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

Article abstract of DOI:10.1039/dt9960002665

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(H₂P₃O ₁₀)], 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(H₃P₃O ₁₀)]⁺ species and 0.84 × 10^-3 s^-1 for β-[Co(NH₃)₅(H₂P₃O ₁₀)], -[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(NH ₃)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.

Full text of DOI:10.1039/dt9960002665

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