The heptadentate ligand OBETA (2,2'-oxybis(ethylamine)-N,N,N',N'-tetraacetic acid) was reported to form complexes with Ln(3+) ions more stable than those formed by the octadentate and more popular congener EGTA (ethylene glycol O,O'-bis(ethylamine)-N,N,N',N'-tetraacetic acid). The structural features leading to this puzzling coordination paradox were investigated by X-ray diffraction, solution state NMR, molecular modeling, and relaxometric studies. The stability constant of Gd(OBETA) (log K-GdL = 19.37, 0.1 M KCl) is 2 orders of magnitude higher than that of the higher denticity analogue Gd(EGTA) (log K-GdL = 17.66, 0.1 M KCl). The half-lives (t(1/2)) for the dissociation reactions of Gd(OBETA) and Gd(EGTA) ([Cu-2+](tot) = 0.2 mM, [Cit(3-)](tot) = 0.5 mM, [PO43-](tot) = 1.0 mM, and [CO32-](tot) = 25 mM at pH = 7.4 and 25 degrees C in 0.1 M KCl solution) are 6.8 and 0.63 h, respectively, reflecting the much higher inertness of Gd(OBETA) near physiological conditions. NMR studies and DFT calculations using the B3LYP functional and a large-core ECP indicate that the [Gd(OBETA)(H2O)(2)](-) complex most likely exists in solution as the (lambda lambda)(delta delta delta delta)(A)/(lambda lambda)(delta delta delta delta)A enantiomeric pair, with an activation free energy for the enantiomerization process of similar to 40 kJ mol(-1). The metal ion is nine-coordinate by seven donor atoms of the ligand and two inner-sphere water molecules. The X-ray crystal structure of [C(NH2)(3)](3)[Lu(OBETA)(CO3)] 2H(2)O is in agreement with the predictions of DFT calculations, the two coordinated water molecules being replaced by a bidentate carbonate anion. The (1)H NMRD and (17)O NMR study revealed that the two inner-sphere water molecules in Gd(OBETA) are endowed with a relatively fast water exchange rate (k(ex)(298) = 13 x 10(6) s(-1)). The higher thermodynamic stability and inertness of Ln(OBETA) complexes, peaking in the center of the 4f series, combined with the presence of two coordinated water molecules suggests that Gd(OBETA) is a promising paramagnetic probe for MRI applications.