Phosphato complexes of cobalt(III). V. The anation of diaquobis(ethylenediamine) cobalt(III) by orthophosphates

Abstract

The anation reaction of [Co en₂(OH₂)₂]³⁺ by PO₄^3-, and their protonic analogues, is shown to proceed in two kinetic steps. The first is the rapid equilibrium formation of ion-pairs, followed by a slow substitution step. Ion-pair formation causes a significant shift in the cis-trans equilibrium position at pH 7. The anation rate exhibits a sharp maximum around pH 7, and kinetic studies demonstrate that the anation proceeds predominantly via the trans-[Co en₂OH.OH₂]²⁺.HPO^2-₄ion-pair. The rate of inner-sphere substitution from this ion-pair (k = 0.255 min^-1 at 47.9º, ΔH? = 30.0 ± 0.6 kcal mole^-1 =, ΔS? = +(19 ± 3) e.u.) is 10-20 times slower than the estimated rate of water exchange with trans-[Co en₂OH.OH₂I²⁺. Specific structures are assigned to the ion-pairs and these structures influence rates of substitution. At high phosphate concentrations, there is spectral evidence for the slower formation of diphosphate complexes. The relevance of these kinetic observations to the generation and preparation of phosphato complexes is discussed.