The self-exchange kinetics of CO ligands in the solvated forms of the commonly commonly used complex [MBr3(CO)(3)](2-) (M = Re, Tc-99) were investigated in-depth by C-13 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 C-13 NMR and IR spectroscopy as well as by single-crystal X-ray diffraction. For technetium, no such intermediate could be identified. The activation parameters (Delta H-double dagger = 110 +/- 7 kJ mol(-1) and Delta S-double dagger = 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)], k(1) = 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.