Labile complex formation Eigen mechanism

The effect of exchange of lactic, mandelic and sulfosalicylic acids on the relaxation of solvent protons gave rate constants (k) of exchange from 1.73 to 0.701 mol-1 s-1.642 Kinetics of complex formation with mandelic (HMDA) and vanillomandelic acids (HVMDA) gave rate constants (1.09 x 103 and 1.13 x 103 mol-1 s 1 for MDA- and VMDA ) consistent with a dissociative (Eigen) mechanism.438 As in the case of oxalic and malonic acids (Section 33.5.5.5.ii Table 27), species with coordinated hydroxyl are labilized. 

For labile complexes, it is often quite difficult to distinguish between inner and outer sphere complexes. To add to this confusion is the fact that formation constants for such labile complexes when determined by optical spectrometry are often lower than those of the same system determined by other means such as potentiometry, solvent extraction, etc. This has led some authors to identify the former as "inner sphere" constants and the latter as "total" constants. However, others have shown that this cannot be correct even if the optical spectrum of the solvated cation and the outer sphere complex is the same (4, 7). Nevertheless, the characterization and knowledge of the formation constants of outer sphere complexes are important as such complexes play a significant role in the Eigen mechanism of the formation of labile complexes (8) This model describes the formation of complexes as following a sequence ... 

Complex Formation Labile Cations. Solvent effects on reactivity in the formation of complexes of metal(n) cations with unidentate ligands have been reviewed, with special reference to magnesium(n) and to the solvents methanol, acetonitrile, DMF, and DMSO. There has been controversy over the mechanism of reaction of thiocyanate with nickel(n) in DMSO, with supporters of the usual Eigen-Wilkins la mechanism and of a D mechanism. The most recent investigators of this reaction report rate constants and activation parameters and favour the la mechanism. There has been further discussion of the mechanism of the reaction between nickel(n) and bipy in DMSO an earlier suggestion that the rate-determining step is ring closure is not supported by recent observations. Rate constants for the reaction of acetate, of other carboxylates, and of pada with nickel(ii) in several non-aqueous solvents have been determined. 

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