Adduct formation between M(ClO4)(2) and nickel(II) complexes of tetradentate Schiff-base ligands ([NiL]) have been investigated in acetonitrile by means of UV-vis spectrophotometric analysis (M2+, Mg2+, Ca2+, Sr2+, Ba2+, Ni2+; [NiL] = [Ni(salen)] = [N,N'-ethylenebis(salicylideneiminato)]nickel(II) [Ni(salphen)] = [N,N'-o-phenylenebis(salicylideneiminato)]nickel(II), [Ni(salpren)] = [N,N'-o-propylenebis(salicylideneiminato)]. [NIL] complexes act as bidentate oxygen-donor ligands forming adducts of the type [(NiL)M](2+) and [(NiL)(2)M](2+) whose general order of increasing stability was correlated to the dimensions of the M2+ cations. Complexes of general composition 2[Ni(salen)]. M(ClO4)(2) were isolated as powders in the solid state(M = Sr, Ca, Mg). X-ray structures of the crystalline complexes [{Ni(salen)}(2)Ba(ClO4)(2)(thf)] (1) (thf = tetrahydrofuran) and [{Ni(salen)}(2)Ni-(H2O)(2)](ClO4)2 (2) are reported. 1 crystallizes in the space group P2(1)/c (a = 10.995 Angstrom; b = 20.830 Angstrom; c = 19.567 Angstrom, beta = 116.94 degrees) and 2 in C2/c (a = 10.3020 Angstrom; b = 21.818 Angstrom; c = 17.796 Angstrom; beta = 118.98 degrees). Two [Ni(salen)] units act as oxygen bidentate ligands in both the compounds. The barium is coordinated to eight oxygen atoms in a distorted square antiprismatic geometry. An 8-fold coordination is completed by a molecule of tetrahydrofuran and two perchlorate anions acting as bi- and monodentate O-ligands, respectively. In 2, the nickel atom, coordinated to two [Ni(salen)] complexes, has a distorted octahedral geometry and completes its coordination by linking two molecules of water.