Preparation and isomerization of dicarbonyldihalogeno[o-phenylenebis-(dimethylarsine)]osmium complexes

Abstract

The preparation of the series of osmium(II) complexes [Os(pdma)X₂(CO)₂][X = Cl, Br, or I; pdma =o-phenylenebis(dimethylarsine)] starting from [OsX₆]^2– is reported, together with their physical and chemical properties. For X = I all three theoretically possible geometrical isomers of the octahedral complex [these being with trans-CO and cis-l ligands, (I); with trans-l and cis-CO ligands, (II); and with cis-CO and cis-l ligands (III)] have been obtained, and the interconversions (I)→(II)(in 2-methoxyethanol as solvent at 104 °C), (I)→(III)(in nitrobenzene at 104 °C), and (II)→(III)(in ethylene glycol at 165 °C) demonstrated. The thermodynamic stability of the isomers lies in the order (I) < (II) < (III). Based on studies of the interconversions at various temperatures, in various solvents, and in the presence of added ligands, it is suggested for [Os(pdma)l₂-(CO)₂] that the transformation (I)→(II) in 2-methoxyethanol takes place via a rhombic-twist mechanism and the transformation (I)→(III) in nitrobenzene by formation of a five-co-ordinate intermediate with unidentate pdma. For X = Cl or Br only isomers (II) and (III) could be obtained, and the conversion (II)→(III) demonstrated.