Electron counting and bonding analysis in triruthenium clusters containing sulfoximido ligands : true or false electron-deficient systems ?

Triruthenium clusters containing a methylphenylsulfoximido cap or bridge, Ru₃(CO)₉(μ₂-H)[μ₃-NS(O)MePh] (<b>1</b>), Ru₃(CO)₁₀(μ₂-H)[μ₃-NS(O)MePh] (<b>2</b>), Ru₃(CO)₈(μ₃-η²-CPhCHBu)[μ₃-NS(O)MePh] (<b>3</b>), Ru₃ (CO)₉(μ₃-η²-PhCCCCHPh)[μ₂-NS(O)MePh] (<b>4</b>), and Ru₃(CO)₇(μ₂-CO)(μ₃-η²-PhCCCCHPh)[μ₃-NS(O)MePh] (<b>5</b>) have been examined by EHT and DFT calculations in order to analyze the bonding present in the clusters and to establish the electron counting. They clearly show that a μ₃-sulfoximido group is not a 3e⁻ ligand as one may be led to think at first sight, but rather acts as a three-orbital/5e⁻ system, i.e. should be considered as isolobal to an N---R⁻ ligand. Because of some delocalization of its π-type orbitals on the sulfur and oxygen atoms, it is expected to bind slightly less strongly to metal atoms than classical imido ligands. Once in a μ₂ coordination mode, the sulfoximido ligand retains a lone pair on its pyramidalized N atom and becomes a two-orbital/3e⁻ ligand. It follows that clusters <b>1</b>, <b>2</b>, <b>4</b> and <b>5</b> are electron-precise, whereas cluster <b>3</b> is electron deficient with respect to the 18e⁻ rule but obeys the polyhedral skeletal electron pair electron-counting rules. Consistently, all the calculated clusters exhibit large HOMO–LUMO gaps and no trace of electron deficiency can be found in their electronic structures.