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.

  • $\begingroup$ 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) $\endgroup$
    – user2117
    Dec 11, 2013 at 12:55

1 Answer 1


There is a slight difference between solvated ions and aqua complexes: The nature of the bond.

When solvating a sodium ion, the electron pairs of the oxygen in water never really interact with the sodium in a way that creates a bond of some sort: It's all just electrostatic interactions.

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.

It is not unimaginable that there then exist further electrostatic interactions between the transition metal aqua complex and the surrounding water molecules, thus leading to the solvation of said complex.


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