KINETIC-STUDY OF FORMATION OF [CO(H2PO4)(H2O)5]2+ AT VARIOUS ACIDITIES AND IONIC STRENGTHS

The reaction of [Co(H2O)6]3+ with the system H2PO4/H2PO4- to yield the 1:1 complex [Co(H2PO4)-(H2O)5]2+ has been studied in aqueous solution at 25-degrees-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(NH2)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.