Question

Jahn-teller effect is not observed is high spin complexes of:
A. $d{}^4$
B. $d{}^9$
C. $d{}^7$
D. $d{}^8$

Answer 1

The jahn-teller effect also known as john teller distortion is used to describe the geometrical distortions of a molecule which is non linear that reduces its symmetry and energy. This is typically observed in the complexes which are octahedral where the axial bonds can be shorter or longer than the equatorial bonds.

Complete step by step answer:
The jahn-teller effect also known as john teller distortion is used to describe the geometrical distortions of molecules or ions that are associated with electronic configurations. The complexes with high spin such as $d{}^3,d{}^5,d{}^8,\& d{}^{10}$do not exhibit the effect of jahn-teller distortions. These configurations correspond to a variety of transition metals. Most common examples are as follows; $Cr{}^{3 + },Co{}^{3 + },\& Ni{}^{2 + }$.

Additional information:
This theorem postulated by Hermann John and Edward teller stated that “stability and degeneracy is not possible simultaneously unless the present molecule is linear” in terms of electronic state. This leads to a break of degeneracy which in turn stabilizes the molecule and by consequence, reduces the symmetry. later in the year $1937$ this theorem was revised and phrased as “ any non-linear molecular system in a degenerate state will be unstable and has to go under distortion forming a system of lower symmetry and low energy, and thereby removing degeneracy. This is commonly seen or observed with transition metal octahedral complexes and also in tetrahedral complexes as well.

Note: This effect is dependent on the electronic state of the system and it can also be observed in complexes which are tetrahedral. It is asymmetrically occupied. In case of the unevenly occupied orbitals, this effect is weak and hence the energy gain is much less.