Kinetic and High-Pressure Mechanistic Investigation of the Aqua Substitution in the Trans-Aquaoxotetracyano Complexes of Re(V) and Tc(V): Some Implications for Nuclear Medicine

Abstract

A kinetic study of the aqua substitution in the [TcO(OH2)(CN)4]− complex by different thiourea ligands (TU = thiourea, NMTU = N-methyl thiourea, NNDMTU = N, N′-dimethylthiourea) yielded second-order formation rate constants (25∘C) as follows [NNDMTU, NMTU, TU, respectively]: kf = 11.5 ± 0.1, 11.38 ± 0.04, and 7.4 ± 0.1 M−1s−1, with activation parameters: ΔHkf#:55±2, 42±3, 35±5 kJ mol−1; ΔSkf#:−40±8, −84±11, −110±17 J K−1mol−1. A subsequent high-pressure investigation of the aqua substitution in the [ReO(OH2)(CN)4]− and [TcO(OH2)(CN4)]− complexes by selected entering ligands yielded ΔVkf# values as follows: Re(V): −1.7±0.3(NCS−), −22.1±0.9 (TU) and for Tc(V): −3.5±0.3(NCS−), −14±1 (NNDMTU), and−6.0±0.5 (TU) cm3mol−1, respectively. These results point to an interchange associative mechanism for the negative NCS− as entering group but even a pure associative mechanism for the neutral thiourea ligands.