Co-ordination chemistry of higher oxidation states. Part 32. Synthetic, spectroscopic, and electrochemical studies of some trans-[OsL2X4]–[OsL2X4]– systems (L = PR3, AsR3, SbR3, SeMe2, or pyridine; X = Cl or Br). Crystal structure of [PPh4][Os(PEt3)2Cl4]

Abstract

The preparations of trans-[OsL₂X₄](L = PEt₃, PPh₃, PEt₂Ph, PEtPh₂, AsEt₃, AsPh₃, SbPh₃, SeMe₂, or pyridine; X = Cl or Br) are described. Reduction of these with ascorbic acid in ethanol in the presence of PPh₄⁺ or NEt₄⁺ ions (and in other ways) leads to [OsL₂X₄]^– ions. Both series of complexes were characterised by analysis, i.r., and u.v.–visible spectroscopy, and magnetic measurements. The paramagnetic [Os(PR₃)₂X₄] show sharp ³¹P-{¹H} n.m.r. resonances with very large low-frequency shifts (ca.–1200 p.p.m.). Assignments are proposed for the major features in the u.v.–visible spectra in terms of intraconfigurational, d–d, and ligand-to-metal charge-transfer transitions. Cyclic voltammetry has been used to study the osmium(IV)–osmium(III) couples in these systems, and the redox potentials correlated with the energies of the lowest charge-transfer transitions in the u.v.-visible spectra. The X-ray structure of [PPh₄][trans-Os(PEt₃)₂Cl₄] has been determined: space group P2₁/n with a= 12.225(2), b= 17.882(1), c= 18.551(2)Å, β= 103.22(1)°, Z= 4; least-squares refinement converged to R= 0.053 (R′= 0.048). The anion has approximate D_4h symmetry with Os–Cl 2.375(3)–2.386(2)Å and Os–P 2.371(3), and 2.386(3)Å.