Complexes of nickel(II) with 2,2′-bis(dimethylarsino)biphenyl. Four-, five-, and six-co-ordinate systems

The chelating diarsine 2,2′-bis(dimethylarsino)biphenyl (dmab) forms a series of complexes with NiII of composition [NiX2(dmab)2](X = Cl, Br, I, or NCS) which are assigned a pseudo-octahedral structure on the basis of spectroscopic and magnetic-moment data. In solution in organic solvents the complexes dissociate to varying extents to form five-co-ordinate species. In addition, a series of diamagnetic five-co-ordinate complexes of composition [NiX(dmab)2][ClO4](X = Cl, Br, or I) have been prepared, and are assigned a trigonal-bipyramidal structure on the basis of electronic spectra. The diarsine also forms a diamagnetic square-planar complex [Ni(dmab)2][ClO4]2 which on treatment with iodide ion is converted into the five-co-ordinate [Nil(dmab)2][ClO4].