Water exchange reactions, paramagnetic

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. 

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