Bridged electron transfer outer-sphere mechanism

The superb elegance of this demonstration lies in the choice of reactants which permits no alternative mechani.sm. Cr" (d ) and Co" (d ) species are known to be substitutionally labile whereas Cr" (d ) and Co " (low-spin d ) are substitutionally inert, Only if electron transfer is preceded by the formation of a bridged internrediate can the inert cobalt reactant be persuaded to release a Cl ligand and so allow the quantitative formation of the (then inert) chromium product. Corroboration that electron transfer does not occur by an outer-sphere mechanism followed by los.s of CP from the chromium is provided by the fact that, if Cl is added to the solution, none of it finds its way into the chromium product. 

Bridge mediation mechanisms in heterogeneous outer sphere electrochemical reactions has also been theoretically treated using the pull—push and push-pull mechanistic concepts [84]. Schmidt [85] has considered theoretically homogeneous inner sphere bridge electron transfer reactions without atom or ion transfer. Bridge mediation in electron transfer reactions may also involve simultaneous atom or ion transfer. Heyrovsky [86] invoked mediation of electron transfer by formation of bridges to explain the enhancement of the rate of electroreduction of indium (III) ions in the presence of specifically adsorbed halide ions on mercury. 

Like all redox reactions26,28,967,968 those of copper(II) may be divided into two types (a) outer sphere mechanisms involving electron (or proton) transfer between coordination shells that remain essentially intact and (b) inner sphere mechanisms in which the oxidizing and reducing species are connected by a bridging ligand, which is common to both metal ion coordination spheres."9... 

A simpler situation from the point of view of a theoretical treatment -although more difficult to study experimentally - is electron exchange between ions which constitute two halves of the same redox couple, e.g. MnO /MnO -, Co(NH3) +/Co(NH3)6+ etc. Two distinct types of mechanism have been postulated. In the outer-sphere mechanism, the coordination spheres of both oxidant and reductant remain intact as electrons are transferred, and the oxidation numbers of the central atoms change. The inner-sphere mechanism describes a situation where a bridged binuclear complex is formed as an intermediate, and the bridging ligand - which may be Cl-, OH etc. or an ambidentate ligand like NCS" - provides a pathway for electron transfer. 

In an outer-sphere mechanism, electron transfer occurs without a covalent linkage being formed between the reactants. In an inner-sphere mechanism, electron transfer occurs via a covalently bound bridging ligand. 

The choice between inner- and outer-sphere mechanisms is difficult to ascertain. In Table 12.14, the outer-sphere mechanism is imposed by the reducing agent. [Ru(NH3)5f + is an inert species and does not allow formation of bridging species. Although [Cr(bipy)3] is formally labile, the chelate effect may predispose this complex to an outer-sphere mechanism. The delocalized tt systems of the bipy ligands of [Cr(bipy)3] may lower the barrier for outer-sphere electron transfer relative to [Ru(NH3)g] MLCT in [Cr(bipy)3] may facilitate the electron transfer. 

The oxidant can also dictate an outer-sphere mechanism. In Table 12.15, [Co(NH3)6] and [Co(en)3] " participate in outer-sphere electron transfer their ligands have no accessible lone pairs with which to form bonds to bridge with the reductant. The electron transfer mechanisms for the other reactions are less certain, although labile CT (aq) is assumed to react by inner-sphere mechanisms when bridging is possible. 

A bridging ligand reduction model vs. the outer-sphere mechanism for electron transfer has been tested using rate constants from Cr(II) systems. A correlation between the rate constant and the gas-phase electron affinity of the bridging group implies an inner-sphere mechanism. If such a correlation is absent an outer-sphere mechanism is assumed. ... 

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