Reduction of Metal Complexes

For the reduction of metal complexes, the half-wave potential is shifted to more negative potentials (vs. the true metal ion), reflecting the additional energy required for the decomposition of the complex. Consider the reversible reduction of a hypothetical metal complex, MLp ... 

In general, metal nanoparticles are obtained via reduction of metal complexes, such as metal chlorides, by chemical agents (chemical reduction), or by electrons (electrodeposition). Hybrids of metal oxides are obtained by oxidation, network formation or precipitation of precursors such as metal nitrates and acetates [144]. 

Figure 2 Biomimetic synthesis strategy for peptide encapsulated nanoclusters. Synthesis begins with complexation of peptide hgand to metal ion, followed by borohydride reduction of metal complex, nucleation, and formation of peptide stabilized nanocluster. (Reprinted with permission from Ref. 1. 2003 American Chemical Society)...

Formation of Bonds between Hydrogen and Transition Metals 1.10.7. by Reduction of Metal Complexes... 

The direct electrochemical reduction of metal-complexes during the preconcentration step is possible only when a sufficient negative potential is applied to the working electrode. By careful selection of the deposition potential only the most... 

Direct, nonenzymatic reduction of metal complexes by the cell wall or cell membrane of T. weissflogii is evidenced in two ways. First, a nonlinear rate of Cu(BPDS)2 reduction occurs initially after addition of the substrate. Second, this reductive capacity of the cells is sensitive to pretreatment with an oxidant such as Cu(II) (Fig. 3b). These observations demonstrate that the reducing sites are irreversibly consumed and are not replaced or regenerated quickly. The copper pretreatment does not interfere with the cell s ability to reduce metal complexes through an enzymatic pathway. 

The equilibrium constant of (12.1.3) favors the hydrated form. Thus the forward reaction in (12.1.3) precedes the reduction of H2C=0, and under some conditions the current will be governed by the kinetics of this reaction (yielding a so-called kinetic current). Other examples of this case involve reduction of some weak acids and the conjugate base anions, the reduction of aldoses, and the reduction of metal complexes. 

Propargylic radicals are generated on reduction of metal-complex-stabilized Nicholas cations with Zn. This principle, which has been found very recently [21], can be used for ring-closure reactions via intramolecular radical recombination [Eq. (12)]. 

Similar experiments, involving electron transfer between an anion and a neutral molecule, yield relative or absolute EAs. The method has been used to determine relative free energies for electron attachment for a variety of metallocenes and /3-diketonate molecules. Electron photodetachment spectroscopy of negatively charged ions is another source for obtaining electron affinities of molecules. These data provide an important component of thermochemical cycles involving oxidation/reduction of metal complexes, and serve as a basis for obtaining other thermochemical values. 

Table 1 Rate constants and thermodynamic parameters for reduction of metal complexes by chromium(ii), vanadium(ii), iron(ii), and europium(u). Second-order rate constants are quoted in 1 mol s at T = 25 "C unless stated. The units of AH and AS are kcal mol and cal deg mol respectively...

The electrochemical route for the synthesis of metal nanomaterials foresees the electrochemical oxidation-reduction of metal complexes accomplished in a simple two- [76] or three- [77] electrode t3 e cell. The electrodes are immersed in an electrolytic solution basically composed of soft-templating molecules which operate in the reaction domain both as shape-inducing reagents stabilizing and delineating the nanoparticles shape and size and furthermore as supporting electrol3 e [76, 78, 79]. A two-electrode setup is sketched in Fig. 10.3a. 

In the one-step synthesis of FePt nanoparticles, platinum acetylacetonate (Pt(acac)2) and iron pentacarbonyl (Fe(CO)5) and Fe(CO)5 was mixed at excess of stabilizers at 100 °C, then the mixture was heated to more than 200 °C, and kept it at that temperature for Ih, before it was heated to reflux [215, 223]. It was found that with benzyl ether as solvent and oleic acid and oleylamine as stabilizers, one-pot reaction of Fe(CO)5 and Pt(acac)2 could give nanosized FePt particles (3 - 4 nm). Size, composition, and shape of the particles were controlled by varying the synthetic parameters such as molar ratio of stabilizers to metal precursor, addition sequence of the stabilizers and metal precursors, heating rate, heating temperature, and heating duration. Monodisperse FePt nanocrystals were prepared by hydrolysis of pentacarbonyl iron and reduction of metal complexes in the presence of oleic acid and oleylamine [215]. 

A related version of great economic interest is the Fischer-Tropsch process for reductive conversion of carbon monoxide to hydrocarbons. This reaction is catalyzed by a number of metals but cobalt and iron have been most closely studied. The key reaction steps are reduction of metal-complexed carbon monoxide and carbonyl insertion reactions. The hydrocarbon chain is built up by a series of successive carbonyl insertion and reduction steps. 

The reductions of metal complexes by carbon monoxide may proceed via elimination of CO2 from intermediates of the type MCO2H. The oxidation of [Co(CN)2(CO)(PEt3)2l by [Fe(CN)s] in alkaline aqueous solution is shown to proceed via a relatively unstable intermediate, [(NC)6Fe iCNCo KCN)2 (C02H)(PEt3)2]. The overall reaction proceeds quantitatively according to the equation... 

The reduction of metal complexes is accompanied often by dimerization and/or by formation of compounds with metal-metal bonds. For example, the reduction of [FeBr(CO)2(il -C5(p-tol)5)] with zinc powder in... 

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