Entropy contribution to the nickel(III)/nickel(II) redox change in six-coordinating systems. Investigation of the role of the negative charge of the ligand in the stabilization of high oxidation states of the metal center

Entropy changes ΔS°rc assocd. with the 2 half-reactions (i) [NiIIIL2]3+ + e- = [NiIIL2]2+ and (ii) [NiIIIY]0 + e- = [NiIIY] (L = 1,4,7-triazacyclononane H3Y = 1,4,7-triazacyclononane-N,N',N''-triacetic acid) were detd. through the investigation of the temp. dependence of the E1/2(NiIII/NiII) parameter over the 0.1-3.5M concn. range of the background electrolyte (NaCl). The ΔS°rc for half-reaction (i) is pos., and its value decreases with an increasing concn. of the supporting electrolyte, whereas ΔS°rc for half-reaction ii is neg. and its value increases (becoming less neg.) with an increasing concn. of NaCl. The above results were interpreted in terms of variation in the size of the hydration sphere of the electroactive species, which are related to changes of the elec. charge on the complexes. In particular, it was shown that such polyneg. charged ligands as Y3- favor access to unusually high oxidn. states of the metal center due a very favorable entropy term: this reflects a substantial increase of translational entropy, assocd. with the release of H2O mols. from the hydration sphere of the complex during the [NiIIY]/[NiIIIY]0 oxidn. process