Reduction/redox organometallic chemistry

It is usually difficult to determine or impose activities, since activation coefficients are nearly always unknown for the redox systems investigated in organic or organometallic chemistry. Thus formal reduction potentials E° are measured, when feasible, rather than... 

The pentafulvalenes have a rich organometallic chemistry, which is the result of the redox relation between pentafulvalene (101) and its reduction product, the bi(cyclopentadienyl) dianion (102). The latter can also be regarded as the product of a double deprotonation of a dihydrofulvalene. In many aspects, this chemistry resembles that of cyclopentadienyl complexes. However, there are a number of features unique to the system of two coupled cyclopentadienyl systems, in particular the chemistry of bimetallic complexes with and without intermetallic bonding and their redox processes [90]. A number of review articles have appeared [91-97] here, only the key aspects and actual developments are summarized. 

Modern electrochemical methods provide the coordination chemist with a powerful means of studying chemical reactions coupled to electron transfer and exploiting such chemistry in electrosynthesis. In addition, the electrochemical generation of reactive metallo intermediates can provide routes for the activation of otherwise inert molecules, as in the reduction of N2 to ammonia,50 and for electrocatalyzing redox reactions, such as the reduction of C02 to formate and oxalate,51 the oxidation of NH3 to N02-,52 and the technologically important oxidation of water to 02 or its converse, the reduction of 02 to water.53 Electrochemical reactions involving coordination compounds and organometallic species have been extensively reviewed.54-60... 

Figure 7 illustrates the electrochemial redox chemistry in acetronitrile for several coordination complexes of iron [Fe (MeCN)4, Fe CL, and Fe (acac)s (acac = acetylacetonate)] in relation to that for two iron organometallics [Fe (Cp)2 and Fe (CO)s (iron-pentacarbonyl) both stable 18-electron systems]. In MeCN, Fe (MeCN)4" is the only charged species of the group. It is reversibly oxidized (II/III couple E1/2, -I-1.6 V vs SCE). The uncharged Fe Cb molecule is reversibly reduced (Ill/n couple Ei/2, -1-0.2 V vs SCE) to giveFe Cl, which is reduced by an irreversible two-electron process to iron metal (Ep,c -L5 V vs SCE). The more basic Fe (acac)3 molecule is reversibly reduced (ni/n couple Ei/2, -0.7 V vs SCE), but does not exhibit a second reduction peak. The III/II reduction potentials for these three coordination complexes are a measure of their relative electrophilicity (Lewis acidity). 

An account of the redox chemistry of binuclear palladium complexes and the role of binuclear intermediates in Pd-catalysed oxidation reactions has been provided. Stoichiometric organometallic studies of the oxidation of binuclear Pd(II) complexes to binuclear Pd(III) complexes and subsequent C-X reductive elimination from the resulting binuclear Pd(III) complexes, which confirmed the viability of C-X bond-forming reactions mediated by binuclear Pd(III) complexes, has been described. The effect of ligand modification on the structure and reactivity of binuclear Pd(III) complexes has been presented to highlight the impact that ligand structure can exert on the structure and reactivity of binuclear Pd(III) complexes." ... 

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