The protonation heats of N-(meta-substituted phenyl) iminodiacetic acid (m-CH3PhIDA), the heats of formation of Ni(II), Zn(II), Co(II)-m-CH3PhIDA binary systems and Ni(II), Zn(II), Co(II)-m-CH3PhIDA-nitrilotriacetic acid (NTA) ternary systems have been determined in aqueous solution at 25.0±0.2℃, I=0.1 mol.dm^-^3(NaClO4) by means of a model RD-1 automatic conduction calorimeter. To compare the formation heats for metal ions, we found that they obey Irving-Williams order, i.e. Co(II)Zn(II). A supposed structure of the hydrated ligand has been offered to discuss the formation heats of ternary complexes M(II)-m-CH3PhIDA-NTA and of binary complexes M(II)-m-CH3PhIDA.

Key words: ZINC COMPLEX, TERNARY COMPLEX, HYDROGEN BONDS, COBALT COMPLEX, NICKEL COMPLEX, METHYLBENZENE P, IMIDO-ACETIC ACID P