Some factors governing the spin-multiplicity of five-co-ordinate complexes of cobalt(II) and nickel(II)

Abstract

A survey of the sets of donor atoms now available for five-co-ordinate complexes of cobalt- and nickel-(II) is made. By ordering the different donor sets according to the values of the overall electronegativities or to the sum of the nucleophilic reactivity constants of the donor groups, the cross-over point between high- and low-spin complexes can be located. The capacity of spin-pairing increases with the nucleophilic activity and decreases with the electronegativity of the donor atoms. For anionic ligands this tendency increases in the order: Cl < Br < I < NCS. The low-spin state is favoured for d⁸ over d⁷ complexes. The symmetry of the chromophore is also an important factor in determining the spin multiplicity of the complexes.