populations suggest weak metal-metal bonds and strong metal-bridging ligand bonds for all of these dimers. 1988, American Chemical Society. All rights reserved.
