Electronic Structure of Nickel Carbonyl Clusters: Chemical Bonding and Spectroscopy of [Ni₅(CO)₁₂]²⁻, [NI₆(CO)₁₂]²⁻, and [Ni₈C(CO)₁₆]²⁻ Studied by the LCGTO-LDF Method

We report the results of linear combination of Gaussian type orbitals (LCGTO) local density functional (LDF) calculations on the electronic structure of naked and carbonylated Ni clusters. We found substantial differences in the electronic structure and bonding characteristics of the two classes of compounds. In bare Ni clusters the bonding is more metallic, while in the corresponding carbonylated compounds a substantial gap separates the occupied and the virtual levels. As a consequence of carbonylation, the average atomic configuration goes from 3d⁹4sp to (formally) 3d¹⁰, and thus the strength of the metal-metal bond changes. This is supported by the analysis of (a) density of states (molecular orbitals), (b) electron density, and (c) energetics of bare and ligated clusters. These results are discussed in view of the proposed “molecular cluster-surface” analogy. The formation of the dianionic [Ni_n(CO)_m]²⁻ cluster from the corresponding neutral form results in a considerable stabilization. It is shown that, in general, the two extra electrons play an important role in bonding the monomeric units into the oligomeric cluster forms. Finally, the solution optical spectrum of [Ni₆(CO)₁₂]²⁻ is reported and interpreted in terms of one-electron transitions.