Ligand exchange reactions outer-sphere complex formation constant

Copper(II) and zinc(II) are two of the more labile divalent metal ions and as a consequence the former is too labile for its water exchange rate to be determined by the NMR methods which utilize the paramagnetism of other divalent first-row transition metal ions, while the latter is diamagnetic and such NMR methods cannot be applied. However, it has been shown that water exchange rates and mechanisms can be deduced with reasonable reliability from simple ligand substitution studies, and this is one of the reasons for a recent variable-pressure spec-trophotometric SF study of the substitution of 2-chloro-l,10-phenanthroline on Cu(II) and Zn(II). The observed rate constants for the complexation reaction (kc) and the decomplexation reaction (k ) and their associated activation parameters for Cu(II) and Zn(II) are kc(298 K) = 1.1 x 10 and 1.1 x 10 dm mol" s", AH = 33.6 and 37.9 kJ mol", A5 = 3 and -2JK- mol", AV = 7.1 and 5.0 cm" mol", k 29S K) = 102 and 887 s", AH = 60.6 and 57.3 kJ mol", A5 = -3 and 4 J K" mol" and A V = 5.2 and 4.1 cm" mol". These data are consistent with the operation of an mechanism for the rate-determining first bond formation by 2-chloro-l,10-phenanthroline with the subsequent chelation step being faster [Eq. (18)]. For this mechanistic sequence (in which [M(H20)6 L-L] is an outer-sphere complex) it may be shown that the relationships in Eq. (19) apply. 

The study of the effect of solvent upon the rate of complexation has revealed details of the reaction mechanism, and it is hoped that a study of the solvent-dependence of activation volumes may permit further elucidation of metal-ligand, metal-solvent and solvent-solvent interactions. According to the Eigen-Wilkins mechanism for ligand substitution reactions ( )(5)5 the first step is formation of an outer-sphere complex, characterised by an equilibrium constant Ki2 Subsequently the ligand enters the first coordination sphere and the forward rate constant for this step is identified with kg for exchange of... 

It has been shown in previous studies by several fast reaction techniques that the complexation reaction between a metal ion and a bidentate ligand involves several discrete steps (5). As shown in figure 2, step 1 represents rapid (diffusion controlled) formation of an outer-sphere complex and step 2 the formation of a monodentate complex for which the forward rate constant can be identified with that for water exchange (kex) measured by n.m.r. methods. Since PADA is a bidentate ligand, a further ring closure step (3) takes place in which the final product is formed. The rate of ring closure is generally assumed to be rapid compared with the rate of dissociation of the monodentate complex (i.e. kj k 2) ... 

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