Why do transition metals form particularly stable complexes?

Question

The pre-transition metal ions can form complexes but the ligands are weakly bound unless they are multidentate and thus benefit from the chelate effect. For example, crown ethers and cryptands.

However, transition metal ions can form a vast array of stable of stable complexes with monodentate and multidentate ligands. Why is there such a difference between the pre transition elements and the transition metals in this respect.

My initial thoughts are that the transition metal ions will have a higher charge and smaller radius so the electrostatic attraction is larger. I also thought about the d-orbitals but the pre transition elements have d orbitals available, they're just not filled. As a result, is increased stability related to exchange energy or perhaps the lower energy d orbitals? Are there any more factors? Are my suggestions valid?

Answer 1

To illustrate my answer, I am going to use a modified version of the picture of a simplified molecular orbitals scheme Professor Klüfers (LMU Munich) uses in his internet scriptum to the coordination chemistry I course.

On the left you can see the metal’s orbitals, on the right the group orbitals of six $\unicode[Times]{x3C3}$ donors (assuming no $\unicode[Times]{x3c0}$ interactions of any kind). $\mathrm{e_{g}}$ and $\mathrm{t_{2g}}$ correspond to the metal’s d-orbitals, $\mathrm{a_{1g}}$ is the 4s orbital and $\mathrm{t_{1u}}$ are the 4p-orbitals.

You can clearly see that the 3d-orbitals are similar in energy to the ligand group orbitals, and that they have the correct symmetry to mix. The greatest stabilisation comes from the $\mathrm{e_g}$ orbitals.

Empty d-orbitals can thus be thought to help with coordination by stabilising the coordination compound. Completely filled d-orbitals (e.g. $\ce{Zn^2+}$) are contraproductive and therefore zinc’s coordination chemistry is much like sodium’s.