Geometrical and Linkage Isomers of [OsCl 2 (dmso) 4 ] − The Complete Picture

The isomerisation between the three known isomers of the [OsCl 2 (dmso) 4 ] complex — trans ‐[OsCl 2 (dmso‐ S ) 4 ] ( 1 ), cis , fac ‐[OsCl 2 (dmso‐ S ) 3 (dmso‐ O )] ( 2 ), and cis ‐[OsCl 2 (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 with 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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)