Kinetic study of formation of [Co(H2PO4)(H2O)5]2+ at various acidities and ionic strengths

Abstract

The reaction of [Co(H₂O)₆]³⁺ with the system H₃PO₄/H₂PO₄^– to yield the 1 : 1 complex [Co(H₂PO₄)(H₂O)₅]²⁺ has been studied in aqueous solution at 25 °C and at different acidities and ionic strengths. The reactivity of this complex has been found to be ca. 4 orders of magnitude greater than that of [Co(NH₃)₅(H₂O)]³⁺. The results are interpreted in terms of a dissociatively activated conjugate-base substitution with an enhanced rate due to a rapid low-spin ↔ high-spin equilibrium for the [Co(H₂O)₆]³⁺ complex. As for other similar cobalt–phosphate systems, only substitution activity due to H₂PO₄^– has been detected. The first acidity constant of [Co(H₂O)₆]³⁺ has also been measured as a function of ionic strength.