Three H₂O₂ molecules are involved in the "fenton-like" reaction between Co(H₂O)₆ ²⁺ and H₂O₂

Co(II) complexes and Co(H₂O)₆²⁺ are used as catalysts in advanced oxidation processes. Therefore it was decided to study the kinetics of reaction of Co(H₂O)₆²⁺ with H₂O₂. Surprisingly, the kinetic results point out that the process involves three consecutive reactions, each of them requiring an H ₂O₂ molecule, i.e. three H₂O₂ molecules ligate to the central cobalt cation prior to the formation of radicals. DFT analysis suggests that the transient (H₂O) ₃Co^II(OOH)₂(H₂O₂) decomposes via: (H₂O)₃Co^II(OOH) ₂(H₂O₂) → (H₂O) ₃Co^II(OOH)(^̇OOH)(OH) + ^̇OH ΔG⁰ = -5.975 kcal mol^-1, with no evidence for the formation of a Co(iii) transient. It is proposed that analogous mechanisms are involved whenever the redox potential of the central cation is too high to enable the reaction: M(H₂O)₆ⁿ⁺ + H ₂O₂ → M⁽ⁿ⁺¹⁾⁺_aq + ^̇OH + OH^-.