The water exchange process on [(CO)(3)Re(H(2)O)(3)](+) (1) was kinetically investigated by (17)O NMR. The acidity dependence of the observed rate constant k(obs) was analyzed with a two pathways model in which k(ex) (k(ex)(298) = (6.3 +/- 0.1) x 10(-3) s(-1)) and k(OH) (k(OH)(298)= 27 +/- 1 s(-1)) denote the water exchange rate constants on 1 and on the monohydroxo species [(CO)(3)Re(I)(H(2)O)(2)(OH)], respectively. The kinetic contribution of the basic form was proved to be significant only at [H(+)] < 3 x 10(-3) M. Above this limiting [H(+)] concentration, kinetic investigations can be unambiguously conducted on the triaqua cation (1). The variable temperature study has led to the determination of the activation parameters Delta H(++)(ex) = 90 +/- 3 kJ mol(-1), Delta S(++)(ex) = +14 +/- 10 J K(-1) mol(-1), the latter being indicative of a dissociative activation mode for the water exchange process. To support this assumption, water substitution reaction on 1 has been followed by (17)O/(1)H/(13)C/(19)F NMR with ligands of various nucleophilicities (TFA, Br(-), CH(3)CN, Hbipy(+), Hphen(+), DMS, TU). With unidentate ligands, except Br(-), the mono-, bi-, and tricomplexes were formed by water substitution. With bidentate ligands, bipy and phen, the chelate complexes [(CO)(3)Re(H(2)O)(bipy)]CF(3)SO(3) (2) and [(CO)(3)Re(H(2)O)(phen)](NO(3))(0.5)(CF(3)SO(3))(0.5).H(2)O (3) were isolated and X-ray characterized. For each ligand, the calculated interchange rate constants k'(i) (2.9 x 10(-3) (TFA) < k'(I) < 41.5 x 10(-3) (TU) s(-1)) were found in the same order as the water exchange rate constant k(ex), the S-donor ligands being slightly more reactive. This result is indicative of I(d) mechanism for water exchange and complex formation, since larger variations of k'(i) are expected for an associatively activated mechanism.
The complex formation in water between the stable tricarbonyltriaqua fac-[(CO)(3)Re(H(2)O)(3)](+) (1) complex and N- and S-donor ligands has been studied by high-pressure (1)H NMR. Rate and equilibrium constants for the formation of [(CO)(3)Re(Pyz)(H(2)O)(2)](+), [(CO)(3)(H(2)O)(2)Re(mu-Pyz)Re(H(2)O)(2)(CO)(3)](2+), [(CO)(3)Re(THT)(H(2)O)(2)](+), and [(CO)(3)Re(DMS)(n)()(H(2)O)(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.; Kölle, U.; Hall, S. Inorg. Chem. 2003, 42, 3516). The calculated interchange rate constant k(1)' (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(1)' = 2.9 x 10(-3) s(-1)) < Br(-) < CH(3)CN < Pyz < THT < DMS < TU (thiourea) (k(1)' = 41.5 x 10(-3) s(-1)). On the other hand, 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 Delta V(++)(f,1) = +5.4 +/- 1.5, Delta V(++)(r,1) = +7.9 +/- 1.2 cm(3) mol(-)(1), for THT as Delta V(++)(f,1) = -6.6 +/- 1, Delta V(++)(r,1) = -6.2 +/- 1 cm(3) mol(-1), and for DMS as Delta V(++)(f,1) = -12 +/- 1, Delta V(++)(r,1) = -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.
The substitution of water in the half-sandwich complexes Cp*Rh(H2O)3 2+ and Cp*Ir(H2O)3 2+ (Cp* = η5-pentamethylcyclopentadienyl anion) by Cl-, Br-, I-, SCN-, py-CN (4-cyanopyridine), py-nia (nicotinamide), py (pyridine), TU (thiourea), and DMS (dimethylsulfide) was studied by stopped-flow spectroscopy at variable concentration, temperature, and pressure. The proton dissociation constants of the triaqua complexes, pK a = 6.47 (for rhodium) and pK a = 3.86 (for iridium), as well as the equilibrium constants for the formation of the dinuclear species (Cp*M)2(μ-OH)3 + were obtained by spectrophotometric titrations. The equilibrium constants K 1 for the formation of the monosubstituted complexes Cp*M(H2O)2L+/2+, as determined for anionic and neutral ligands L, lie in the range 102−105 M-1 and follow the sequences K(Cl-) < K(Br-) < K(I-) and K(py-CN) < K(py-nia) < K(py) < K(TU,DMS). Assuming the Eigen−Wilkins mechanism for the formation of the monosubstituted complexes, second-order rate constants k f,1 were corrected for outer sphere complex formation and for statistical factors to obtain rate constant k i‘ for the interchange step. The interchange rates k i‘ are nearly independent of the nature of L and very close to the rate of water exchange (k ex(Rh) = (1.6 ± 0.3) × 105 s-1 and k ex(Ir) = (2.5 ± 0.08) × 104 s-1). In all cases, i.e., for M = Rh and Ir and for L = anionic or neutral, the volume of the transition state is larger than that of the triaqua species. These findings support the operation of an I d mechanism without excluding a D mechanism. For a given ligand L, the substitution of another water molecule in the complexes Cp*M(H2O)2L+/2+ is by 1 order of magnitude slower than the substitution of the first water molecule in the triaqua species Cp*M(H2O)3 2+, as verified, for example, by k f,1 = 2.61 × 103 and k f,2 = 3.09 × 102 M-1 s-1 for M = Ir and L = py.
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