Co-ordination chemistry of higher oxidation states. Part 40. Spectroscopic, electrochemical and structural studies of cationic osmium-(III) and -(IV) diphosphine and diarsine complexes

Abstract

The osmium(III) complexes trans-[Os(L–L)₂X₂]BF₄[X = Cl or Br; L–L =o-C₆H₄(PMe₂)₂, o-C₆H₄(AsMe₂)₂, o-C₆H₄(AsMe₂)(PMe₂), o-C₆F₄(AsMe₂)₂, Ph₂PCH₂CH₂PPh₂, Me₂PCH₂CH₂PMe₂, Ph₂AsCHCHAsPh₂, o-C₆H₄(PPh₂)₂ or o-C₆F₄(PPh₂)₂] have been prepared by dilute HNO₃ oxidation of the osmium(II) analogues in aqueous HBF₄. Concentrated HNO₃ converts most of these complexes into Os^IV, but the unstable trans-[Os(L–L)₂X₂][ClO₄]₂ were isolated as solids only for L–L =o-C₆H₄(PMe₂)₂, o-C₆H₄(AsMe₂)₂ or o-C₆H₄(AsMe₂)(PMe₂). Cyclic voltammetry reveals that the Os^II–Os^III and Os^III–Os^IV couples are generally reversible. Assignments of the UV/VIS spectra of the osmium-(III) and -(IV) complexes are proposed. The X-ray structure of trans-[Os{o-C₆H₄(AsMe₂)₂}₂Cl₂]ClO₄ is reported: monoclinic, space group C2/m, a= 13.890(2), b= 10.381(2), c= 11.682(3)Å, β= 113.86(2)°, Z= 2, and with R= 0.039. Osmium L_III-edge (and where appropriate bromine K-edge) extended X-ray absorption fine structure data for the complexes trans-[Os{o-C₆H₄(AsMe₂)₂}₂X₂]ⁿ⁺(n= 0–2) are reported and the trends in Os–As and Os–X bond lengths with changing osmium oxidation state are discussed.