Transition metals water orbitals

The distribution of the d electrons between these various orbitals is determined by the differences in energy level between them. The energy difference is directly proportional to the electric field strength of the ligand systems taking part in complex formation. Since the transition-metal ions do not exist in isolation, they must be evaluated in association with counter-ions or solvents such as water or other ligand systems present. 

Although this example, at face value, looks to be a case of the use of the absorption of UV/visible radiation to determine the concentration of a single ionic species (the Cu2+ ion) in solution, and, therefore, the province of the previous chapter, it is, in fact, the quantification of a molecular absorption band. In a sulfate solution, the copper ion actually exists, not as a bare ion, but as the pentaquo species, in which the central copper ion is surrounded by five water molecules and a sulfate ion in an octahedral structure (Fig. 4.1). The color of the transition metal ions arises directly from the interaction between the outer d orbital electrons of the transition metal and the electric field created by the presence of these co-ordinating molecules (called ligands). Without the aquation... 

Somewhat better data are available for the enthalpies of hydration of transition metal ions. Although this enthalpy is measured at (or more property, extrapolated to) infinite dilution, only six water molecules enter the coordination sphere of the metal ion lo form an octahedral aqua complex. The enthalpy of hydration is thus closely related to the enthalpy of formation of the hexaaqua complex. If the values of for the +2 and +3 ions of the first transition elements (except Sc2, which is unstable) are plotted as a function of atomic number, curves much like those in Fig. 11.14 are obtained. If one subtracts the predicted CFSE from the experimental enthalpies, the resulting points lie very nearly on a straight line from Ca2 lo Zn2 and from Sc to Fe3 (the +3 oxidation state is unstable in water for Ihe remainder of the first transition series). Many thermodynamic data for coordination compounds follow this pattern of a douUe-hunped curve, which can be accounted for by variations in CFSE with d orbital configuration. 

Hameed, A., M.A. Gondal and Z.H. Yamani (2004). Effect of transition metal doping on photocatalytic activity of WO3 for water splitting under laser illumination Role of 3d-orbitals. Catalysis Communications, 5(11), 715-719. 

---