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 degrees 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: Delta H(#) (k(f) ) : 55 +/- 2, 42 +/- 3, 35 +/- 5 kJ mol(-1); DeltaS(#) (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 DeltaV(#) (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.