Phosphato complexes of cobalt(III). II. Reversible chelation and ring-opening reactions

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Abstract
The bidentate phosphato complex Co en2PO40; has been shown to exist in rapid reversible equilibrium with the monodentate complex Co en2OH.HPO40 A detailed kinetic analysis of the opposed ring-opening and ring-closing (or chelation) reactions has evaluated rate constants for each of the protonated forms in acid and basic media. The ring-opening reaction is subject to both acid and base catalysis. However, within the pH range 6-9, the bidentate form containing a four-member phosphate ring is thermodynamically more stable than the monodentate form. This is ascribed to a rapid chelation reaction arising from a favourable cis hydrogen-bonded transition state.