Electron transfer bridged

Electrogenerated monovalent Co complexes of the well-known open chain N202 Schiff base ligands salen (8), salphen (9), and their substituted derivatives undergo oxidative additions with alkyl halides. Reactions of the complex with substrates within the series RBr (R = Pr, Bu, t-Bu) proceed at different rates. The reaction occurs by an inner-sphere alkyl-bridged electron transfer, with a Co1- R+- X-transition state, which is sensitive to distortions of the complex in different configurations.124... 

Of course, in inner sphere bridged electron transfer reactions, one of the steps in the mechanistic sequence is the substitution of one of the complexes into the coordination shell of the second. Thus, if the electron transfer process that follows is fast enough, the substitutional step may become rate determining. I am not sure that there is any clear cut evidence that this is the case for any systems actually examined. I did cite a case of a bridged electron transfer reaction that proceeded with a rate constant of 109—i.e.,... 

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. 

Other binuclear complexes with clearly defined structures have been characterized as products of bridged electron transfer reactions, as for example (NC)5Fe(II)CNCo(III)(edta)4 from the reaction of Fe(CN)h3 and Co(II)(edta)2- 63), and (H20)5Cr(III)ClIr(III)Cl5 from Cr2 +- aq and IrCl62 " 132). Charge transfer spectra and photo-electron transfer reactions have recently been reported 141a). 

Although the reaction in equation (2) has long been a paradigm for efficient, single-atom-bridged electron-transfer reactions, few simple model complexes exist that have a similarly linked donor and acceptor. A problem in such systems is that ligand-substitution rates tend to be very large when the d <7-orbitals are partly or fiiUy occupied. ... 

Ligand replacement by CO followed by disproportionation leads to oxidation-reduction because the exchange is followed by ligand-bridged electron transfer between the two species first formed. This behavior has been observed for CN replacement in [Co(CN)5] to give [Co(CN)3(CO)2] and [Co(CN)6] whereas only replacement without disproportionation takes place for K[Pd(CNXCO)] formed from K2[Pd(CN)2] . ... 

Conceivably, redox reaction (g) [and Eq. (e) as well] could proceed via a chloride-bridged, electron-transfer step. ... 

STRONGLY COUPLED, BRIDGED ELECTRON-TRANSFER SYSTEMS... 

Figure 10 Comparison of orbital and electronic state diagrams for three-state, bridged electron transfer...

The vast majority of studies of bridged electron transfer systems have employed polyatomie bridging ligands. This very important class of electron transfer systems is discussed in Sections 7.11.4.4 and 7.11.4.5. 

Figure 12 Correlation of absorption maxima and oscillator strengths for bridged electron transfer systems...

The range of behavior and the complexities of bridged electron transfer systems is overwhelming. This section has attempted an internally consistent organization and treatment of the properties of these systems that encompasses the full range of the topic. Time, space, and personal bias have dictated appreciable selectivity in the systems discussed. It has not been possible to cite all the important contributions or even to do full justice to every aspect of the cited observations. 

The effect is described as synergistic , i.e. the two catalysts together are more effective than either one alone, and this is explained by the fact that isonicotinamide has an effective lead-in group for a bridged electron transfer, so that step 1 is faster than the reaction V + Eu +, and the redox potentials of the catalysts are such that the steady-state concentration of is greater than that of... 

Ion pairs are formed in [Fe(CN)6] reduction of [Co(NH3)sL] complexes where L is a substituted pyridine ligand. The magnitudes of the association constants, Table 2.2, are consistent with approaches of [Fe(CN)6] to the coordinated-ammonia side of the oxidant. The electron transfer rates are relatively insensitive to structure once thermodynamic forces have been accounted for and are similar to corresponding rates for bridged electron transfer. 

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