# Difference between sodium ion and a transition metal ion dissolving in water?

In class, I learned that when $\ce{NaCl}$ is dissolved in water, the partially negative oxygen part of the water molecules surround the $\ce{Na+}$ ion and energy is given off (hydration enthalpy)

Likewise, when a transition metal ion like $\ce{Fe^2+}$ is dissolved in water, water molecules surround the ion. However, in this case, we say that a complex ion is formed.

Can someone please tell if there is really a difference in the two cases and if yes, could someone specify them.

• I think that in transition metal complexes the donor atom (the center transition metal ion in the complex) has a large amount of ligands and that sodium has much less ligands (however (Na(NH3)6)+ is a complex)
– user2117
Dec 11 '13 at 12:55

However, when a transition metal ion gets surrounded with these water molecules, the lone pair electrons of the oxygen form a bond with the ion. This is because they interact with the $d$ orbitals of the transition metal (which usually also contains some electrons), thus changing the energetic state of the ion itself. First and second group elements don't have that energy-stabilizing effect because they don't have electrons in the orbitals that get split up by the formation of the complex.