Ligand effects upon the reactions of Ni(II) with sodium tetrahydroborate: Ni(I) complexes of bipyridyl and 1,10-phenanthroline

Abstract

The reactions between NaBH4 and Ni(II) have been studied in the presence of a variety of ligands in an effort to determine (a) the conditions under which reduction occurs, (b) the extent of the reduction (e.g. to Ni(I), Ni(0) complexes, or to Ni metal or boride), and (c) whether intermediate Ni complexes can be isolated.With ligands having no π-bonding capabilities (NH3, ethylenediamine, edta, citrate), reduction depends upon the Ni:ligand ratio and, in the presence of an excess of the ligand, reduction of Ni(II) is very slow. When vacant coordination sites exist on the metal through dissociation of, for example, Ni(NH3)62+, allowing for the interaction of the BH4 group with the metal, then rapid reduction to the metal (or boride) takes place.With the π-bonding N-donor ligands 1,10-phenanthroline (phen) and 2,2′-bipyridyl (bipy) reduction of the stable ML32+ complexes probably occurs via an outer-sphere electron transfer mechanism but, in these cases, new species of the type NiL2X (L = phen, bipy; X = BH4, PF6, BPh4), which formally contain Ni(I), have been isolated.