Electronic structures of tetrahedral iron, cobalt, and nickel clusters. Partial quenching of magnetism in partially carbonylated derivatives

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Abstract
Spin-polarized calculations of tetrahedral Fe4, Ni4, and Co4 clusters have been performed by the self-consistent field discrete variational Xα method. Partially carbonylated C3v clusters Co4(μ−CO)3 and (OC)3CoCo3(μ−CO)3 have also been examined to explore how magnetic moments vary with cluster coordination environment. The bare tetrahedral clusters are magnetic containing 12, 6, and 2 unpaired electrons for Fe4, Co4, and Ni4, respectively. Exchange splittings also decrease (2.68, 1.60, and 0.56 eV) along this series. Analysis of density of states plots for these clusters supports the notion that 4s and 4p mixing into the metal d band yields hybrid orbitals better suited for metal–metal bonding. Electronic transitions are predicted to occur in the near IR absorption spectra of the M4 clusters, a region yet to be studied experimentally. Calculations for C3v Co4(μ−Co)3 yield six unpaired electrons with a large spin density on the unligated apical cobalt. In (OC)3CoCo3(μ−CO)3, where the apical cobalt is coordinately saturated, all the spin density (four unpaired electrons) is localized in the basal plane. Bridging CO groups in the basal plan are antiferromagnetically coupled to cobalt.