The planar aromatic tridentate ligand 2,6-bis(1-methylbenzimidazol-2-yl)pyridine (L-1) reacts with Ln(III) (Ln = La-Lu) in acetonitrile to give the successive complexes [Ln(L-1)(m)](3+) (n = 1-3). Stability constants determined by spectrophotometry and potentiometric competitive titrations with Ag-1 show that the 1:1 and the 1:2 complexes display the usual thermodynamic behavior associated with electrostatic effects while the 1:3 complexes exhibit an unusual selectivity for the midrange Ln(III) ions (Delta log K-3(Gd-Lu)approximate to 4). A detailed investigation of the solution structure of [Ln(L-1)(3)](3+) (Ln = La-Dy) reveals that the closely packed triple-helical structure found in the crystal structure of [EU(L-1)(3)](3+) is retained in acetonitrile for the complete series. A sharp control of the coordination cavity results from the interstrand pi-stacking interactions which appear to be optimum for Gd-III. For Yb-III, for instance, a 1:2 complex, only could be isolated, which crystallizes as a hydroxo-bridged dimer [Yb(OH)(L-1)(2)](2)-(ClO4)(4)(HClO4)(0.5)(CH3(C)N)(7.32)(L-1)(0.5) (triclinic, P (1) over bar, a = 13.250(2) Angstrom, b = 16.329(2) Angstrom, c = 27.653(3) Angstrom, alpha = 99.941(9)degrees, beta = 93.393(9)degrees, gamma = 108.114(9)degrees, Z = 2). The binding of bulky substituents to the nitrogen atoms of the benzimidazole side arms in L-4 (i) severely affects the wrapping process, (ii) leads to less stable triple-helical building blocks, and (iii) removes the size-discriminating effect. The last can however be restored if a strong electron-donor group is connected to the central pyridine ring in L-8. Stability and solution structure data for [Ag-2(Li)(2)](2+) (i = 1, 4, 8) are also reported and discussed.