Question

Which of the following complexes will show strong Jahn-Teller distortion?
A. ${\left[ {Cu{{\left( {{H_2}O} \right)}_6}} \right]^{2 + }}$
B. ${\left[ {Cr{{\left( {{H_2}O} \right)}_6}} \right]^{3 + }}$
C. ${\left[ {Mn{{\left( {{H_2}O} \right)}_6}} \right]^{2 + }}$
D. ${\left[ {Co{{\left( {{H_2}O} \right)}_6}} \right]^{3 + }}$

Answer 1

Cobalt, manganese, copper, and chromium are d- block metals. The d block elements are found in the group 3,4,5,6,7,8,9,10,11 and 12 of the periodic table. These are also known as transition metals. The d orbital is filled with an electronic shell $n - 1$ . There are a total of 40 d block elements.

Complete step by step answer:
Ligands bind with the central atom to form a coordination complex. This type of bonding usually takes place when there is a formal donation of one or more pairs of electrons of the ligand. Ligands can be mainly classified into two main types: Strong field ligands and weak field ligands.
There is a repulsion between the d orbitals of the central metal atom and the d orbitals of the ligands because of their similar electronic signature. Because of this, $d$ electrons closer to the ligands will have higher energy than those further away, which results in the d orbitals splitting in energy. The splitting of the d-orbitals into different energy levels in transition metal complexes has important consequences for their stability, reactivity, and magnetic properties.
When d orbitals split into different energy levels if there is unsymmetrical configuration is present in any energy level Jahn teller diction occurs. Due to this distortion, the bond along the z-axis gets disturbed. And that distortion is called z-out distortion or z in distortion.
Among the given options, the complex ${\left[ {Cu{{\left( {{H_2}O} \right)}_6}} \right]^{2 + }}$ showed Jahn teller distortion. As in this complex, copper has an unsymmetrical configuration as follows,
${t_2}{g^6}\,e{g^3}$ .

So, the correct option is A.

Additional information:
Let us discuss a little more about these two types of ligands.
Strong field ligands cause a large splitting in the given chemical species. This means strong field ligands exert strong ligand electrical fields. This property of strong-field ligands makes them capable of forming low spin complexes. This means that strong-field ligands disobey Hund’s multiplicity rule and causes the pairing of electrons even before the entire subshell is filled with electrons of a specific spin character. On the other hand, weak-field ligands obey Hund’s rule.

Note: It should be known that in complexes of weak field ligands, ${\Delta _0} < P$ (pairing energy), the energy difference between ${t_2}g$ and e.g. sets are relatively less. Under the influence of strong-field ligands, ${\Delta _0} > P$ (pairing energy), the energy difference between ${t_2}g$ and e.g. sets are relatively high.