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A u.v.–vis spectrophotometric study of the adduct formation of the nickel(II) Schiff base complexes,([NiL]) where L = [3-methoxysalophen, N,N′-bis(3-methoxysalicylidene)-1,2-phenylenediimine] (1), [4-methoxysalophen, N,N′-bis(4-methoxysalicylidene)-1,2-phenylenediimine] (2), [5-methoxysalophen, N,N′-bis(5-methoxysalicylidene)-1,2-phenylenediimine] (3) and [Salophen, [N,N′-bis(salicylaldehydo)-1,2-phenylenediimine] (4) as donors with R₂SnCl₂ (R = methyl, phenyl and n-butyl) as acceptors have been investigated in chloroform, as solvent. Adducts have been characterized by ¹H, ¹³C and ¹¹⁹Sn NMR, IR and electronic spectroscopy and CHN elemental microanalysis. The formation constants and the thermodynamic free energies were measured using u.v.–vis spectrophotometry titration for 1:1 adduct formation at various temperatures (T = 278 to 308 K). The trend of the adduct formation of the nickel …