Reactions Between Two Metal Complexes

Oironiiuni(li) — Rate constants and activation parameters for reactions of chro-mium(n) and of other reductants are given in Table 1 on p. 17. 

The factors responsible for the enormous variations in rate when bifunctional organic ligands mediate in electron-transfer reactions are by no means understood, but in the case of simple monocarboxylato-ligands the details of the adjacent-attack mechanism seem particularly well worked out. Sykes has reviewed the contribution that dinuclear complexes have played in this. In all simple monocarboxylato-com-plexes of the type (5) the point of Cr + attack is the carbonyl oxygen and its accessibility is determined by the size of R. The conformation (5) is more strongly demanded 

Reductions of the di-/i-hydroxo-p-benzoato-bis[triamminecobalt(m)] complex and the o-chloro-substituted analogue were investigated to see whether any advantage 

Complete reduction of the dinuclear complex (9) by Cr + proceeds in two stages,  

The classic way of demonstrating a mechanism, viz. by product analysis, is not always possible, especially with slow reactions, and the use of linear free-energy relationships is now much in evidence. Sutin has developed an alternative approach to the problem - consideration of the magnitude of the catalytic effect of added anions. This effect is known for the M + reduction (M = V, Cr, or Fe) of a hard oxidant by the inner-sphere mechanism, and a particularly useful fact comes from the observation that N3 is ca. 10 times more effective than SCN in catalysing the rate. The effect of Cl and SCN on the rates of known outer-sphere reactions are given, using the reductions of [Co(NH3)8] +, [Co(en)3] +, and [Co(phen)3] + by Cr and V. At low concentrations of added anion, X , the rate law is observed to be 

As in the previous volume, an attempt has been made to include all work on the kinetics or mechanisms of metal-metal electron-transfer reactions published during the period under review, though for reasons of space some of the references are limited to entries in the Tables, which are collected together at the end of the chapter. A few of the journals for 1977 were still not available at the time of writing, but will be dealt with in the next volume. 

Books on the kinetics of oxidation-reduction reactions of actinide ions and on the radiation chemistry of metal ions may be mentioned here. Other books and reviews are cited at the appropriate places in the text.  

Volume 82 of the Journal of Electroanalytical Chemistry consists of papers dedicated to Professor V. G. Levich on the occasion of his 60th birthday, prefaced by a short biographical notice. The contributors to the volume express the hope that Professor Levich will soon be able to resume fully his scientific work. Two of the papers are concerned with homogeneous electron-transfer reactions.  

Chromium(II).—Quantitative data for this and for the other reducing species are tabulated in Tables 1 and 2 on pp. 17 and 24. 

The outer-sphere reductions of penta-ammine pyridine and alkyl-substituted pyridine complexes have been investigated and have been compared with similar reactions involving Eu + as reductant. For the Cr reactions, the rate constants are in the ran 10 —10 1 mol s K In the reactions with the 4-ethoxycarbonylpyridine, 2-methylpyrazine, and 2,6-dimethylpyrazine (1) complexes, where there is a site for inner-sphere reduction and electron 

The role of co-ordinated water as a bridging ligand in redox systems has been discussed, the data for the reaction 

The reduction of thioether and carboxylate chelates of cobalt(in) by Cr has been investigated. The complexes [(en)2Co(NH2CH2C02)] + and 

The reductions of di- and tetra-nuclear cobaJt(in) complexes have been studied by Sykes and his co-workers. In the reaction with amido-At-formato-bis[tetra-amminecobalt(ra)] the only observable stage in the reaction may be represented by the rate law 

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Solid evidence for this type of chain mechanism (differing only in the chain propagation steps) had been earlier obtained by Margerum and his co-workers jjj study of coordination chain reactions between two metal complexes each containing one multidentate ligand, edta, trien and so on, e.g.. 

Reactions Between Two Metal Complexes Table 1 cont.)... 

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