Study Of The Zn（Ⅱ）-NH₃-H₂O Complex In Ammoniacal Solution By Quantum Chemical Calculation

The "ammoniacal leaching-solvent extraction-acidic electrowinning" is one of the most promising technologies to treat the low-grade complex oxide minerals. But the extraction ratio of zinc is not high and the extraction process is not well revealed yet. In order to explore the mechanism of the extraction process, it’s very important to learn the species of the ammoniacal solution and their structure and properties.In this paper, quantum chemistry calculations is applied to explore the structure and properties of zinc species in ammoniacal solution. The main contents and conclusions are as follows:(l)Through comparative studies, it was found that Dunning double-ζ plus polarization and diffuse function (DZP diffuse) basis sets for oxygen, nitrogen, hydrogen and Stuttgart/Dresden ECP (SDD) basis sets for zinc ion are good choices for Zn(II)-NH3-H2O system because of their high accuracy and low resource consumption.(2) With ammonia molecules gradually take the place of water molecules in [Zn(H2O)n]2+(n=4,5,6), the bond length of Zn-O and Zn-N increases but the bond length of Zn-O is always longer than that of Zn-N bond because of their different affinity. The structural symmetry of the complexes associates with the spatial arrangement of ammonia molecules and water molecules. The more symmetrical ammonia molecules and water molecules are in complex, the more symmetrical the structure of complex is. With the increase of m in [Zn(NH3)m(H2O)n]2+(m+n=4,5,6) complexes, the binding energy of the complexes becomes larger. As the number of ammonia molecules increases, the vibration of Zn-O and Zn-N bonds trends to lower frequencies. Meanwhile, the vibrational spectra of O-H bond and N-H bond will have a little blue-shift. The charge of zinc ions is mainly transferred to the hydrogen atoms of ammonia molecules which have taken the place of water molecules. With the replacement of water molecules by ammonia molecules, the binding energy of four-coordinate complex increases, that of five-coordinate complex increases first and then decreases and that of six-coordinate complex decreases constantly.(3)Comparing four, five and six-coordinate complex of [Zn(NH3)m(H2O)n]2+(m+n=4,5,6) complex with the same number of ammonia molecules, it was found that the five-coordinate complex is most stable when m is less than two, and the four-coordinate complex is most stable when m is more than one.