Redox-induced electron rearrangements

There are many examples of redox-induced structural rearrangements in organometallic complexes the reader is referred to a review by Connelly. The simplest rearrangement is cis/trans or fac/mer isomerization, usually induced by oxidation. Thus, cw-[Mn(CO)2(dppe)2] slowly converts to the trans isomer with a rate constant of 10 s at room temperature. Upon oxidation, the cis — trans isomerization increases in rate by 7 powers of 10. The process is not catalytic, so that stoichiometric oxidation followed by reduction is required to synthetically utilize the increased reactivity of the radicals in the conversion of 18-electron cis-[Mn(CO)2(dppe)2l to frani-[Mn(CO)2(dppe)2]. An example of oxidatively induced fac mer isomerization is given in Scheme 10. The fac mer reaction for the neutral 18-electron isomer is slow, with 2 = 2 X 10 s , K2 = 4. The reaction fac mer is much faster and... 

It is interesting to note that [Fe3(CO)9(p.3-EtC2Et)], which has the alkyne orthogonal to one metal-metal bond of the trinuclear cluster, also undergoes redox-induced rearrangement two-electron reduction gives the dianion [Fe3(CX))9(li3-EtC2Et)]2- in which the alkyne is parallel to the same metal-metal bond. ... 

When covalently attached to electron transfer active subunits, the DHA-VHF couple can facilitate chemical and physical switching of electronic properties, as a result of photochemically induced rearrangement accompanied by a change in the redox potential. An interesting example of such a switching system is the compound containing a dihydroazulene component and a covalently attached anthraquinone moiety.1311 This system is able to act as a multimode switch, assisted by various processes such as photochromism, reversible electron transfer, and protonation-deprotonation reactions (Scheme 8). 

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