Reaction of the bis-tridentate ligand bis{1-ethyl-2-[6'-(N,N-diethylcarbamoyl)pyridin-2'-yl]benz-imidazol-5-yl }methane (L2) with Ln(CF3SO3)(3)(.)xH(2)O in acetonitrile (Ln = La-Lu) demonstrates the successive formation of three stable complexes [Ln(L2)(3)](3+), [Ln(2)(L2)(3)](6+), and [Ln(2)(L2)(2)](6+). Crystal-field independent NMR methods establish that the crystal structure of [Tb-2(L2)(3)](6+) is a satisfying model for the helical structure observed in solution. This allows the qualitative and quantitative (beta(23)(bi),(Ln1Ln2)) characterization of the heterobimetallic helicates [(Ln(1))(Ln(2))(L2)(3)](6+). A simple free energy thermodynamic model based on (i) an absolute affinity for each nine-coordinate lanthanide occupying a terminal N6O3 site and (ii) a single intermetallic interaction between two adjacent metal ions in the complexes (DeltaE) successfully models the experimental macroscopic constants and allows the rational molecular programming of the extended trimetallic homologues [Ln(3)(L5)(3)](9+).