Mn2+ has five unpaired d-electrons, a long electronic relaxation time, and labile water exchange, all of which make it an attractive candidate for contrast agent application in medical magnetic resonance imaging. In the quest for stable and nonlabile Mn2+ complexes, we explored a novel dimeric triazacyclononane-based ligand bearing carboxylate functional groups, H(4)ENOTA. The protonation constants of the ligand and the stability constants of the complexes formed with some endogenously important metals (Ca2+, Cu2+, Zn2+), as well as with Mn2+ and Ce3+, have been assessed by NMR methods, potentiometry, and UV-vis spectrophotometry. Overall, the thermodynamic stability of the complexes is lower as compared to that of the corresponding NOTA analogues (H(3)NOTA, 1,4,7-triaazacyclononane-1,4,7-triacetic acid). The crystal structure of Mn-2(ENOTA)(H2O)center dot 5H(2)O contains two six-coordinated Mn2+, in addition to the three amine nitrogens and the two oxygens from the pendent monodentate carboxylate groups, and one water (Mn2) or one bridging carboxylate oxygen (Mn1) completes the coordination sphere of the metal ion. In an aqueous solution, this bridging carboxylate is replaced by a water molecule, as evidenced by the O-17 chemical shifts and proton relaxivity data that point to monohydration for both metal ions in the dinuclear complex. A variable-temperature and -pressure O-17 NMR study has been performed on [Mn-2(ENOTA)(H2O)(2)] to assess the rate and, for the first time on a Mn2+ chelate, also the mechanism of the water exchange. The inner sphere water is slightly more labile in [Mn-2(ENOTA)(H2O)(2)] (k(ex)(298) = 5.5 x 10(7) s(-1)) than in the aqua ion (2.1 x 10(7) s(-1), Merbach, A. E.; et al. Inorg. Chem. 1980, 19, 3696). The water exchange proceeds via an almost limiting associative mechanism, as evidenced by the large negative activation volume (Delta V-not equal = -10.7 cm(3) mol(-1)). The proton relaxivities measured on [Mn-2(ENOTA)(H2O)(2)] show a low-field dispersion at similar to 0.1 MHz arising from a contact interaction between the Mn-II electron spin and the water proton nuclear spins.