The [N,N'-bis(Salicyliden)-2,3-diaminopyridine]tributylphosphinecobalt(III) perchlorate monohydrate, [Co(Salpyr)(PBu3)]ClO4 center dot H2O, was synthesized and characterized. For the new five coordinate complex and some others, such as [N,N'-bis(Salicyliden)-1,2-phenylenediamine]tributylphosphinecobalt(III) perchlorate monohydrate, [Co(Saloph)(PBu3)]ClO4 center dot H2O, [N,N'-bis(4-methylSalicyliden)-1, 2-phenylenediamine]tributylphosphine cobalt(III) perchlorate monohydrate, [Co(4-MeSaloph)(PBu3)]ClO4 center dot H2O, [N,N'-bis(4-nitroSalicyliden)-1,2-phenylenediamine]tributylphosphinecoba lt(III) perchlorate monohydrate, [Co(4-NO(2)Saloph)(PBu3)]ClO4 center dot H2O, and [N, N'-bis(Salicyliden)-1,2-phenylenediamine]dimethylphenylphosphinecobalt(I II) perchlorate monohydrate, [Co(Saloph)(PMe2Ph)]ClO4 center dot H2O, the formation constants and the thermodynamic parameters of the interaction of Co(III) Schiff bases with amines were measured spectrophotometrically. Furthermore, the formation constant of these 1: 1 adducts in 96% ethanol solvent and in constants ionic strength (I = 0.1 M NaClO4) and at different temperatures were determined using the Ketelaar equation. Also the adduct formation of [Co(Saloph)(PBu3)]ClO4 center dot H2O, with Imidazol, 2-MeImidazol, 4-MeImidazol, 2-EtImidazol, and BzImidazol in 96% ethanol was studied. The formation constants and the thermodynamic parameters for [Co(Salpyr)(PBu3)]ClO4 center dot H2O as an acceptor with some aliphatic amines, such as n-butylamine, sec-butylamine, tert-butylamine, benzylamine, and diethylamine as donors were determined in DMA. The comparison of formation constants and the thermodynamic parameters show that the trend of formation constants of pentacoordinate cobalt(III) Schiff base complexes toward a given donor at lower temperature of the range used (283 and 293) K is as follows: 4-MeSaloph > Saloph > 4-NO(2)Saloph > Salpyr. Also the following binding trend of the donors toward a given cobalt(III) Schiff base complex is in sequence: Im > 4-MeIm > 2-MeIm > 2-EtIm > BzIm and for aliphatic amines is as follows: Benzylamine > sec-butylamine similar to n-butylamine > tert-butylamine > diethylamine. The formation constants for a given donor change according to the following trend for phosphine axial ligands: PBu3 < PMe2Ph. (c) 2007 Elsevier Ltd. All rights reserved.