Reactions Proceeding by Mechanisms of Uncertain Classification

The early lore on the rate ofFe++ —Fe+ + + electron exchange is confused by paradoxes 19,68, 79,132,136). Partly because of this, the problem challenged a large number of skillful experimenters, and a general growth of activity in the entire field ensued. The first definitive experiments were done by Dodson (S ) his results are consistent with those of Linnenbom and Wahl (79) and with those of Betts et al. (19), but not with those of van Alten and Rice (132), or those of Kierstead (68). 

In Table VII is a summary of the data which have been obtained on the Fe++ — Fe+++ reaction. The observation that the values of E are so nearly the same for a variety of paths has inclined some authors (112) to the view that a common process, assumed to be H-atom transfer between the hydration spheres, is taking place for all the systems. This view has been strengthened by the observation made by Hudis and Dodson (62) that the specific rate coefficients for the terms 

Since the Fe(III) complexes in question are substitution labile, the composition of the activated complex does not establish its geometry, nor can the form of the rate law (FeX++) (Fe++), for example, even be taken to mean that Feflll) brings the group X into the activated complex. Thus, for the rate term (FeX++) (Fe++), all of the following formulations will satisfy equally the kinetic requirements  

Observations made by Lewis et al. (75), Anderson and Bonner 6), and others (9B), for systems of the charge type -1-2, -fS, show that when substitution in the first sphere of coordination of the oxidizing agent is excluded, Cl does not exert its usual catalytic effect. Thus it seems likely that, in the Fe++ — Fe+ + + systems, activated complexes (a) and (c) can 

At best, the kind of argument used here can indicate that a major part of the reaction takes place by a particular reaction path. Based as it is on gross comparisons of rate, it does not make possible an assessment of the contribution to the total reaction of rival, but minor, paths. And yet, only such a complete description can be regarded as satisfactory. To take a particular example, we are perhaps prepared to accept the conclusion that with X = F a part of the reaction takes place by the bridged activated 

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