Metal deposition and dissolution

On a liquid metal electrode all surface sites are equivalent, and the deposition of a metal ion from the solution is conceptually simple: The ion loses a part of its solvation sheath, is transferred to the metal surface, and is discharged simultaneously; after a slight rearrangement of the surface atoms it is incorporated into the electrode. The details of the process are little understood, but it seems that the discharge step is generally rate determining, and the Butler-Volmer equation is obeyed if the concentration of the supporting electrolyte is sufficiently high. For example, the formation of lithium and sodium amalgams [1] in nonaqueous solvents according to: . . .Li + + e- ⇌ Li(Hg) Na+ = e- ⇌ Cd(Hg) . . . (10.1) obey the Butler-Volrner equation with transfer coefficients that depend on the solvent. On the other hand, the deposition of multivalent ions may involve several steps. Thus, the formation of zinc amalgam from aqueous solutions, with the overall reaction: . . . zn2+ + 2e- ⇌ Zn (Hg) . . . (10.2) occurs in two steps: First, Zn2+ is reduced to an intermediate Zn+ in an electron transfer step, and then the univalent ion is deposited [2]. In contrast, the surface of a solid metal offers various sites for metal deposition. Figure 10.1 shows a schematic diagram for a crystal surface with a quadratic lattice structure. A single atom sitting on a flat surface plane is denoted as an adatom; several such atoms can form an adatom cluster. A vacancy is formed by a single missing atom; several vacancies can be grouped to vacancy clusters. Steps are particularly important for crystal growth, with kink atoms, or atoms in the halfcrystal position, playing a special role. When a metal is deposited onto such a surface, the vacancies are soon filled. However, the addition of an atom in the kink position creates a new kink site; so at least on an infinite plane the number of kink sites does not change, and the current is maintained by incorporation into these sites. Similarly metal dissolution takes place predominantly at half-crystal positions, since the removal of a kink atom creates a new kink site.

Compounds of Cobalt(III) With trans-6,13-Diamino-6,13-dimethyl-1,4,8,11-tetraazacyclotetradecane

Compounds are described of cobalt(III) with the macrocycle trans-6,13-diamino-6,13-dimethyl-1,4,8,11-tetraazacyclotetradecane ( diam ) of the types [Co(diam-N6)]3+, with diam in octahedral coordination, with the primary amino substituents coordinated trans; [Co(diam-N5)X]2,3+ and [Co(diamH-N5)X]3,4+, with diam in square pyramidal pentacoordination , with a ligand X,X-coordinated trans to the primary amino substituent and with the non-coordinated amino substituent neutral or monoprotonated, respectively; and trans-[Co(diamH-N4)X2]2+ and trans-[Co(diamH2-N4)X2]3+, with diam in planar tetracoordination, with trans ligands X-, and with one or both amino groups protonated , respectively. The diam and [Co(H2O)6](ClO4)2 in water exposed to the air react to sequentially form [Co( diam )]2+, trans-[{Co(diam-N5)}2(O2)]4+, trans-[Co(diam-N5)(OH)]2+ and [Co(diam-N6)]3+ cations. The μ- peroxo or hydroxo compounds can be used to prepare other N5-cations including [Co(diamH-N5)(H2O)]4+, [Co(diamH-N5) Cl ]3+ and [Co(diam-N5) Cl ]2+. [Co(diamH2-N4)]4+ formed by reduction of [Co(diam-N6]3+ with zinc amalgam in hydrochloric acid reoxidizes in air to form trans-[Co(diamH2-N4)Cl2]3+ and a [Co(diamH-N5) Cl ]2+ cation isomeric with that obtained from the hydroxo compound. The N4-tetra- and N5-penta-coordinated forms are stable in acid solution, but in base convert sequentially into the N6-hexacoordinated form in equilibrium with trans-[Co(diam-N5)(OH)]2+. Preparations, spectroscopic properties, and interconversion reactions of compounds of these cations, and some bromo, thiocyanato and acetato analogues are reported.