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

The self-exchange kinetics of CO ligands in the solvated forms of the commonly used complex [MBr3(CO)3](2-) (M = Re, (99)Tc) were investigated in-depth by (13)C 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 (13)C 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.