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Among $\ce{Mn^2+}$, $\ce{V^2+}$, $\ce{Ni^2+}$ and $\ce{Ti^2+}$, which cation is having the highest hydration energy to form the aqua complex $\ce{[M(H2O)6]^2+}$?

I guess $\ce{Mn^2+}$ should be the answer as it has the most stable electronic configuration.

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Note that hydration energy is the energy released when water molecules are adsorbed on the surface of the cation. Hence the cation should have a high charge density, which means it should have a small size. So, the smaller, the better.

Here, $\ce{Ni}$ has the smallest atomic radii compared to all other options due to greater effective nuclear charge, so it's ionic radii would also follow the same trend. Hence, $\ce{Ni^2+}$ would have greater hydration enthalpy.

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