Ligand exchange and complex formation kinetics studied by NMR exemplified on fac-[(CO)3M(H2O)]+ (M = Mn, Tc, Re)
In this review ligand exchange and complex formation reactions on fac-[(CO)3M(H2O)3]+ (M = Mn, Tc, Re) and on fac-[(CO)2(NO)Re(H2O)3]2+ are presented. A variety of experimental NMR techniques are described and it is shown that sometimes combinations of techniques applied at variable temperature or variable pressure allowed to measure exchange rate constants and their activation parameters as well as thermodynamic parameters. Furthermore, the use of uncommon nuclei for NMR like 17O or 99Tc extends considerably the range of applications especially in aqueous solutions when 1H NMR is often not very useful. Tricarbonyl triaqua complexes of technetium(I) and rhenium(I) became important precursors for a variety of radiopharmaceuticals under development. It has been shown that the fac-[(CO)3M]-unit is kinetically inert and thatwater molecules bound to it can be easily replaced. Reactivity of the ReI complexes is one to two orders of magnitude slower than its TcI analogues. Furthermore, it shows a marked acidity dependence which has not been observed for TcI and MnI species.