Nooshin Keshtkar - Department of Chemistry, Firoozabad branch, Islamic Azad University, P.O.Box ۷۴۷۱۵-۱۱۷, Firoozabad. Iran
Alison Zamanpour - Department of Chemistry, Firoozabad branch, Islamic Azad University, P.O.Box ۷۴۷۱۵-۱۱۷, Firoozabad. Iran
Sheida Esmaielzadeh - Department of Chemistry, Darab branch, Islamic Azad University, P.O. Box ۷۴۸۱۷۸۳۱۴۳-۱۹۶, Darab, Iran

To investigate the stability of [H2L] Schiff base ligand (where: L = derived methyl-2-(1-methyl-2 -aminoethane)amino-1-cyclopentenedithiocarboxylate) and their Ni(II) and Cu(II) complexes in gas phase, some properties such as HOMO-LUMO energies, chemical hardness and binding energy were obtained by using the gradient corrected density functional theory (DFT) method with the B3LYP functional [1] and LANL2DZ and 6-311G٭ basis sets. The optimized geometries of [H2L] shows the mentioned ligand can act as an NNOS tetradentate chelating agent using the thion sulfur atom (S), the nitrogen atom (N) from (N=CCH3) group, the primary nitrogen atom (N) and the hydroxyl oxygen atom (O). The title ligand contains two planar fragments, vis. the cyclopentene and salicylate derivatives moieties. The ligand uses its four atoms to link the metal ion. Therefore, the metal centre is in a tetracoordinated environment and the geometry about the metal ions could be described as square planar for [NiL] and square planar distorted for [CuL]. A large HOMO-LUMO energy gap increase stability and decrease chemical reactivity [2]. The calculation results confirm that the [CuL] complex is the most stable than [NiL] complex. This finding is further supported by the fact that the experimental formation constants of these complexes