Formation of Metal Complexes with Ethylenediamine: a Critical Survey of Equilibrium

At present ethylenediamine is one of the more studied nitrogen— —ligands. The main physico—chemical properties of this diamine are reported in this survey. The fundamental techniques for its purification and storage and its acid—base characteristics in aqueous solution are described. The formation constants of the complexes with various metal ion, the thermodyna mic values relative to the reactions of formation of the complexes and their stereochemistry are presented. The protonation constants of the diami me and the corresponding enthalpic and entropic values obtained in various ionic media have been considered and a critical examination of the resp.lts is presented. The values of the stability constants of the complexes and the thermodynamic quantities relative to the present equilibria have been critically examined. Such values are recommended when the metallic systems, also in their proton— and hydroxide—form, are well determined and results by different authors are in agreement. For cases lacking accurate data tentative values are suggested or further investigations are recommended. Errors on the recommended values have been estimated. 1 — Physical and chemical properties of ethylenediamine NH —CS -CH —NH 1 2—diaminoethane ethylenediamine (en) 2 2 2 2 Ethylenediamine is colourless at room temperature having the characteristic smell of an amine. 0 . . . 0 -, It has a melting point of 11.1 C and a boiling point of 116.9 C at 101325 Pa (1 atm.). uome physico—chemical constants of pure ethylenediamine are reported in Table 1. TABLE 1. Physico-chemical constants of ethylenediamine Ref. 1) Molecular weight 60.099 2) Temperature of phase transition C(II) ÷ C(I) -8.2 °C 15M 3) Melting point 11.1 °C 75M ) Boiling point 116.9 °C 75M 5) Enthalpy of transition 87 J mol 1 15M 6) Enthalpy of fusion 22.6 kJ mol 1 15M i) Enthalpy of vaporization 5.69 kJ mol 1 69F 8) Heat capacity of crystal(II) at 1 °C 90.1 JK1mol T5M 9) Heat capacity of crystal(I) at 0 —1 —1 —1.5 C 102.3 JK mol 15M 10) Heat capacity (liquid) 19.65—25.0800x10 2T+1.00x10 T2 —13 —1 —1 —3.5970x10 T JK mol 15M