Outer-sphere redox reactions of [CoIII(NH3)5(H x P y O z )](m? 3)? complexes. A temperature- and pressure-dependence kinetic study on the influence of the phosphorus oxoanions

Abstract

Outer-sphere redox reactions between [CoIII(NH3)5(HxPyOz)](m– 3)–(HxPyOzm–= H2PO2–, H2PO3–, HPO32–, HP2O73–, P2O74–, γ-H2P3O103–, -HP3O104–, -P3O105–, β-H3P3O102–, -H2P3O103–, -HP3O104– or -P3O105–) and [Fe(CN)6]4– have been studied as a function of pH, HxPyOzm– oxoanion, temperature and pressure. The effect of the oxidation state, size, geometry and extent of protonation of the HxPyOzm– 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, KOS, are the same except for the non-linear β-H3P3O102–, -H2P3O103–, -HP3O104– and -P3O105– 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(HxPyOz)](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-P3O105– 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.