Kinetics and Mechanism of CO Exchange in <i>fac</i>-[MBr₂(solvent)(CO)₃]⁻ (M = Re, ⁹⁹Tc)

The self-exchange kinetics of CO ligands in the solvated forms of the commonly used complex [MBr3(CO)3]2– (M = Re, 99Tc) were investigated in-depth by 13C NMR spectroscopy in organic solvents such as dimethylformamide and methanol. The two homologues exhibit surprisingly different chemical behavior. In the case of rhenium, the stable intermediate [NEt4]­[ReBr2(CO)4] was isolated and characterized by 13C NMR and IR spectroscopy as well as by single-crystal X-ray diffraction. For technetium, no such intermediate could be identified. The activation parameters (ΔH⧧ = 110 ± 7 kJ mol–1 and ΔS⧧ = 127 ± 22 J mol–1 K–1) and the observed influences of different ligands and solvents suggest a dissociative-interchange-type mechanism with a second-order rate constant for the formation of [NEt4]­[ReBr2(CO)4], k1 = 0.039 ± 0.001 M–1 s–1 at 274 K. On the basis of variable-temperature NMR experiments, kinetic simulations, and density functional theory calculations, a complete model for the CO self-exchange, including all respective rate constants, is reported.