The linear μ-oxo-bis[pentachlororuthenate(IV)] anion. Molecular orbital calculations

Ab initio calculations were done on the dinuclear linear diamagnetic [Cl5RuORuCl5]4− anion. The optimized geometry agrees with the X-ray structure. The energy sequence obtained for the molecular orbitals explains the diamagnetic properties of this anion and allows the assignment of the electronic transitions. The lowest allowed energy for the electronic transition is assigned as π*RuORu←πRu,Cl. The calculated bond order between the ruthenium atoms and the oxygen in the RuORu μ-oxo bridge is close to 2. This supports the idea that this fragment is linear. The results show that the LUMO-eu has antibonding character between the ruthenium atoms and the oxygen. This helps to explain why compounds in which the ruthenium atoms have oxidation states lower than IV are not linear. As the LUMO is populated the double bonds to the oxygen atom tend to a single bond character and the linear arrangement is no longer required. In addition, on decreasing the symmetry, the degeneracy of the eu orbital is removed and this stabilizes any additional electrons.