Hydration of zinc ions: theoretical study of [Zn(H2O)4](H2O)82+ and [Zn(H2O)6](H2O)62+

Herein, a quantum-mechanical (QM) investigation of the title complexes is reported. Several different levels of QM theory (HF/6-31G∗, B3LYP/6-31G∗, MP2/6-31G∗, etc.) were employed in order to gain insights into the origin of the preference for six-coordination of hydrated Zn2+ ions which has been observed experimentally. B3LYP/6-311+G(2d,2p)//B3LYP/6-31G∗ calculations predict that the four coordinated structure [Zn(H2O)4](H2O)82+ is the most favorable one by 8 kJ/mol. At MP2/6-311+G(2d,2p)//MP2/6-31G∗, the [Zn(H2O)6](H2O)62+ complex is 28 kJ/mol more stable than the four-coordinated configuration [Zn(H2O)4](H2O)82+, thereby, satisfactorily reproducing the experimental observed preference for six-coordination. According to our analyses, the HF and B3LYP methods overestimate the contribution of hard and soft interactions to the Zn–water interactions, respectively, whereas the MP2 method gives a more balanced description.