Metal redox rearrangements

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The first reduction, which is likely centred on the Cr(III)— Cr(II) process, has substantial electrochemical reversibility (A2sp = 70 mV, at 0.2 V s-1), thus suggesting that no significant structural rearrangements accompany such redox step. Unfortunately, no further investigations have been carried out to clarify whether the successive reductions are centred on the metal or on the ligand. 

Rearrangements of clusters, i.e. changes of cluster shape and increase and decrease of the number of cluster metal atoms, have already been mentioned with pyrolysis reactions and heterometallic cluster synthesis in chapter 2.4. Furthermore, cluster rearrangements can occur under conditions which are similar to those used to form simple clusters, e.g. simple redox reactions interconvert four to fifteen atom rhodium clusters (12,14, 280). Hard-base-induced disproportionation reactions lead to many atom clusters of rhenium (17), ruthenium and osmium (233), iron (108), rhodium (22, 88, 277), and iridium (28). And the interaction of metal carbonyl anions and clusters produces bigger clusters of iron (102, 367), ruthenium, and osmium (249). 

Ascorbic Acid Is Required to Maintain the Enzyme that Forms Hydroxyproline Residues in Collagen Vitamin B12 Coenzymes Are Associated with Rearrangements on Adjacent Carbon Atoms Iron-Containing Coenzymes Are Frequently Involved in Redox Reactions Metal Cofactors Lipid-Soluble Vitamins... 

The chemistry of cluster complexes, e.g. of the sort [FeitSi, (SR) i,] 2, is of particular interest since such complexes are known to be close representations or synthetic analogues of the redox centres present in various iron-sulphur proteins. It is important to know whether the valence electrons are localized or delocalized in such complexes - in fact several studies by e.s.r., n.m.r., and, more recently, resonance Raman spectroscopy have shown that such clusters are delocalized rather than trapped-valence species. This result is linked with the most important biophysical property of iron-sulphur proteins, viz. that of electron transfer. Rapid electron transfer is possible if any consequential geometric rearrangements around the metal atom sites are small, as implied by many resonance Raman results on such cluster complexes (cf. the small-displacement approximation, which provides a basis for enhancement to fundamental but not to overtone bands) (22). Initial studies of [MSi,]2- ions (M = Mo or W) (23,24) have since been supplemented by studies of dinuclear species e.g. [(PhS)2FeS2MS2]2 (25) and cluster species... 

Another possible two-electron mechanism involves the direct transport of two electrons from a mononuclear transition metal complex to a substrate (S). Such a transport alters sharply the electrostatic states of the systems and obviously requires a substantial rearrangement of the nuclear configuration of ligands and polar solvent molecules. For instance, the estimation of the synchronization factor (asyn) for an octahedral complex, with Eq. 2.44 shows a very low value of asyn = 10 7to 10 8 and, therefore, a very low rate of reaction. The probability of two-electron processes, however, increases sharply if they take place in the coordination sphere of a transition metal, where the reverse compensating electronic shift from the substrate to metal occurs. Involvement of bi- and, especially, polynuclear transition metal complexes and clusters and synchronous proton transfer in the redox processes may essentially decrease the environment reorganization, and, therefore, provide a high rate for the two- electron reactions. 

The free-radical activity is conceived as being generated by an "internal redox" reaction, in which the central metal ion is reduced to a lower valency state (in this case cobalt(II)) by an electron transfer from the ligand, the latter then acquiring a free-radical site by electronic rearrangement. Polymerisation then proceeds by propagation from the free-radical site so generated. 

These possibilities arise because of the presence of species, such as water and surfactant, with which the primary radical can Interact. Three possibilities are Illustrated in Figure 18. In each case, the radical activity is associated in the first Instance with a species in which cobalt(III) has been reduced to cobaltCII) and the acetylacetonate ligand has rearranged to give a free radical on the methylenic carbon atom. In the first possibility, the monomer reacts directly with this species, and propagation then proceeds in the normal way. The consequence of such a mechanism would be that the polymer produced would contain both cobalt (albeit perhaps more loosely bound than in an acetylacetonate) and a moiety derived from acetylacetone. In the second possibility, the species which results from the Internal redox reaction interacts with another molecule in the reaction system (such as water) in such a way that the radical-bearing entity is displaced from the metal complex. 

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