Density functional study of linkage isomerism in dimethyl sulfoxide Ru(III) and Rh(III) complexes

The different stability of S and O bonded dimethyl sulfoxide Ru(III) and Rh(III) complexes, [MClmL5−m(DMSO)]n, has been studied by the density functional method (B3LYP/6-31 G(d,p)). The calculated binding energies indicate that the S-bonded isomers are more stable than the O-isomers when L is a σ donor ligand, while the stability order is reversed when L is a π accepting ligand. This trend can be rationalized in terms of the absolute electronegativity of the square-pyramidal complex [MClmL5−m]n, rather than in terms of its global softness. S and O bonding are definitely favored, respectively, by highly negative and highly positive electronegativities. The trend of the MS and SO distances indicates the absence of significant π back-bonding M→DMSO, even in the case of σ donors, in contrast with the behavior of Ru(II) complexes.