A structure and reactivity analysis of monomeric Ni(II)-hydroxo complexes prepared from water.

The nickel(ii) chemistry with the tridentate ligands bis[(N'-R-ureido)-N-ethyl]-N-methylamine (H(4)(R), R = isopropyl, tert-butyl) is described. The Ni(ii)-OH complexes, [Ni(II)H(2)(R)(OH)](-) were generated using water as the source of the hydroxo ligand. These complexes are pseudo-square planar, in which the primary coordination sphere contains three nitrogen donors from [H(2)(R)](2-) and the oxygen atom from the hydroxide (Ni-O(H), 1.857(1) A). The Ni(ii)-OH unit also is involved in two intramolecular hydrogen bonds between the urea groups of the [H(2)(R)](2-) and the hydroxo oxygen atom. Attempts to deprotonate the Ni(ii)-OH unit to produce Ni(ii)-oxo complexes were unsuccessful. A variety of bases with pK(a) of less than 15 (in DMSO) were unable to deprotonate the hydroxo ligand. Treating the Ni(ii)-OH complexes with KOBu(t) (pK(a) approximately 29) afforded the ligand substitution product, [Ni(II)H(2)(R)(OBu(t))](-). Ni(ii)-siloxide complexes were isolated when the [Ni(II)H(2)(R)(OH)](-) complexes were allowed to react with K[N(TMS)(2)].