Phosphonate complexes. Part 6. Influence of steric effects, solvation, and chelation on stability

Abstract

The interaction of some phosphonic acids RPO(OH)₂, chosen to introduce different controlling factors (R = Bu^t, cyclo-C₆H₁₁, Ph, n-C₁₀H₂₁, Me₂NCMe₂, CH₂SEt, CH₂CH₂CO₂^–), with Ca^II and Cu^II has been investigated by potentiometry at 25 °C, I= 0.1 mol dm^–3(K[NO₃]). The selection of the species and the refinement of the stability constants illustrates the applicability of a new version of the program MUCOMP. Examination of linear free-energy plots indicates that factors other than polar effects are of significance, i.e. chelate formation with the potentially bidentate ligands and particularly solvation effects which preferentially stabilize the complexes of α-branched-chain phosphonates. Other features (steric hindrance, predominance of some microscopic forms) are discussed.