Geometrical and linkage isomers of [OSCl2(dmso)4] -: The complete picture

The isomerisation between the three known isomers of the [OsCl2(dmso)(4)] complex - trans-[OsCl2(dmso-S)(4)] (1), cis,-fac-[OsCl2(dmso-S)(3)(dmso-O)] (2), and cis-[OsCl2(dmso-S)(4)] (3) - has been investigated by NMR spectroscopy, X-ray crystallographic analysis, and DFT calculations. We show that the two dmso-linkage isomers 2 and 3 equilibrate slowly at room temperature in solutions of both dimethyl sulfoxide (DMSO) and light-protected chloroform. Although crystals of 2 precipitate from DMSO solutions, crystals of 3 were obtained from chloroform solutions. Compound 2 isomerises to 1 after exposure to sunlight, while, 1 transforms into 2 in hot DMSO solution. The molecular energies were calculated by DFT methods, which show that in the gas phase there are small differences in the total energies (E-t), with 1 exhibiting the lowest energy, in contrast with its low population in solution. This result indicates that the stability of each isomer in solution is not simply determined by E-t, but that the solvent plays an important role. In fact, quantum chemical calculations for the isomerisation process 1 --> 2 show a marked increase in the equilibrium constant with increasing the solvent polarity. The calculation of the metal-dmso binding energies, in combination wit h the X-ray data, shows a significantly higher strength of Os - S bonds with respect to Ru - S bonds, which explains the different behaviour of otherwise analogous osmium-dmso and ruthenium-dmso complexes. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003).