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.

Hydrolytic pathways of the organometallic aqua ion [Re(CO)3(H2O)3]+ (2) in aqueous media were investigated by means of potentiometric titration experiments. The aqua complex 2 was obtained quantitatively by dissolving (NEt4)2[Re(CO)3Br3] (1) in water. Conventional alkalimetric titrations (0.1 M KNO3, 25 °C) allowed the determination of the formation constants of [Re3(CO)9(μ2-OH)3(μ3-OH)]- (3) and [Re2(CO)6(μ2-OH)3]- (4). The neutral dinuclear [Re2(CO)6(μ2-OH)2(H2O)2] (5) was observed as a minor species. A fast titration technique was used to investigate a rapid preequilibrium, consisting of the formation of the mononuclear deprotonation products [Re(CO)3(OH)(H2O)2] (6) and [Re(CO)3(OH)2(H2O)]- (7). The corresponding pK a values are 7.5(2) and 9.3(3). The immediate extraction of an aqueous solution of 2 with diethyl ether after base addition (1 equiv) led to the quantitative isolation of the well-known cubane cluster [Re(CO)3(OH)]4 (8), which was obtained as a DMF (8b) or OPPh3 adduct (8c). Attempts to isolate the aqua complex 2 as a crystalline material by precipitating the bromo ligands of 1 with AgCF3COO resulted in the formation of (NEt4)2[Re(CO)3(CF3COO)3] (9). The structures of 8b, 8c, and 9 were elucidated by single-crystal X-ray analysis.

A variety of trinuclear complexes [M(3)(H(-)(3)L)(2)](3+) [M = Y, La, Eu, Gd, Dy; L = 1,3,5-triamino-1,3,5-trideoxy-cis-inositol (taci) and 1,3,5-trideoxy-1,3,5-tris(dimethylamino)-cis-inositol (tdci)] was prepared as solid materials of the composition M(3)(H(-)(3)L)(2)X(3).pH(2)O.qEtOH (X = Cl, NO(3); 2.5

The trinuclear [Gd(3)(H(-)(3)taci)(2)(H(2)O)(6)](3+) complex has been characterized in aqueous solution as a model compound from the point of view of MRI: the parameters that affect proton relaxivity have been determined in a combined variable temperature, pressure, and multiple-field (17)O NMR, EPR, and NMRD study. The solution structure of the complex was found to be the same as in solid state: the total coordination number of the lanthanide(III) ion is 8 with two inner-sphere water molecules. EPR measurements proved a strong intramolecular dipole-dipole interaction between Gd(III) electron spins. This mechanism dominates electron spin relaxation at high magnetic fields (B > 5 T). Its proportion to the overall relaxation decreases with decreasing magnetic field and becomes a minor term at fields used in MRI. Consequently, it cannot increase the electronic relaxation rates to such an extent that they limit proton relaxivity. [Gd(3)(H(-)(3)taci)(2)(H(2)O)(6)](3+) undergoes a relatively slow water exchange (k(ex)(298) = (1.1 +/- 0.2) x 10(7) s(-1)) compared to the Gd(III) aqua ion, while the mechanism is much more associatively activated as shown by the activation volume (DeltaV () = (-12.7 +/- 1.5) cm(3) mol(-)(1)). The lower exchange rate, as compared to [Gd(H(2)O)(8)](3+) and [Gd(PDTA)(H(2)O)(2)](-), can be explained with the higher rigidity of the [Gd(3)(H(-)(3)taci)(2)(H(2)O)(6)](3+) which considerably slows down the transition from the eight-coordinate reactant to the nine-coordinate transition state. The unexpectedly low rotational correlation time of the complex is interpreted in terms of a spherical structure with a large hydrophobic surface avoiding the formation of a substantial hydration sphere around [Gd(3)(H(-)(3)taci)(2)(H(2)O)(6)](3+).