Metal ions reduction mechanism

Although electroless deposition seems to offer greater prospects for deposit thickness and composition uniformity than electrodeposition, the achievement of such uniformity is a challenge. An understanding of catalysis and deposition mechanisms, as in Section 3, is inadequate to describe the operation of a practical electroless solution. Solution factors, such as the presence of stabilizers, dissolved O2 gas, and partially-diffusion-controlled, metal ion reduction reactions, often can strongly influence deposit uniformity. In the field of microelectronics, backend-of-line (BEOL) linewidths are approaching 0.1 pm, which is much less than the diffusion layer thickness for a... 

There is compelling evidence that reducing agent oxidation and metal ion reduction are, more often than not, interdependent reactions. Nonetheless, virtually all established mechanisms of the electroless deposition fail to take into account this reaction interdependence. An alternative explanation is that the potentials applied in the partial solution cell studies are different to those measured in the full electroless solution studies. Notwithstanding some differences in the actual potentials at the inner Helmholtz plane in the full solution relative to the partial solutions, it is hard to see how this could be a universal reason for the difference in rates of deposition measured in both types of solution. 

Pulse radiolysis permits the study, via the time-resolved technique, of the detailed mechanisms of metal ion reduction in different environments (ligand, solvent, support,...),... 

Note that solvated electrons can also be produced by photodetachment of electrons from certain anions or photoionization of molecules using UV excitation.Then the mechanism of metal ion reduction is expected to be quite similar to the radiolytic processes of Figures 2a, 2b and 2c. However, the matrices used as cluster hosts are generally not transparent to light, and the reduction is often restricted to the surface. 

After the initial stage of the metal ion reduction on the catalytic sites, a further metal plating and film growth proceed via the so-called autocatalytic mechanism. Simply, the reduction of metal ions is autocatalyzed by the metal being deposited. 

Metal-ion reduction of both mononuclear and dinuclear carboxylatocobalt(m) complexes have been reviewed. There continues to be substantial interest in this subject and in particular the adjacent-attack mechanism for complexes with simple monocarboxylate ligands now appears to be well understood. The importance of the iimer-sphere mechanism in reductions by Eu + has been amply demonstrated. Gould has also illustrated the usefulness of the comparison of rate data from the reactions of a common oxidant by several reductants. Many workers are currently involved in attempts to measure first-order rates of electron transfer within precursor complexes. In the search for likely systems, the one chosen by Taube ... 

The rates of reduction of a number of pyridine derivatives by and Eu + have been determined from a study of catalysis in the reduction of cobalt(iii) complexes by metal-ion reductants. - If an outer-sphere mechanism were operating in both cases, the ratio of rate constants for and reductions would be around 0.26 whereas the observed ratios are 10 —10 . An inner-sphere mechanism is proposed for the europium(ii) reactions to give Eu + and a pyridine-related radical. Values for the rate constants (Table 5) show considerable variation with substrate structure, but for all are less than 401 mol s and consequently no distinction between an inner-sphere and an outer-sphere process may be made in this case. The relative rates for reduction of bipyridyl derivatives do, however, lie within the range for outer-sphere reactions. 

Alkyl hydroperoxides give alkoxy radicals and the hydroxyl radical. r-Butyl hydroperoxide is often used as a radical source. Detailed studies on the mechanism of the decomposition indicate that it is a more complicated process than simple unimolecular decomposition. The alkyl hydroperoxides are also sometimes used in conjunction with a transition-metal salt. Under these conditions, an alkoxy radical is produced, but the hydroxyl portion appears as hydroxide ion as the result of one-electron reduction by the metal ion. ... 

Various other observations of Krapcho and Bothner-By are accommodated by the radical-anion reduction mechanism. Thus, the position of the initial equilibrium [Eq. (3g)] would be expected to be determined by the reduction potential of the metal and the oxidation potential of the aromatic compound. In spite of small differences in their reduction potentials, lithium, sodium, potassium and calcium afford sufficiently high concentrations of the radical-anion so that all four metals can effect Birch reductions. The few compounds for which comparative data are available are reduced in nearly identical yields by the four metals. However, lithium ion can coordinate strongly with the radical-anion, unlike sodium and potassium ions, and consequently equilibrium (3g) for lithium is shifted considerably... 

Tl(III) < Pb(IV), and this conclusion has been confirmed recently with reference to the oxythallation of olefins 124) and the cleavage of cyclopropanes 127). It is also predictable that oxidations of unsaturated systems by Tl(III) will exhibit characteristics commonly associated with analogous oxidations by Hg(II) and Pb(IV). There is, however, one important difference between Pb(IV) and Tl(III) redox reactions, namely that in the latter case reduction of the metal ion is believed to proceed only by a direct two-electron transfer mechanism (70). Thallium(II) has been detected by y-irradiation 10), pulse radiolysis 17, 107), and flash photolysis 144a) studies, butis completely unstable with respect to Tl(III) and T1(I) the rate constant for the process 2T1(II) Tl(III) + T1(I), 2.3 x 10 liter mole sec , is in fact close to diffusion control of the reaction 17). 

Actually, it is recognized that two different mechanisms may be involved in the above process. One is related to the reaction of a first deposited metal layer with chalcogen molecules diffusing through the double layer at the interface. The other is related to the precipitation of metal ions on the electrode during the reduction of sulfur. In the first case, after a monolayer of the compound has been plated, the deposition proceeds further according to the second mechanism. However, several factors affect the mechanism of the process, hence the corresponding composition and quality of the produced films. These factors are associated mainly to the com-plexation effect of the metal ions by the solvent, probable adsorption of electrolyte anions on the electrode surface, and solvent electrolysis. 

When ceric ions were substituted for chromic acid, the reaction was still zero-order with respect to metal ion, the rate of reduction of which was unchanged. The mechanism favoured by the authors depends on formation of a complex of silver ions and hydrogen, viz. 

The mechanism suggested by these kinetics depends on the simultaneous oxidation of two ions in substrate-metal ion complexes so that free radicals are not produced. A few data on Cr(II) reduction of these unsaturated acids indicate simple second-order kinetics ... 

The major conclusion from the smdies of the mechanism of O2 reduction by CcO is that formation of a peroxo-level intermediate bridging two metal ions is not a prerequisite for four-electron reduction, at least in molecular complexes. 

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