Calorimetric titrations have been performed in anhydrous acetonitrile at 25 degrees C to give the complex stability constants (K-s) and the thermodynamic parameters for the stoichiometric 1:1 complexation of light lanthanoid(III) nitrates (La-Gd) with N-methoxyethylaza-4,7,11,14-tetraoxacyclohexadecane (aza-16-crown-5 lariat) (1). Using the present and reported data, the complexation behavior of C- and N-pivot lariats with 16-crown-5 skeletons is discussed comparatively and globally from the thermodynamic point of view. Possessing structural flexibility as compared with lariat ether 15-(2,5-dioxahexyl)-15-methyl-1,4,7,10,13-pentaoxacyclohexadecane (16-crown-5 lariat) (2), aza-16-crown-5 lariat gave the highest K-s for Nd3+ among the light lanthanoid nitrates. The complex stability sequence as a function of reciprocal ionic diameter of lanthanoid showed a characteristic peak profile at Nd3+ for 1, th, relative cation selectivity for Nd3+ over the neighboring Pr3+ and Sm3+ amounted to 9. Thermodynamically, the complexation of light lanthanoid nitrates with the lariat ethers is mainly enthalpy-driven in acetonitrile, but the cation selectivity is entropy-governed. The high cation selectivity of aza-16-crown lariat (1) is attributed to the entropic loss that is minimized only when a strict size match is materialized between the ionic diameter of the lanthanoids and the relatively flexible three-dimensional cavity induced upon lariat ligation. Therefore, the three-dimensional induced fit plays an important role in the recognition of trivalent lanthanoid ions by lariat ethers with structural flexibility.