The co-ordination number of transition-metal ions. Part VII. An evaluation of steric factors in the stabilisation of high-spin five-co-ordinate nickel(II) complexes of multidendate α-pyridyl ligands

Abstract

Nickel(II) complexes of six new potentially ter- or tetra-dendate α-pyridyl-containing ligands are described. Co-ordination number is assigned on the basis of the physical properties of solids and solutions. Both five- and six-co-ordinate compounds are characterised. The five-co-ordinate compounds, which are all high-spin, are of two kinds, (i) neutral mononuclear complexes containing the N₃X₂(X = halide) set of donor atoms, and (ii) uniunivalent salts in which the complex cation contains the N₄X donor set. It is shown that the potentially tetradendate ligands may function either in a terdendate or in a tetradendate fashion, and a useful i.r. spectral criterion for unco-ordinated pyridyl groups is described. The change in co-ordination number from six to five accompanies a progressive increase in the steric demands of the ligands; in several instances five- and six-co-ordinate complexes co-exist in equilibrium in solution. It is shown that steric effects constitute a major basis, though not a sufficient one, for the stabilisation of the high-spin five-co-ordinate configuration for nickel(II). The polarisability of the anionic ligand is an important associated factor.