Applying the Jellium model to octacarbonyl metal complexes

The recently reported octacarbonyl metal complexes M(CO)₈ (M = Ca, Sr, Ba) feature interesting bonding structures. In these compounds, the bond order is 7, while accommodating 8 lone pairs of ligands in forming octa-coordinated complexes or ions. Here, by comparing [Ba(CO)₈]^2- and metal clusters of [BaBe₈]^2- analogically, we demonstrate that the Jellium model can not only be applied on metal clusters, but is also a useful tool to understand the electronic structures of [M(CO)₈]^q (M, q = Ca, 2-; Sc, 1-; Ti, 0; V, 1+; Cr, 2+; Ba, 2-). By applying the Jellium model, we find that a 20-e model with the configuration |1S²|1P⁶|1D¹⁰|1F²| is an appropriate description of the valence bonding structures of M(CO)₈ species, where each coordinative bond contains 7/8ths of the bonding orbitals and 1/8th non-bonding orbitals.