Co-ordination chemistry of higher oxidation states. Part 17. Synthesis and properties of nickel(IV) complexes with neutral ligands, and structures of [Ni{o-C6H4(EMe2)2}2Cl2][ClO4]n(E = P or As; n= 1 or 2) by nickel K-edge extended X-ray absorption fine structure

Abstract

Pseudo-octahedral nickel(IV) complexes [Ni(L–L)₂X₂][ClO₄]₂[X = Cl or Br; L–L =o-C₆H₄(PMe₂)₂, o-C₆F₄(PMe₂)₂, o-C₆H₄(PMe₂)(AsMe₂), or o-C₆H₄(AsMe₂)₂] have been prepared by HNO₃–HX oxidation of [Ni(L–L)₂X₂], followed by treatment with HClO₄, and [Ni(Me₂PCH₂CH₂PMe₂)₂Cl₂][BF₄]₂ by Cl₂–CCl₄ oxidation of [Ni(Me₂PCH₂CH₂PMe₂)₂][BF₄]₂. The complexes have been characterised by elemental analysis, i.r., u.v.–visible, and e.s.r. spectroscopy and magnetic measurements. Cyclic voltammetry has been used to study the Ni^II–Ni^III–Ni^IV redox potentials as a function of L–L. Extended X-ray absorption fine structure of the nickel K-edge has led to the first coordination sphere bond lengths for the title complexes: [Ni{o-C₆H₄(PMe₂)₂}₂Cl₂]⁺(Ni–P = 2.25, Ni–Cl = 2.41 Å), [Ni{o-C₆H₄(AsMe₂)₂}₂Cl₂]⁺(Ni–As = 2.34, Ni–Cl = 2.42 Å), [Ni{o-C₆H₄(PMe₂)₂}₂Cl₂]²⁺(Ni–P = 2.225, Ni–Cl = 2.275 Å), [Ni{o-C₆H₄(AsMe₂)₂}Cl₂]²⁺(Ni–As = 2.39, Ni–Cl = 2.27 Å), which are interpreted as evidence for metal centred oxidation {n= 1 →n= 2 in [Ni(L–L)₂X₂]ⁿ⁺}.