Co-ordination chemistry of higher oxidation states. Part 22. trans-Dioxo-osmium(VI) complexes of diamines, diphosphines, and diarsines

Abstract

The reactions of tertiary arsines and diarsines with a mixture of OsO₄, concentrated HCl, and ethanol at low temperatures produce the dioxo-osmium(VI) complexes [OsO₂Cl₂L₂](L = AsEt₃, AsMe₂Ph, or AsPh₃) and [OsO₂Cl₂(L–L)][L–L =o-C₆H₄(AsMe₂)₂,o-C₆H₄(AsPh₂)₂, Ph₂AsCH₂CH₂AsPh₂, or Ph₂AsCHCHAsPh₂]. The corresponding reactions at higher temperatures lead to mixtures containing Os^VI complexes and chloro complexes of Os^III and Os^IV. The diphosphine complexes [OsO₂Cl₂(L–L)][L–L =o-C₆H₄(PMe₂)₂, Me₂PCH₂ CH₂PMe₂,PH₂PCH₂CH₂PPh₂, or Ph₂PCHCHPPh₂] are prepared similarly, but are very difficult to obtain pure, and the bromo complexes [OsO₂Br₂(L–L)](L–L = diphosphine or diarsine) are obtained from OsO₄, concentrated HBr, EtOH, and L–L. NNN′N′-Tetramethylethylenediamine (tmen) produces [OsO₂X₂(tmen)], and [H₂tmen][OsO₂Cl₄] has been characterised as a minor by-product in the case of X = Cl. Bis-ligand complexes [OsO₂(L–L)₂]²⁺ have been obtained only with L–L =o-C₆H₄(PMe₂)₂. The complexes have been characterised by i.r., u.v.–visible, ¹H, and ³¹P n.m.r. spectroscopy and microanalysis. Decomposition in solution is suggested to involve oxygen-atom transfer from the OsO₂²⁺ group to the neutral ligand. The properties and stabilities of the complexes are discussed and the ease of oxidation of the neutral ligands is suggested to be a major factor determining relative stabilities.