Journal of the Chemical Society, Dalton Transactions p. 2665 - 2671 (1996)
Update date:2022-08-04
Topics:
Martinez, Manuel
Pitarque, Mari-Angel
Van Eldik, Rudi
Outer-sphere redox reactions between [CoIII(NH3)5(HxPyO z)](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(H2P3O 10)], 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(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.
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