Inner-sphere electron transfer parameters

From the temperature variation of the equilibrium constant, thermodynamic parameters for the reaction were also obtained. The extent of formation of [Mo(CO)5l]" was found to be cation-dependent, and while equilibrium constants of 39 and 21 atm L moF were obtained for Bu4P and pyH+, none of the anionic iodide complex was observed for Na. Despite this variation, there seemed to be no correlation between the concentration of [Mo(CO)5l]" and the rate of the catalytic carbonylation reaction. It was proposed that [Mo(CO)5] and [Mo(CO)5l] are spectator species, with the catalysis being initiated by [Mo(CO)5]. Based on the in situ spectroscopic results and kinetic data, a catalytic mechanism was suggested, involving radicals formed by inner sphere electron transfer between EtI and [Mo(CO)5]. 

The reactivity pattern of the second reaction step observed at pH >3, fe2(obs). is very similar to that observed for the reaction between the [Co(P)(N02 )(H20)] complex and NO under similar conditions. Importantly, the observed rate constant for this reaction does not depend significandy on the NO2 concentration, but it depends linearly on the NO concentration. This finding together with the dissociative nature of the activation parameters determined for this reaction leads to the conclusion that in the presence of NO the [Co(P)(N02 )(H20)] complex must exist in equilibrium with small amounts of the [Co(P)(N02 )(NO)] intermediate (see lower part of Scheme 12). The last intermediate slowly decomposes through an inner-sphere electron-transfer reaction to generate the final products [Co (P) (NO )] and NO2. Since the latter reaction represents the rate-determining step for the second reaction, the proposed mechanism seems to be consistent with the observed NO and N02 concentration dependences, as well as with the activation parameters determined in this study (126). 

The rate constant and activation parameters for the electron-transfer reactions are given in Table I. The reaction rate of the Cu/3 complex via the inner-sphere was smaller than that of the Cu/ethylimidazole complex. The coordination of the substrate to Cu(II) ion was enthalpically unfavored as compared to the homogeneous Cu complex. On the other hand, the outer-sphere reaction with Fe(II)(phenanthroline)3 proceeded faster for the Cu/3 system than for the homogeneous Cu/ethylimidazole complex. 3 made a significant favorable entropic contribution to the outer-sphere electron-transfer reaction. 

Only in a limited number of instances will the value of k and its associated parameters be useful in diagnosing mechanism. When the redox rate is faster than substitution within either reactant, we can be fairly certain that an outer-sphere mechanism holds. This is the case with Fe + and RuCP+ oxidation of V(II) and with rapid electron transfer between inert partners. On the other hand, when the activation parameters for substitution and redox reactions of one of the reactants are similar, an inner-sphere redox reaction, controlled by replacement, is highly likely. This appears to be the case with the oxidation by a number of Co(III) complexes of V(II), confirmed in some instanees by the appearance of the requisite V(III) complex, e.g. 

The above treatment neglects nuclear-tunneling effects. The nuclear-tunneling factors calculated for a (hypothetical) electron transfer having the inner-sphere parameters of and [Fe(HjO)jp ions (see Table 1 in 12.2.3.3.4) and no solvent re-... 

A study of the irreversible reduction of several Co ", Rh" and Ir" complexes revealed no correlation between the polarographic Ey and several spectroscopic parameters but, interestingly, it was found that a linear correlation existed for several of the Co " complexes between the y, and In where was the rate constant for homogeneous electron transfer, when [Ru(NH3)6] was used as reductant. The theoretical foundation for this relationship is that Ey is linearly related to In (the heterogeneous rate constant for electrochemical reduction) and, from the theories of Marcus and Hush, the ratio of k for a series of compounds is the same as the ratio of the rate constants k for a constant reductant provided both pathways are outer sphere. The mechanistic implication of the relationship is not clear it may simply mean that both pathways proceed via an outer sphere mechanism as no correlation was found between y, and the values of kgx for reduction by which can undergo homogeneous electron transfer by an inner sphere mechanism. 

The product of the reduction of [Co(bpy)3] by Cr, upon aerial oxidation, is a red dimeric species, postulated to have the structure [(H20)4Cr(/x-OH)2Cr(OH2)2]. This product and the stoichiometry of the reaction suggests a two-electron process, with the bpy ligand serving as a temporary bridging radical. An investigation of the Cr(II) reduction of [Co(pd)3] (pd = pentane-2,4-dione) in water/acetone mixtures reveals outer-sphere, and mono- and di-bridged ([H ] dependent) pathways.The effect of the cosolvent on the activation parameters is observed at an acetone mole fraction of 0.06, at which point its solvation of the activated complex becomes important. The reduction of [Co(en)2(dppd)] (dppd = l,3-diphenylpropane-l,3-dione) by Cr occurs by a multistep mechanism in which the first step is the formation of the [Co(en)2(dppd )] radical, which catalyzes the inner-sphere Co(III)/Cr(II) electron transfer process. " A molecular orbital study indicates that the [Co(en)2(dppd)] reduction likely involves attack of Cr " at the methine carbon of dppd, in contrast to the attack on an oxygen in the [Co(en)(pd)2] reduction. 

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