AZOOXIMATES OF BI-VALENT AND TRI-VALENT NICKEL

The reaction of arylazooximes, RC(NOH)NNPh (HL(R), R = Me or Ph), with nickel(II) acetate tetrahydrate in methanol under anaerobic-condition s afforded [NiL(3)(R)](-) isolated as the NEt(4)(+) salt. One (L(Ph))( -) ligand in [NiL(3)(Ph)](-) underwent facile displacement by L-L liga nds like 2,2'-bipyridine (bipy) furnishing [NiL(2)(Ph)(bipy)]. The Ni- III-Ni-II reduction potential of [NiL(3)(R)](-) in acetonitrile is app roximate to 0.1 V vs. saturated calomel electrode. The trivalent compl ex [NiL(3)(R)] was quantitatively isolated via constant-potential elec trolysis at 0.3 V. The Ni-IV-Ni-III couple of the tris chelate was obs erved near 0.9 V, but the nickel(IV) complex could not be isolated in the solid state. The relatively low metal reduction potential allowing facile preparation of the stable [NiL(3)(R)] system is attributed to the strong-field nature of the oximato-N atom. In going from [NiL(3)(P h)](-) to [NiL(2)(Ph)(bipy)] the Ni-III-Ni-II reduction potential incr eases by approximate to 0.3 V showing that (L(Ph))(-) is a much better stabiliser of Ni-III than is bipy. The crystal structures of [NEt(4)] [NiL(3)(Ph)] and [NiL(2)(Ph)(bipy)] have been determined. The geometry of [NiL(3)(R)] (S = 1/2) was studied with the help of its EPR spectru m (d(z), ground state) in the [CoL(3)(R)] lattice. Both [NiL(3)(R)](-) and [NiL(3)(R)] have exclusive meridional geometry consistent with st eric and angular-overlap considerations. In [NiL(2)(Ph)(bipy)] the two anionic oximato functions are placed in mutually trans positions. The oximato-N ligand displays substantial trans influence. Thus in [NiL(3 )(Ph)](-) the Ni-N (azo) bond lying trans to Ni-N (oxime) is approxima te to 0.05 Angstrom longer than the other two mutually trans Ni-N (ate ) bonds. The average Ni-N (ate) distance in [NiL(2)(Ph)(bipy)] is appr oximate to 0.04 Angstrom shorter than that in, [NiL(3)(Ph)](-) because none of the Ni-N (azo) bonds in the former complex, is subject to the trans influence of Ni-N (oxime). In both complexes the Ni-N (oxime) l engths are significantly shorter than the Ni-N (azo) lengths, I consis tent with stronger Ni-N (oxime) sigma bonding which is also a reason b ehind the: strong-field nature of the oximate ligand.