The nickel(II) complex of 3-methoxysalicylaldimine, L2(2)Ni, reacts with lithium, sodium, and potassium salts to afford dimetallic complexes L22NiNaY and L22NiLiY (Y = ClO4) and L22NiKI. Structural analysis performed both in the solid state (X-ray diffraction) and in solution (UV-vis, H-1, C-13, and Na-23 NMR spectroscopy, magnetic measurements) points to an almost perfect similitude with the simple model of self-assembling ionophore, initially suggested. The two steps of the process are structurally characterized. L22Ni.2H2O (1) crystallizes in the monoclinic space group P2(1)/n (No. 14) with two formula weights in a cell having the dimensions a = 11.084(1) angstrom, b = 4.8503(8) angstrom, c = 15.519(1) angstrom, and beta = 97.85(1)degrees. The dimetallic complex L22NiNaClO4.H2O (2) crystallizes in the orthorhombic space group Pbca (No. 61) with eight formula weights in a cell having the dimensions a = 11.232(1) angstrom, b = 15.530(1) angstrom, and c = 22.698(2) angstrom. In the first complex two imine molecules assemble themselves around a nickel ion to form the monometallic complex L(2)2Ni with a trans-NiN2O2 chromophore. Upon addition of lithium, sodium, or potassium cations, the chromophore adopts a cis configuration with the formation of an outer O4 site able to bind an alkali-metal cation. Evaluation of the stability constants shows that the monometallic species L2(2)Ni displays a substantial selectivity for sodium ions.