Electronic Configuration in Electrodeposition from Aqueous Solutions

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Abstract
Consideration of electronic structures of metal ions in aqueous solutions indicates that metals are electrodeposited from ions in which coordinate linkages involve only orbitals of the outermost electronic shell. When coordination involves the penultimate shell also, that is, with Taube's “inner orbital” complexes, the metal is not electrodeposited. Apparently the energy required to break such hybridization exceeds that required for cathodic discharge of hydrogen from these solutions. Platinum metals are exceptions, probably because of extraordinary stability of the metallic state for these elements; deposition, however, requires high activation, as shown by over‐ potentials and low current efficiencies. Between different oxidation states, electrolytic oxidation or reduction is irreversible if the electronic configuration must be changed significantly. Irreversible deposition is observed for transition metals or when the ion is bound in a hydrolyzed aggregate. Since inner orbital hybridization is associated with a lack of substitutional lability of the coordinated groups, whereas lability of outer orbital complexes probably results from formation of a dissociation interm iate, it is likely that such an intermediate is also important in the cathode process. These considerations are used to account for and extend the Piontelli electrolytic classification of metals. In considering the effects of anions, it is shown that aquo complexes are reduced with highest irreversibility, which is attributed to high activity of water; the influence of halide ions may be labilization by the trans effect.

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