The electronic and steric effects of some Schiff bases and the solvent on the thermodynamic parameters of the pentacoordinate Co(III) Schiff base complexes were studied. The formation constants and the thermodynamic parameters were measured spectrophotometrically for 1:1 adduct formation of the complexes as acceptors with tributylphosphine (PBu 3 ) as donor, in some solvents (acetonitrile, tetrahydrofuran, butanol, ethanol and N,N-dimethylformamide) in constant ionic strength (I = 0.01 M, sodium perchlorate) and at various temperatures. The trend of the reactivity of the pentacoordinate cobalt(III) Schiff base complexes toward tributylphosphine according to the solvent is as follows: acetonitrile > tetrahydrofuran > butanol > ethanol > N,N-dimethylformamide. The trend of the reactivity of pentacoordinate cobalt(III) Schiff base complexes toward the donor in a given solvent according to the equatorial Schiff base is as follows: BBE > BAE > Salen.

Some new unsymmetrical diimino tetradentate Schiff bases derived from 3,4-diaminobenzophenone, (N-salicyliden-N'-5-OMe.salycyliden)-3,4-diaminobenzophenone (H 2 sal-5-OMe.sal.dabp) (H 2 L 1 ), (H 2 sal-5-Br.sal.dabp) (H 2 L 2 ), (H 2 sal-5-Cl.sal.dabp) (H 2 L 3 ) and (H 2 sal-5-NO 2 .sal.dabp) (H 2 L 4 ) and their Ni(II) and Cu(II) complexes were synthesized and characterized by elemental analysis, IR, 1 H NMR, UV-Vis spectra and mass spectra. The thermodynamic formation constants of the complexes were determined spectrophotometrically at constant ionic strength (0.1 M NaClO 4 ) at 25 ºC in DMF and their free energies of formation were calculated at 25 ºC.

The trend of the formation constants of cobalt(III) Schiff base complexes toward a given donor according to the axial ligand is as follow: PBu3 > PMePh2. Also, the following binding trend of the donors toward a given cobalt(III) Schiff base complex is obtained: benzylamine > n-butylamine > sec-butylamine > tert-butylamine.