Metal-ligand complexes, oxidation potentials

Most polypyridine complexes of second- and third-row transition metals also display a predominantly metal-localized oxidation at positive potentials which are chemically either reversible or partly reversible. Further one-electron oxidations often occur at more positive potentials in liquid SO2 [57]. The first oxidation potential depends on the metal atom (for example, Ru > Os), the ancillary hgands in [M(W)(X)(Y)(Z)(N,N)j or [M(X)(Y)(N,N)2] and, also, on the structure of the polypyridine ligand. Empirically, oxidation potentials can be calculated using additive Lever electrochemical parameters which quantify the influence of the metal atom and individual hgands on metal-centered redox couples [9, 157, 220]. 

The dendrimer-type tetranuclear Ru(II)-Os(II)3 complex (22, protonated form) shows an interesting electrochemical behavior due to the presence of free basic sites in its bridging ligands [41]. The protonated form shows a 3-1 oxidation pattern due to the simultaneous oxidation of the three Os-based units, followed by the one-electron oxidation of the Ru-based unit. On addition of base, the six chelating moieties (three on the Ru center and one on each Os center) undergo deprotonation. This causes changes in the oxidation potential of the metal ions, with a consequent switching from 3-1 to 1-3 in the oxidation pattern. 

On oxidation of mononuclear complexes in the potential window <+1.6 V, only one metal-based process is observed. The oxidation potential depends strongly on the nature of the metal ion (Os is oxidized at less positive potentials compared to Ru ) and, less dramatically, on the nature of the coordinated ligands. Because of the previously discussed electronic properties of the isolated components and of the stabilization of the LUMO of 2,3-dpp and 2,5-dpp on coordination to a second metal center, it can be expected that the oxidation potential of the metal-containing building blocks increases in the order ... 

In this expression, y is the charge of the complex, A is a constant that depends upon the metal, the solvent, and the reference electrode, and (d /djc)L can be considered as a measure of the L ligand effect, that is, the variation of the oxidation potential per each CO replacement by L. A separation between the ligand and the metal center effects is also evident. 

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