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

A kinetic study of the aqua substitution in the [TcO(OH(2))(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]: k(f) = 11.5 ± 0.1, 11.38 ± 0.04, and 7.4 ± 0.1 M(−1)s(−1), with activation parameters: Δ H(#) (k(f) ) : 55 ± 2, 42 ± 3, 35 ± 5 kJ mol(−1); ΔS(#) (k(f) ) : − 40 ± 8, − 84 ± 11, − 110 ± 17 J K(−1)mol(−1). A subsequent high-pressure investigation of the aqua substitution in the [ReO(OH(2))(CN)(4)](−) and [TcO(OH(2))(CN)(4)](−) complexes by selected entering ligands yielded ΔV(#) (k(f) ) 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) cm(3)mol (−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.