Metals/metalloids reduction-oxidation reactions

Figure 19. Transformations of Fe(II, III) at an oxic anoxic boundary in the water or sediment column (modified from Davidson, 1985). Peaks in the concentration of solid Fe(III) (hydr)oxides and of dissolved Fe II) are observed at locations of maximum Fe(III) and Fe(II) production, respectively. The combination of ligands and Fe(ll) produced in underlying anoxic regions are most efficient in dissolving Fe(III) (hydr)oxides. Redox reactions of iron—oxidation accompanied by precipitation, reduction accompanied by dissolution—constitute an important cycle at the oxic-anoxic boundary which is often coupled with transformations (adsorption and desorption) or reactive elements such as heavy metals, metalloids, and phosphates.

Cross-coupling reactions all have very similar mechanisms. The first step is oxidative addition to Pd of an R-X species followed by transmetallation of an R group from an R -metal/metalloid species. Reductive elimination of R-R from palladium completes the cycle. 

We have found that the main group metal and metalloid reductants mocury, bismuth, and antimony are highly effective " in reducing WCIe or M0CI5 at surprisingly lower temperatures than commonly used in the solid-state synthesis of early transition metal cluster halides. BorosUicate ampules can be substituted for the more expensive and less easily sealed quartz ampules at these lower temperatures, and the metals and metalloids are not as impacted by oxide coatings that inhibit sohd-state reactions with more active metals. These lower temperatures may allow access to kinetic products, such as trinuclear clusters, instead of thermodynamic products. 

Changes in the oxidation state of trace metals can occur depending on the redox condition of the environment. Redox reactions are thus important in influencing the chemical speciation of a number of metals and metalloids, notably Hg, As, Se, Cr, Pu, Co, Pb, Ni, and Cu (Oscarson et al., 1981 Bartlett and James, 1993 Alloway, 1995 Myneni et al., 1997 Huang, 2000 James and Bartlett, 2000 Adriano, 2001 Sparks, 2003). Redox reactions also exert a great influence in the transformation and reactivity of Fe and Mn oxides in soils tliat have an enormous capacity to adsorb metals and metalloids (Huang and Germida, 2002). Furthermore, reduction of sulfate to sulfide in an anerobic environment also affects... 

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