The kinetics of the reaction between [S2MoS2Cu(SC6H4R-4)]2−
(R = MeO, H, Cl or NO2) and CN− to form [S2MoS2CuCN]2− have been studied in MeCN using stopped-flow spectrophotometry. In all cases, the rate law is of the form, Rate =
{kR1
+
k2R[CN−]}[S2MoS2Cu(SC6H4R-4)2−]. It is proposed that both kR1 and kR2 correspond to associative substitution mechanisms. The kR2 pathway involves attack by CN− at the copper site followed by dissociation of the thiolate. The kR1 pathway involves attack of the solvent (MeCN) at the copper site, followed by dissociation of the thiolate to form [S2MoS2Cu(NCMe)]−. Subsequent rapid substitution of the coordinated solvent by cyanide produces [S2MoS2CuCN]2−. The evidence that both the kR1 and kR2 pathways involve associative mechanisms are: (i) the 4-R-substituent on the thiolate ligand has a similar effect on both kR1 and kR2, with electron-withdrawing 4-R-substituents facilitating substitution; (ii) both the kR1 and kR2 pathways are associated with similar activation parameters (for k1H: ΔH‡
= 5.5 ± 0.5 kcal mol−1, ΔS‡
=
−23.9 ± 2.0 cal deg−1 mol−1; for k2H: ΔH‡
= 2.3 ± 0.5 kcal mol−1, ΔS‡
=
− 23.9 ± 2.0 cal deg−1 mol−1) and (iii) addition of C6H5S− results in a similar increase in both kR1 and kR2.
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