The hydration energy is the energy released when ions are surrounded by water molecules and form hydrated ions. It is an important factor in determining the stability and solubility of ionic compounds in water. In general, the hydration energy of ions follows a predictable trend based on their charge and size. However, there are some discrepancies in the hydration energy of the complex of divalent metal ions of the 3d-series transition elements.

1. Hydration energy trend:
- In general, the hydration energy of ions increases with increasing charge and decreasing size. This is because ions with higher charge attract more water molecules and ions with smaller size can be more effectively hydrated.
- For example, the hydration energy of the divalent ions Mg2+ and Ca2+ follows this trend. Mg2+ has a higher hydration energy than Ca2+ because it has a higher charge and a smaller size.

2. Discrepancies in the 3d-series transition elements:
- The 3d-series transition elements include elements from scandium (Sc) to zinc (Zn) in the periodic table.
- The ions of these elements, such as Sc2+, Ti2+, V2+, Cr2+, Mn2+, Fe2+, Co2+, and Ni2+, show discrepancies in their hydration energy compared to the general trend.
- These ions have partially filled d-orbitals, which can interact with water molecules and affect the hydration energy.
- The presence of partially filled d-orbitals can lead to the formation of coordination complexes with water molecules, where water molecules directly coordinate with the metal ion.
- The formation of these coordination complexes can increase the stability and decrease the hydration energy of the ions.
- As a result, the hydration energy of these ions may be lower than expected based on their charge and size.

In conclusion, the divalent metal ions of the 3d-series transition elements show discrepancies in their hydration energy compared to the general trend. These discrepancies are due to the presence of partially filled d-orbitals, which can lead to the formation of coordination complexes with water molecules and decrease the hydration energy of the ions.