Natural Bond Orbital Analysis of [Fe(H 2O)6]2+/3+ and  ( )  ( )n + 2 6 2 2 Zn H O H O ; N=0-4

Metals are found in different forms in nature. One of the major forms is a molecule resulted from the coordination of metals with other molecules or ions. Such associations are often referred as coordination complexes. These coordinated complexes play very important roles in the physiological activities of human and plant lives. For example, haemoglobin, a macromolecule responsible for the transportation of oxygen and carbon dioxide in blood, consists of coordination of iron with nitrogen bases around it. Oxygen is transported by the first coordination sphere of the complex, whereas CO2 is transported by the second coordination sphere of the iron complex called heme. In plants, photosynthesis is carried out by the pigments called chlorophyll where coordination of magnesium with other molecules plays a vital role. Such prevalence of metal complexes indubitably make them a wide branch of study. The transition metal complexes are rich in structural diversity. 

Many successful computational study of aqua complexes have validated the outcomes of ab-initio methods using density functional theory. Structure and vibrational properties of the complexes [Fe(H2O)6]2+ and [Fe(H2O)6]2+ have been fastidiously studied by Jarzecki and co-workers with DFT approach. The geometries of the complexes are precisely determined computationally at B3LYP level of approximation. With the same DFT approach the aqua complexes of Zn2+ ion have been studied by Rul´ısekˇ and Havlas.