The synthesis and characterization of N,N'-bis(2-pyridylmethyl)trans-1,2-diaminocydohexane-N,N1-diacetic acid (H(2)bpcd) cationic complexes of La(III), Nd(III), and Sm(III) are reported. The Ln(III)-bpcd(2-) complex ions, where bpcd(2-) stands for N,N'-bis(2-pyridylmethyl)-trans-1,2-diaminocydohexane-N,N'-diacetate, were isolated as PF6- salts. These salts were characterized by elemental analysis, X-ray crystallography, IR, and H-1 and C-13 NMR spectroscopy. Binuclear [La-2(bpcd)2(H2O)(2)](2+) crystallized from an aqueous solution in the monoclinic P2(1)/c space group as a cocrystallate with Na(2)bpcd and NaPF6, nominally Na-2.34[La-1.22(C22H26N4O4)(2)(H2O)(2)][PF6](2). 2H(2)O, with a = 11.3343(6) angstrom, b = 17.7090(9) angstrom, c = 15.0567(8) angstrom, beta = 110.632(3)degrees, and Z = 4 (Z' = 2). La is eight-coordinate with distorted dodecahedral coordination geometry provided by a N4O4 donor atom set. In addition to four N atoms from the bpcd(2-) ligand, La's coordination sphere includes 0 atoms from a water molecule and three acetate groups (one 0 atom from singly bound acetate and two 0 atoms from acetate groups that bridge the La centers). The H-1 and C-13 assignments for H(2)bpcd and the metal-bpcd(2-) complexes were made on the basis of 2D COSY and HSQC experiments, which established H-1-H-1 and H-1-C-13 correlations. The NMR spectral data were used to establish the symmetry of the cationic complexes present in aqueous solution. The data indicate that the La(III)bpcd2- and Sm(III)-bpcd2- complexes are present in solution as a single species with C-2 symmetry. The H-1 NMR spectrum of [Nd(bpcd)]PF6 in D2O consists of eight considerably line-broadened, paramagnetic-shifted singlets. The ab initio quantum mechanical calculations at the PCM/MP2/SDD//HF/SDD level,, which were established previously for determining isomerization energies for octahedral M(III)-bpad(2-) complex ions, were used to determine the relative free energies of the geometric isomers possible for eight-and nine-coordinate La(III)-bpcd(2-) cationic aqua complexes in aqueous solution, i.e., [La(bpcd)(H2O)(2)](+) and La(bpcd)(H2O)(3)](+).