Disproportionation mechanism homogeneous solution reactions

In homogeneous solutions NO disproportionation maybe promoted by transition metal complexes, and a variety of mechanisms seem to be available owing to the many possible modes of coordination. One example is the reaction of NO with nickel carbonyl shown in Eq. (35) (76),... 

Another general problem, perhaps with somewhat more impact on homogeneous solution chemistry, is whether the reactions of radical ions with nucleophiles or electrophiles, depending upon the charge, go directly via the radical ions or if reactive doubly charged ions must first be formed by disproportionation. The reaction of thianthrene radical cation with water (Murata and Shine, 1969 Shine and Murata, 1969) is a typical example which was proposed to involve the disproportionation mechanism (71 and 72). The... 

The disproportionation reaction (Eq. 2) of two tetrazolinyl radicals was studied by Umemoto [18a] and it was concluded that this reaction is a slow process, and therefore this process should also be ruled out as a fast d-step. Importantly, the difference between the redox potentials of cR and R (AEp = 0.25 V) favors the backward reaction. The feasibility of the backward reaction is substantiated by ESR experiments by Maender and Russell [20] who found that the mixture of formazan and tetrazolium salt gave rise to tetrazolinyl radicals. Finally, the solution electron transfer (Eq. 3) is possible as a homogeneous electron transfer (d-step) since it would be reasonable to expect that the redox potentials of the reacting species are very close. However, this reaction would imply dEp/dlogv slope of 19.7 mV (Table 1) which was not observed. Taking all the arguments into account it can be concluded that the mechanism shown in... 

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