Triruthenium clusters containing a methylphenylsulfoximido cap or bridge, Ru 3(CO) 9(μ 2-H)[μ 3-NS(O)MePh] ( 1), Ru 3(CO) 10(μ 2-H)[μ 3-NS(O)MePh] ( 2), Ru 3(CO) 8(μ 3-η 2-CPhCHBu)[μ 3-NS(O)MePh] ( 3), Ru 3(CO) 9(μ 3-η 2-PhCCCCHPh)[μ 2-NS(O)MePh] ( 4), and Ru 3(CO) 7(μ 2-CO)(μ 3-η 2-PhCCCCHPh)[μ 3-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 μ 3-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 NR − 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 μ 2 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.