Octahedral and trigonal-bipyramidal complexes of osmium(II) with 1,2-bis(dicyclohexylphosphino)ethane. Stereochemistry of addition reactions to five-co-ordinate osmium(II) derivatives and to their ruthenium(II) analogues

Abstract

The complexes cis- and trans-[OsX₂(dcpe)₂][X = Cl or Br; dcpe = 1,2-bis(dicyclohexylphosphino)-ethane] have been prepared, and their physical and chemical properties investigated. The cis isomers were found to dissociate quickly in polar solvents to give [OsX(dcpe)₂]⁺, which have been isolated in the solid state as the tetraphenylborate salts. On the basis of ³¹P n.m.r. and u.v.–visible spectra, the d⁶ cations are assigned a trigonal-bipyramidal structure with the halide in an equatorial position. The unsaturated complexes [OsX(dcpe)₂]⁺ readily and stereoselectively add halide or neutral ligands L (CO, MeCN, or PhCN) in dichloromethane solution to form the corresponding cis derivatives. The cis-[OsX(L)(dcpe)₂]⁺ cations can be thermally converted into the trans isomers. The analogous [RuX(dcpe)₂]⁺ species, previously reported to give trans addition with X^– and L, were found to form cis-[RuX₂(dcpe)₂] and cis-[RuX(L)(dcpe)₂]⁺ at –60 °C in dichloromethane or acetone solution. Also these species are thermally converted into the thermodynamically stable trans isomers. The complexes cis-[RuX₂(dcpe)₂] were isolated, and the stereochemistry of the various six-co-ordinate complexes of osmium(II) and ruthenium(II) established by ³¹P n.m.r. and i.r. spectroscopy. The cis–trans interconversion of these osmium(II) and ruthenium(II) isomeric pairs in various solvents was also examined.