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

Triruthenium clusters containing a methylphenylsulfoximido cap or bridge, R u-3(CO)(9)(mu (2)-H)[mu (3)-NS(O)MePh] (1), Ru-3(CO)(10)(mu (2)-H)[mu (3)-N S(O)MePh] (2), Ru-3(CO)(8)(mu (3)-eta (2)-CPhCHBu)[mu (3)-NS(O)MePh] (3), R u-3(CO)(9)(mu (3)-eta (2)-PhCCCCHPh)[mu (2)-NS(O)MePh] (4), and Ru-3(CO)(7) (mu (2)-CO)(mu (3)-eta (2)-PhCCCCHPh)[mu (3)-NS(O)MePh] (5) have been exami ned 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 mu (3)-sulfoximido group is not a 3e(-) ligand as one may be led to thin k at first sight, but rather acts as a three-orbital/5e(-) system, i.e. sho uld be considered as isolobal to an N-R- ligand. Because of some delocaliza tion of its pi -type orbitals on the sulfur and oxygen atoms, it is expecte d to bind slightly less strongly to metal atoms than classical imido ligand s. Once in a Ir, coordination mode, the sulfoximido ligand retains a lone p air on its pyramidalized N atom and becomes a two-orbital/3e(-) ligand. Tt 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 polyhe dral skeletal electron pair electron-counting rules. Consistently, all the calculated clusters exhibit large HOMO-LUMO gaps and no trace of electron d eficiency can be found in their electronic structures. (C) 2001 Elsevier Sc ience B.V. All rights reserved.