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] (1), Ru3(CO)10(μ2-H)[μ3-NS(O)MePh] (2), Ru3(CO)8(μ3-η2-CPhCHBu)[μ3-NS(O)MePh] (3), Ru3(CO)9(μ3-η2-PhCCCCHPh)[μ2-NS(O)MePh] (4), and Ru3(CO)7(μ2-CO)(μ3-η2-PhCCCCHPh)[μ3-NS(O)MePh] (5) 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 1, 2, 4 and 5 are electron-precise, whereas cluster 3 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.