Outer-sphere redox reactions of [CoIII(NH3)5(HxPyOz)](m– 3)– complexes. A temperature- and pressure-dependence kinetic study on the influence of the phosphorus oxoanions

Abstract

Outer-sphere redox reactions between [Co^III(NH₃)₅(H_xP_yO_z)]^{(m– 3)–}(H_xP_yO_z^m–= H₂PO₂^–, H₂PO₃^–, HPO₃^2–, HP₂O₇^3–, P₂O₇^4–, γ-H₂P₃O₁₀^3–, -HP₃O₁₀^4–, -P₃O₁₀^5–, β-H₃P₃O₁₀^2–, -H₂P₃O₁₀^3–, -HP₃O₁₀^4– or -P₃O₁₀^5–) and [Fe(CN)₆]^4– have been studied as a function of pH, H_xP_yO_z^m– oxoanion, temperature and pressure. The effect of the oxidation state, size, geometry and extent of protonation of the H_xP_yO_z^m– oxoanions on the precursor-complex formation constant, electron-transfer rate constant, and thermal and pressure activation parameters has been investigated. The values obtained indicate that all the precursor-complex formation equilibrium constants, K_OS, are the same except for the non-linear β-H₃P₃O₁₀^2–, -H₂P₃O₁₀^3–, -HP₃O₁₀^4– and -P₃O₁₀^5– oxoanions, where the values are consistently larger, indicating that hydrogen bonding plays a very important role. The electron-transfer rate constant for a series of [Co(NH₃)₅(H_xP_yO_z)]^{(m– 3)–}, with linear oxoanions, increases on decreasing the negative charge on the complex {k³⁰⁸= 0.73 × 10^–3 and (8.5–11)× 10^–3 s^–1 for the γ-[Co(NH₃)₅(P₃O₁₀)]^2– and γ-[Co(NH₃)₅(H₂P₃O₁₀)], respectively}. For the non-linear β-P₃O₁₀^5– oxoanions a threshold is observed when the external oxo groups are protonated {k³⁰⁸= 20 × 10^–3 for β-[Co(NH₃)₅(H₃P₃O₁₀)]⁺ species and 0.84 × 10^–3 s^–1 for β-[Co(NH₃)₅(H₂P₃O₁₀)], -[Co(NH₃)₅(HP₃O₁₀)]^– or -[Co(NH₃)₅(P₃O₁₀)]^2–}. The ΔH^‡ values are within the range expected, while those of ΔS^‡ and ΔV^‡ vary considerably with the extent of protonation of the phosphorus oxoanionic ligands, being 13 J K^–1 mol^–1 and + 36 cm³ mol^–1 and 69 J K^–1 mol^–1 and + 13 cm³ mol^–1, respectively for the [Co(NH₃)₅(HP₂O₇)]–[Co(NH₃)₅(P₂O₇)]^– couple. The ΔV^‡ values depend strongly on the oxo group distribution of the oxophosphorus ligand {+ 13 and + 32 cm³ mol^–1 for β- and γ-[Co(NH₃)₅(P₃O₁₀)]^2–, respectively}. Hydrogen bonding and solvent reorganization play a key role in the interpretation of the activation parameters.