The structures and properties of the octahedral M-ethylenediaminetetraacetate complexes (M＝Co^2＋, Ni^2＋, Cu^2＋, Zn^2＋, Cd^2＋) have been studied using density functional theory method at the B3LYP/LanL2DZ level. Apart from the M—O(5) of Cu(EDTA)^2－ which is obviously elongated because of Jahn-Teller effect, the bond lengths of M—N(1) and M—O(5) are increased gradually in the order of Cu^2＋＜Ni^2＋＜Co^2＋＜Zn^2＋＜Cd^2＋while the bond lengths of M—O(3) are increased in the order of Cu^2＋＜Zn^2＋＜Ni^2＋＜Co^2＋＜Cd^2＋. The NBO analysis shows that the interaction between the non-bonded electrons of nitrogen and oxygen atoms and the vacant orbitals of metals is vital for the complexation. Based on the analysis of bond lengths and NAO bond orders of N(1)—C(7), N(1)—C(9) and N(1)—C(15), the fact that the complexed and uncomplexed EDTA turn out different products in the degradation process is illustrated on the level of molecule. According to the thermodynamic theory with considering the IEFPCM model of self-consistent reaction field (SCRF) as well, we have designed reactions and a thermodynamic cycle for the hydrate metal ions and EDTA in aqueous solution. The energetic results indicate that the sequence of binding energies of cations and EDTA (i.e. complexation stability) is Cd^2＋＜Zn^2＋＜Co^2＋＜Ni^2＋＜Cu^2＋. The calculated Gibbs free energies of hydration for metal ions are in agreement with their experimental values as well as the order of complexation Gibbs free energies of studied complexes. Based on the vibrational analysis upon the optimized geometry in gas phase, some important peaks of the infrared spectra are assigned. The IR result shows that the ν(M—N) and ν(M—O), the IR activitied metal sensitive vibrations of M(EDTA)^2－ complexes, are shifted towards higher along with increase of complexation stability.

Key words: density functional theory, ethylenediaminetetraacetate, complexes, degradation