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 NiII–NiIII–NiIV 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+}.