Chelation reduction-oxidation potentials

If the potential becomes too negative at the cathode, the Ni ion from our alloy sample might commence to deposit and form an alloy deposit. Note that the order of selectivity is not always that predicted by the standard potentials in the activity series. Control of the pH and the chelates or complexing agents in a solution may alter the reduction-oxidation potentials from those of an uncomplexed species. 

Metal Deactivators. The abiUty of metal ions to catalyse oxidation can be inhibited by metal deactivators (19). These additives chelate metal ions and increase the potential difference between the oxidised and reduced states of the metal ions. This decreases the abiUty of the metal to produce radicals from hydroperoxides by oxidation and reduction (eqs. 15 and 16). Complexation of the metal by the metal deactivator also blocks its abiUty to associate with a hydroperoxide, a requirement for catalysis (20). 

ReCl3(PPh3)(benzil)] reacts with bipy and related ligands or terpy to form a number of rhe-nium(III) and rhenium(II) compounds which are useful precursors for the synthesis of lower-valent rhenium complexes. " Thus, reduction of [Re(bipy)3][PF6]2 with zinc amalgam results in the rhenium(I) compound [Re(bipy)3][PF6] in excellent yields. The corresponding terpyridyl bis-chelate [Re(terpy)2][PF6] has been prepared in a similar manner. " The electrochemistry of the products provides a convenient measure of the chemical reactivity associated with the redox processes. Thus, the one-electron oxidation of [Re(bipy)3]" is reversible at -0.33 V, whereas the Re"/Re" redox couple is irreversible and occurs at relatively low potentials (-1-0.61 V) which is consistent with the instability of [Re(bipy)3] + in solution. However, in the presence of a small coordinating molecule such as CNBu, oxidation to the rhenium(III) state is readily available by the formation of seven-coordinate complexes of the composition [Re(bipy)3(L)]. " ... 

This is to be regarded as an overall reaction, and the individual steps are not discussed by Beck. The really potential-dependent reaction is then the electrochemical reduction of the oxidized metal chelate ... 

A similar mechanism could operate in the reduction of oxygen on chelate catalysts, as in the organic cathodes with air regeneration described by Alt, Binder, Kohling and Sandstede 13-40>. These cathodes contain a reversible insoluble quinone/hydroquinone system. The quinone, which is electrochemically reducible, can be obtained either by electrochemical oxidation or by purely chemical oxidation with H2O2 or oxygen (air). A cathodic current is observed in these systems only at potentials below the redox potential, and unusually hard current/ voltage characteristic curves are obtained. 

The earliest high-oxidation-state complex of nickel reported was the heteropoly(molybdate) (132, 133) complex [NilvMo90 32]6. which contains nickel(IV) in an octahedral Ni06 coordination environment. There is no evidence for the corresponding nickel(III) species but further work on nickel(IV) complexes of this type has been reported recently (134). Nickel(III) can be prepared in a six-coordinate oxygen donor environment (135) as a tris chelate with 2,2 -bipyridine-l,T-dioxide (bpy02). The complex has a rhombic EPR spectrum and a reduction potential of 1.7 V, from which an estimate of the reduction potential of the ion [Nini(H20)6]3+ of 2.5 V (versus nhe) has been calculated. 

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