A new water-soluble N4O3 tripodal amine phenol ligand, tris(((2-hydroxy-5-sulfobenzyl)amino)ethyl)amine (H(3)TRNS) has been synthesized, and its complexation properties with six lanthanide(III) ions (Ln, Nd, Gd, Ho, Yb, Lu) have been probed. In water, the ligand coordinates in a tridentate fashion through the three phenolate oxygen atoms to give mono- and bis(ligand) complexes. The bis complexes are proposed to be 7-coordinate containing one bound water as evinced by O-17 NMR experiments on the Dy3+ complex. while the mono complexes are proposed to have a coordination number of eight or nine. The stepwise formation constants for the 1:1 and 2:1 (L:M) complexes have been measured at 25 degrees C (mu = 0.16 M NaCl) for Nd3+ (log K-1 = 6.41, log K-2 6.34), Gd3+ (log K-1 = 6.67, log K-2 = 7.69), Ho3+ (log K-1 = 7.67, log K-2 = 8.75), and Yb3+ (log K-1 = 8.53, log K-2 = 9.73). The formation constants show an increasing affinity and an unprecedented selectivity for the heavier lanthanides (beta(2(yb)) - beta(2(Nd)) = 10(5.5)). Furthermore, the stepwise stability constant for the 2:1 complex is increasingly greater than that for the 1:1 complexes of Gd3+, Ho3+, and Yb3+, while K-1 and K-2 for the 1:1 and 2:1 complexes of Nd3+ are approximately the same. Solution calorimetry shows that the selectivity for the heavier lanthanides is an enthalpic effect (Delta H-beta 2 = -45.11 (Yb) and -13.71 kJ . mol(-1) (Nd)), while the anomalous behavior in formation constants (K-2 > K-1) is predominantly entropic (Delta S-2 > Delta S-1) and is rationalized in terms of salvation of the complexes, Lanthanum(III) forms only the 1:1 complex (log K = 5.65) which precipitates from solution.