Characterization of inner- and outer-sphere complexes by thermodynamics and absorption spectra. Part 1. Sulphato-complexes of the first-row transition elements

Abstract

The sulphato-complexes of the transition-metal ions Mn^II, Fe^II, Co^II, Ni^II, and Cu^II have been shown to be predominantly outer sphere, and [Fe(SO₄)]⁺ has been classified as inner sphere, for I= 0 and 5 mol dm^–3 at 25 °C. At I= 0, a linear free-energy relatron (I.f.e.r.) between log K₁for the formation of the sulphato- and for the mono-hydroxo-complexes allows the classifications to be rationalised. The I.f.e.r. also allows the sulphate ligand to be tentatively identrfied as unidentate in the inner-sphere complexes from comparisons with n-dentate ligands (n= 1–3 ). The Fuoss equation adequately describes the K₁ values for those complexes of sulphate and other ligands that are predominantly outer sphere in character. The inner- and outer-sphere character of the species at I= 5 mol dm^–3, deduced from spectrophotometric measurement of the formation constants, is similar to the pattern at I= 0. Large decreases in the entropy of formation of both inner- and outer-sphere species are attributed mainly to the reduced influence of the sulphate ligand on the structure of the medium.