Characterisation of edge-sharing decahalogenodiosmate complexes in multiple oxidation states; synthesis, magnetochemistry, voltammetry and associated spectroelectrochemistry of [PPh4]2[Os2(µ-X)2X8] (X = Br or Cl)†

Abstract

Organo-soluble salts of the edge-sharing bioctahedral [Os₂(µ-X)₂X₈]^2– complexes (X = Br or Cl) have been prepared by an improved procedure and investigated electrochemically. Their solution redox behaviour spans five levels, [Os₂(µ-X)₂X₈]^z– (z = 1–5), complementing pentavalent [Os₂(µ-Cl)₂Cl₈] known only in the solid phase. Chilled solutions of electrogenerated [Os₂(µ-X)₂X₈]^3– and [Os₂(µ-X)₂X₈]^4– have been characterised by UV/VIS/NIR spectroscopy (213 K; 45 000 to 5000 cm^–1). In contrast, [Os₂(µ-X)₂X₈]^– and [Os₂(µ-X)₂X₈]^5– rearrange rapidly to the face-sharing nonahalides, [Os₂(µ-X)₃X₆]^– and [Os₂(µ-X)₃X₆]^4–, losing ½ X₂ or X^– as appropriate. The redetermined magnetic moments of the [Os₂(µ-X)₂X₈]^2– dimers (as their [PPh₄]⁺ salts) are convergent at 1.52 and 1.48 µ_B per Os (296 K) for X = Br and Cl respectively. Cleavage reactions of [Os₂(µ-X)₂X₈]^z– (X = Br or Cl) are valence-dependent, leading to [OsX₅L]^– (L = MeCN) for z = 2, [OsX₆]^2– and trans-[OsX₄L₂]^– for z = 3, and exclusively trans-[OsX₄L₂]^– for z = 4.