Electronic Configuration in Electrodeposition from Aqueous Solutions

Abstract

Consideration of electronic structures of metal ions in aqueous solutions indicates that nmtals 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 overpotentials 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 late, 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 lrans effect.

Cystine as an Addition Agent in the Electrodeposition of Copper

“…in concentration polarization, plus a component of the polarization arising froin the obstructive iilii~~ence of adsorbed cystine or clrstine-copper complexes. Virtual elimination of this component of the polarization by chloricle might be esplained by facilitated electron transfer through an electron bridge, in the manner suggested by I-Ieyrovsliy (5), or by the forination of inore readily clischargeable chlorocystine-copper con~plexes, in accordance with I.,yons' views (6,7). T h e teilclency for the initial activation overpotei~tial in the cystine-chloride system t o attain, with excess chloride, a lower value than that coi-responding to discharge of the aquo-complex, as discussecl previously, gives some support, pel-haps, to the second suggestion.…”

Dissolution of Cadmium in Chromic Chloride Solutions

“…The problem here is similar to that found in the electrodeposition of chromium from aqueous Cr(III) ion, which involves reduction to Cr(II) as the first step. Lyons (11) has pointed out that, in general, halo-aquo ions require tess activation energy than aquo-ions for electroreduction.…”

Cathode Overpotential and Surface-Active Additives in the Electrodeposition of Copper

“…It is generally believed that the effect of addition agents on cathode overpotential (or polarization) during electrodeposition of metals arises primarily because of adsorption of the additive on the cathode surface (1)(2)(3)(4)(5)(6)(7)(8)(9). Possible coordination with the metal ions in solution might also have an effect (6)(7)(8)(10)(11)(12)(13)(14). In addition, the ability of the additive to impede metalion transport in the cathode film could play a more minor role (15).…”