Symmetry and stability of a molecule (Jahn-Teller Distortion)

Question

I've taught in my class that if a molecule posses high degree of symmetry it's stability will be more,in other words symmetry and stability are directly proportional. Is this statement correct? And if this is correct than why Jahn teller distortion stabilizes a molecule when it's symmetry gets low eventually because of the distortion.

Answer 1

The symmetry and stability are not directly related per se; is benzene more stable that naphthalene for example? Benzene is more 'symmetric' than the naphthalene but both are stable molecules; $D_{6h}$ vs $D_{2h}$ point groups.

Jahn and Teller discovered that in a non-linear molecule in an orbitally degenerate electronic state, say $e_g$ or $t_{2g}$ in an octahedral complex, that there is an interaction between the electronic orbital motion and nuclear vibrational motion, which causes the minimum energy position to differ from the symmetric structure. This motion lifts the orbital degeneracy and lowers the energy. Thus the lower symmetry state has lower energy.

An example might be the lengthening of a pair of axial bonds in a octahedral complex, $\ce{[CoF6]}^{3-}$ for example, however, the J_T theorem does not give any indication of how big the distortion might be, i.e. it does not predict the geometrical nature of the distortion. There is also a restriction which is that if the degenerate structure has a centre of symmetry so does the distorted structure.

note: The electronic spin of the molecule (singlet, triplet etc.) is not important it is the orbital degeneracy only that matters. The Jahn-Teller effect is a breakdown of the Born-Oppenheimer approximation which is based on separating nuclear and electronic motion.

Answer 2

Well generally you can say that symmetry contributed to the stability. But in most cases you will find exception. The Jahn–Teller effect is most often encountered in octahedral complexes of the transition metals. In octahedral complexes, the Jahn–Teller effect is most pronounced when an odd number of electrons occupy the eg orbitals. This situation arises in complexes with the configurations d9, low-spin d7 or high-spin d4 complexes, all of which have doubly degenerate ground states. In such compounds the eg orbitals involved in the degeneracy point directly at the ligands, so distortion can result in a large energetic stabilisation and is not related to symmetry.