Three new flexidentate 5-substituted salicylaldimino Schiff base ligands (L-1-OH-L-3-OH) based on 1-(2-aminoethyl)piperazine (X = H, L-1-OH; X = NO2, L-2-OH; and X = Br, L-3-OH) and their nickel(II) complexes (1a, 1b, 2, and 3) have been reported. The piperazinyl arm of these ligands can in principle have both boat and chair conformations that allow the ligands to bind the Ni(II) center in an ambidentate manner, forming square-planar and/or octahedral complexes. The nature of substitution in the salicylaldehyde aromatic ring and the type of associated anion in the complexes have profound influences on the coordination geometry of the isolated products. With the parent ligand L-1-OH, the product obtained is either a planar red compound [Ni(L-1-O)](+), isolated as tetraphenylborate salt (1a), or an octahedral green compound [Ni(L-1-NH)(H2O)(3)](2+), isolated with sulfate anion (1b); both have been crystallographically characterized. In aqueous solution, both these planar (S = 0) and octahedral (S = 1) forms are in equilibrium that has been followed in the temperature range 298-338 K by H-1 NMR technique using the protocol of Evans's method. The large exothermicity of the equilibrium process [Ni(L-1-O)](+) + 3H(2)O + H+ 1 = [Ni(L-1-NH)(H2O)(3)](2+) (DeltaHdegrees = -46 +/- 0.2 kJ mol(-1) and DeltaSdegrees = -133 +/- 5 J K-1 mol(-1)) reflects formation of three new Ni-OH2 bonds in going from planar to the octahedral species. With the 5-nitro derivative ligand L-2-OH, the sole product is an octahedral compound 2, isolated as a sulfate salt while with the bromo derivative ligand L-3-OH, the exclusive product is a planar molecule 3 with associated tetraphenylborate anion. Both 2 and 3 have been structurally characterized by X-ray diffraction analysis.