Synthesis, properties and structures of eight-coordinate zirconium(IV) and hafnium(IV) halide complexes with phosphorus and arsenic ligands

Eight-coordinate [MX4(L - L)(2)] (M = Zr or Hf; X = Cl or Br; L - L = o-C6H4(PMe2)(2) or o-C6H4(AsMe2)(2)) were made by displacement of Me2S from [MX4(Me2S)(2)] by three equivalents of L - L in CH2Cl2 solution, or from MX4 and L - L in anhydrous thf solution. The [MI4( L - L)(2)] were made directly from reaction of MI4 with the ligand in CH2Cl2 solution. The very moisture-sensitive complexes were characterised by IR, UV/Vis, and H-1 and P-31 NMR spectroscopy and microanalysis. Crystal structures of [ZrCl4{o-C6H4(AsMe2)(2)}(2)], [ZrBr4{o-C6H4(PMe2)(2)}(2)], [ZrI4{o-C6H4(AsMe2)(2)}(2)] and [HfI4{o-C6H4(AsMe2)(2)}(2)] all show distorted dodecahedral structures. Surprisingly, unlike the corresponding Ti(IV) systems, only the eight-coordinate complex was found in each system. In contrast, the ligand o-C6H4(PPh2)(2) forms only six-coordinate complexes [MX4{o-C6H4(PPh2)(2)}] which were fully characterised spectroscopically and analytically. Surprisingly the tripodal triarsine, MeC(CH2AsMe2)(3), also produces eight-coordinate [MX4{MeC(CH2AsMe2)(3)}(2)] in which the triarsines bind as bidentates in a distorted dodecahedral structure. There is no evidence for seven-coordination as found in some thioether systems.