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

Ab initio calculations were done on the dinuclear linear diamagnetic [Cl5Ru ORuCl5](4-) anion. The optimized geometry agrees with the X-ray structure. The energy sequence obtained for the molecular orbitals explains the diamag netic properties of this anion and allows the assignment of the electronic transitions. The lowest allowed energy for the electronic transition is ass igned as pi (*)(Ru-O-Ru) <-- pi (Ru.Cl). The calculated bond order between the ruthenium atoms and the oxygen in the Ru-O-Ru mu -oxo bridge is close t o 2. This supports the idea that this fragment is linear. The results show that the LUMO-e, has antibonding character between the ruthenium atoms and the oxygen. This helps to explain why compounds in which the ruthenium atom s have oxidation states lower than IV are not linear. As the LUMO is popula ted 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 t he symmetry, the degeneracy of the e(u) orbital is removed and this stabili zes any additional electrons. (C) 2001 Elsevier Science B.V. All rights res erved.