Oxidation and Reduction of Co-ordinated Ligands

A few studies have appeared dealing with substitutions brought about by oxidation and reduction of co-ordinated ligands. Mention has already been made in this section of the reactions of co-ordinated azides, amides, and isocyanates with NO+. The reverse process (attack at co-ordinated NO+) is postulated to have a reactivity pattern which correlates with the i.r. N—O stretching frequencies. Co-ordinated NO+ behaves as an electrophile if the N—O stretching frequency is less than 1886 cm and the force constant is less than 13.8 mdyn A . A study of the rate of reaction of cis- and /ra -[Co(en)2(N02)2]+ ions with hydrazoic acid has also appeared ... 

Fe complex of 2,2 -bipyridine-3,5 -dicarboxylic acid but the pyridine-4-carboxylic acid was observed not to be photosensitive in the presence of Fe ions. The photochemistry of low-spin Fe complexes with macrocyclic ligands continues to be a field that attracts interest. Irradiation of the charge-transfer bands of [Fe(TIM)(OMe)(MeOH)] + (TIM = 2,3,9,10-tetramethyl-l,4,8,ll-tetraazacyclotetradeca-l,3,8,10-tetraene) and of [Fe(DMG)2(OMe)(MeOH)] (DMG = dimethylglyoxime), was found to lead to oxidation of the co-ordinated MeOH and reduction of the metal centre. These photoreactions have been attributed to the population of methoxy-to-Fe CT states. 

Perhaps the simplest reaction to envisage is the alkylation of a co-ordinated amine. These reactions are well-known and usually occur under strongly basic conditions. It is most likely that these reactions involve deprotonated amido intermediates, and are considered in that context. As we have seen in Chapter 2, the acidity of an amine proton should increase upon co-ordination to a metal centre, and with the charge on that metal. As a consequence, we might expect to see new types of reaction products derived from the amido ligand, particularly with high oxidation state metal complexes. The former effect is indeed the case, and dramatic reduction of the pKa of ammonia and amines is observed upon co-ordination to a metal ion (Table 5-1). 

The crystal structure of bis(NN-di-isobutyldithiocarbamato)nickel(ii). [Ni(S2-CNBu 2)2], shows that nickel is approximately square planar and co-ordinated by two symmetric bidentate ligands (Ni—S = 2.20 A) the ligand symmetry approximates to 2- The reduction mechanism of a series of nickel(ii) dithiocarbamates has been investigated in DMSO at the mercury electrode it is claimed to involve a dissociation to a nickel species which is more easily reduced than the nickel(ii) dithiocarbamate. An e.p.r. study of the reversible electrochemical reduction of nickel(ii) diethyldithio-carbamates in the presence of 2,2 -bipyridyl show that a bipy radical anion is formed initially. Ligand alkylation occurs when ao -dibromo-o-xylene is added to bis-(NiV-diethyldithiocarbamato)nickel(ii). The electron-transfer properties of 16 nickel(ii) dithiocarbamate complexes have been studied in acetone at a platinum electrode. Their oxidation is difficult and irreversible the overall process is ... 

Meerwein-Ponndorf-Verley-Oppenauer (MPVO) reactions are usually mediated by metal alkoxides such as Al(0/-Pr)3. The activity of these catalysts is related to their Lewis-acidic character in combination with ligand exchangeability. The mechanism of these homogeneous MPVO reactions proceeds via a cyclic six-membered transition state in which both the reductant and the oxidant are co-ordinated to the metal center of the metal alkoxide catalyst (Scheme 1). The alcohol reactant is co-ordinated as alkoxide. Activation of the carbonyl by co-ordination to Al(III)-alkoxide initiates the hydride-transfer reaction from the alcoho-late to the carbonyl. The alkoxide formed leaves the catalyst via an alcoholysis reaction with another alcohol molecule, usually present in excess [Ij. 

Addition of hydrogen to Wilkinson s catalyst promotes oxidative addition of hydrogen. Dissociation of a bulky phosphine ligand and co-ordination of the alkene is followed by stepwise stereospecific cis transfer of the two hydrogen atoms from the metal to the alkene by way of an intermediate with a carbon-metal bond (7.26). Diffusion of the saturated substrate away from the transfer site allows the released complex to combine with dissolved hydrogen and repeat the catalytic reduction cycle. 

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