Mechanistic changeover for the water substitution on fac-[(CO)3Re(H2O)3]+ revealed by high-pressure NMR

The complex formation in water between the stable tricarbonyltriaqua fac-[(CO)(3)Re(H2O)(3)](+) (1) complex and Nand S-donor ligands has been studied by high-pressure H-1 NMR. Rate and equilibrium constants for the formation Of [(CO)(3)Re(Pyz)(H2O)(2)](+), [(CO)(3)(H2O)(2)Re(mu-Pyz)Re(H2O)(2)(CO)(3)](2+), [(CO)(3)Re(THT)(H2O)(2)](+), and [(CO)(3)Re(DMS)(n)(H2O)(3-n)](+) (n = 1-3) (Pyz = pyrazine, THT = tetrahydrothiophene, DMS = dimethyl sulfide) have been determined and are in accord with previous results (Salignac, B.; Grundler, P. V.; Cayemittes, S.; Frey, U.; Scopelliti, R.; Merbach, A. E.; Hedinger, R.; Hegetschweiler, K.; Alberto, R.; Prinz, U.; Raabe, G.; Kolle, U.; Hall, S. Inorg. Chem. 2003, 42, 3516). The calculated interchange rate constant K-i (Eigen-Wilkins mechanism) increases from the hard O- and N-donors to the soft S-donors, as exemplified by the following series: TFA (trifluoroacetate) (K-i = 2.9 x 10(-3) s(-1)) < Br- < CH3CN < Pyz < THT < DMS < TU (thiourea) (K-i = 41.5 x 10(-3) s(-1)). On the other hand, K-i values remain close to that of water exchange k(ex) on 1 (k(ex) = 6.3 x 10(-3) s(-1)). Thus, an I-d mechanism was assigned, suggesting however the possibility of a slight deviation toward an associatively activated mechanism with the S-donor ligands. Activation volumes determined by high-pressure NMR, for Pyz as DeltaV(f,1)(double dagger) = +5.4 +/- 1.5, DeltaV(r,1)(double dagger) = +7.9 +/- 1.2 cm(3) mol(-1), for THT as DeltaV(f,1)(double dagger) = -6.6 +/- 1, DeltaV(r,1)(double dagger) = -6.2 +/- 1 cm(3) mol(-1), and for DMS as DeltaV(f,1)(double dagger) -12 +/- 1, DeltaV(r,1)(double dagger),l = -10 +/- 2 cm(3) mol(-1) revealed the ambivalent character of 1 toward water substitution. Hence, these findings are interpreted as a gradual changeover of the reaction mechanism from a dissociatively activated one (I-d), with the hard O- and N-donor ligands, to an associatively activated one (I-a), with the soft S-donor ligands.