Spectrophotometric Study of the Equilibria between Nickel(II) Schiff-Base Complexes and Alkaline Earth or Nickel(II) Cations in Acetonitrile Solution

Adduct formation between M(ClO(4))(2) and nickel(II) complexes of tetradentate Schiff-base ligands ([NiL]) have been investigated in acetonitrile by means of UV-vis spectrophotometric analysis (M(2+)= Mg(2+), Ca(2+), Sr(2+), Ba(2+), Ni(2+); [NiL] = [Ni(salen)] = [N,N'-ethylenebis(salicylideneiminato)]nickel(II), [Ni(salphen)] = [N,N'-o-phenylenebis(salicylideneiminato)]nickel(II), [Ni(salpren)] = [N,N'-propylenebis(salicylideneiminato)]nickel(II)). [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 M(2+) cations. Complexes of general composition 2[Ni(salen)].M(ClO(4))(2) were isolated as powders in the solid state (M = Sr, Ca, Mg). X-ray structures of the crystalline complexes [{Ni(salen)}(2)Ba(ClO(4))(2)(thf)] (1) (thf = tetrahydrofuran) and [{Ni(salen)}(2)Ni(H(2)O)(2)](ClO(4))(2) (2) are reported. 1 crystallizes in the space group P2(1)/c (a = 10.995 Å; b = 20.830 Å; c = 19.567 Å, beta = 116.94 degrees ) and 2 in C2/c (a = 10.3020 Å; b = 21.818 Å; c = 17.796 Å; 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.