Co-ordination chemistry of higher oxidation states. Part 13. Synthesis and properties of alkali-metal hydroxo-oxo-osmate(VIII) compounds and the molecular structure of Cs[O4Os(µ-OH)OsO4]

Abstract

The reaction of concentrated aqueous solutions of caesium or rubidium hydroxide with osmium tetraoxide, in various ratios produced the osmates(VIII), M₂[OsO₄(OH)₂], M[OsO₄(OH)], and M[O₄Os(OH)OsO₄](M = Rb or Cs). Potassium and sodium hydroxide produced only K₂[OsO₄(OH)₂] and Na₂[OsO₄(OH)₂]·2H₂O, respectively. Vibrational spectra show the [OsO₄(OH)₂]^2- ions to have cis-octahedral structures. The structure of the title complex was established by a single-crystal X-ray study and shown to be monoclinic, space group Cc, with a= 14.304(4), b= 5.798(1), c= 10.527 (4)Å, β= 109.92(3)°, and Z= 4. The structure was refined to R= 0.059 from 710 reflections. The anion contains distorted trigonal-bipyramidal co-ordination (O₅) about the osmium atoms, which are linked via a single bent hydroxo-bridge Os–O (H)–Os, 133(1)°. The terminal OsO bonds lie in the range 1.62(4)—1.77(3)Å and Os–OH 2.21 (2) and 2.22(2)Å. Vibrational spectra are reported for M[OsO₄(OH)], M[O₄Os(OH)OsO₄](M = Cs or Rb), and for barium and strontium osmates(VIII), and possible structures proposed.