Cobalt complexes—continued mechanism

Studies of Ln(in) nitrilotriacetate (539) and ethylenediamine (540) complexes have been reported. In the latter case a contact mechanism is responsible for the shifts in the lighter lanthanides (up to Ho) whereas a pseudocontact shift is dominant for the remainder of the series. Some novel lanthanide-cobalt sandwich complexes have also been reported. (587) Uranium(iv) complexes continue to be the most extensively studied of the actinide ion complexes. Appreciable dipolar interactions are expected to arise from the orbitally degenerate ground term arising from the 5f configuration of U(iv). However, in practice isotropic shifts of U(iv) complexes are not dominated by dipolar interactions. 

C tereochemistry has played a major role in the development of chemistry, and it continues to be most significant. Werner made extensive use of the information available to him on the stereochemistry of metal complexes in developing his coordination theory. He made the first meaningful attempt to understand the mechanisms of substitution reactions of these systems on the basis of the stereochemical changes accompanying such reactions. The paper 49) he wrote in 1912 is a real milestone and should be read by anyone interested in octahedral substitution reactions. It is valuable because of the large amount of experimental data it contains on reactions of cis and [Pg.408]

This promise has been only partially fulfilled because of the difficulty of interpreting anation mechanisms where second order kinetics, first order in entering anion and first order in complex, are often found because of ion association which contributes a term in anion concentration to the rate law. A further difficulty, emphasised by Archer in his recent review on the stereochemistry of octahedral substitution reactions, is found in cobalt(III) chemistry because of the difficulty in isolating trans solvent-containing species. This results in continued doubt in the study of such systems as ... 

Chromium(ni) continues to be the second most widely studied metal ion after cobalt(m). The question of dissociative (/a) or associative (/a) mechanisms for substitutions at chromium(m) has been clarified somewhat by comparing data for the formation and dissociation of the [Cr(H20)5X] + and [Cr(NH8)5X] + ions (X = unidentate leaving anion). Recent studies indicate that an associative mechanism is important for aqua-chromium(m) complexes, but for the [Cr(NHa)6X] + ions a dissociative-interchange mechanism is favoured. A summary of kinetics and thermodynamic data for the formation and aquation of [Cr(NH3)5X] + ions is given in Table 12. 

The use of activation volumes in the diagnosis of mechanism has continued to provide much valuable information. Activation volumes for substitution at octahedral complexes have formed the subject of a well-referenced review,in which the importance both of intrinsic and of solvation contributions is recognized. The topics of most relevance to this chapter include isomerization and racemization reactions of cobalt(III) complexes, aquation of cobalt(III) and of iron(II) complexes, and base hydrolysis of cobalt(III) complexes. Merbach s continuing investigations into the effects of pressure on rates of solvent exchange at 2-h and 3+ transition metal cations, while not being always strictly... 

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