A Proton Magnetic Resonance Study of Ligand Exchange on Tetrakis(tetramethylthiourea)zinc(II) Ion

Abstract

The rate of ligand exchange on the tetrakis(tetramethylthiourea) zinc(II) ion, determined by ¹H N.M.R. methods, in CD₂Cl₂ solution is found to be independent of free-tetramethylthiourea concentration. This is consistent with a dissociative ligand-exchange mechanism being operative. Typically for a solution in which [Zn{S=C(NMe₂)₂}₄²⁺] and [S=C(NMe₂)₂] were 0·00101 and 0·00376 respectively, k_ex(200 K) = 16·0±0.6 s^-1, ΔH^‡ = 64·9±O.8 kJ mol^-1 and ΔS^‡ = 106±4 J K^-1 mol^-1 where the observed ligand-exchange rate is given by

rate = 4k_ex[Zn {S=C(NMe₂)₂}₄²⁺]

The preparations of [Zn(O=CHNMe₂)₆](ClO₄)₂, [Zn{O=CMe(NMe₂)}₆](Cl0₄)₂, and [Zn{O=C- (NMe₂)₂}₄](ClO₄)₂ are also reported. The rate of ligand exchange upon these three species in CD₂Cl₂ solution was found to be in the fast-exchange limit of the N.M.R. time scale.