Chemistry · Coordination Compounds · NEET
If the electrons in the d-orbitals of a metal complex are spread unevenly, the shape of the complex is not fully stable. So the complex bends slightly (distorts) to lower its energy and become more stable. This bending is the Jahn-Teller effect. It usually stretches or squeezes the two bonds along one axis (the z-axis), turning a perfect octahedron into a slightly stretched one.
Strong distortion happens when the eg orbitals are unevenly filled: d9, high-spin d4, and low-spin d7. In these cases the two eg orbitals (dz2 and dx2-y2) do not hold the same number of electrons, so the shape must distort. Uneven filling of only the t2g orbitals causes very weak distortion that we usually ignore in NEET.
The eg orbitals (dz2 and dx2-y2) point straight at the ligands. So if they are unevenly filled, the metal-ligand bonds along different axes feel very different pull, and the shape distorts strongly. The t2g orbitals point between the ligands, so uneven t2g filling only causes a tiny distortion. This is why NEET focuses on the eg rule.
Cu2+ is d9. In an octahedral field its eg set holds 3 electrons (eg3), so one eg orbital has 2 electrons and the other has 1. That is uneven filling of eg, which forces a strong distortion. This is why almost every Cu2+ complex is tetragonally distorted (two bonds longer, four bonds shorter).
There is no Jahn-Teller distortion when the eg orbitals are evenly filled (empty, half, or full each). High-spin d8 is a common example: its config is t2g6 eg2, so both eg orbitals hold exactly one electron each - even filling, no distortion. Fully symmetric cases like d3, low-spin d6, and d10 also do not distort.
Write the octahedral d-configuration, then look only at the eg part (the last two orbitals). If the two eg orbitals hold an unequal number of electrons, there is distortion. If they are equal (0-0, 1-1, or 2-2), there is no distortion. Do this fast and you can answer any NEET Jahn-Teller question.
Jahn-Teller effect is not observed in high spin complexes of:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is closely linked to Crystal Field Theory in the Coordination Compounds chapter. NEET has directly asked about it (2016), so you should know which configurations distort and the eg rule, even though NCERT covers it briefly.
It means the octahedron is stretched or squeezed along one axis (usually z). Two bonds along that axis become longer (or shorter) while the other four change the opposite way. This lowers the total energy and stabilises the complex.
In theory it can, but it is very weak and not important for NEET. NEET questions almost always deal with octahedral complexes, so focus on the eg rule for octahedral fields.
Remember Cu2+ (d9). Its eg3 filling is always uneven, so Cu2+ complexes are the textbook example of Jahn-Teller distortion. If you remember this one case, the concept is easy to recall in the exam.
