Question

Among the following species the one which causes the highest CFSE, ${ \Delta }_{ o }$ as a ligand is:
(a) ${ CN }^{ - }$
(b) ${ NH }_{ 3 }$
(c) ${ F }^{ - }$
(d) CO

Answer 1

CFT is a theory that is used to describe the stability of the metal complexes. It revolves around the loss of orbital degeneracy because of the uneven charge distribution due to the approach of the ligands towards the metal ion in specific directions to give the required geometry. Also strong field ligands will cause more splitting and hence the complex will be more stable.

Complete step by step solution:
Before solving this question, let us first understand the crystal field theory. Coordination compounds are formed by cations and the ligands coordinated with them. These ligands remove the orbital degeneracy in transition metal ions as they approach it. This phenomenon is described as crystal field theory developed by Hans Bethe and John Hasbrouck van Vleck. It describes the stabilization of the metal-ligand complexes qualitatively. This change in orbital degeneracy is further used to describe other phenomena such as magnetic properties and the colour of the complex. There are very few coordination compounds of large cations of low charge, such as ${ K }^{ + }$ and ${ Na }^{ + }$. However that is not the case for transition metal cations since they have electrons in d orbitals that are not spherically symmetric in nature. Their shape and occupation becomes a determining factor of how they are going to split up for a particular symmetry. For a particular transition metal ion, the five d-orbitals are degenerate. When ligands approach the metal ion in order to form a complex, some experience more repulsion from the d-orbital electrons than others based on the direction along which they are approaching in order to form a complex with a particular geometry. This leads to a split due to the electrostatic environment which is not uniform. Let us consider a complex with octahedral geometry. Ligands approach the metal ion along the x, y and z direction. Due to this approach, the electrons in the ${ d }_{ z^{ 2 } }$ and ${ d }_{ x^{ 2 }-{ y }^{ 2 } }$ orbitals (which lie along the direction along which the ligands are approaching) experience more repulsion than other orbitals. This leads to a split between the d-orbitals which is known as crystal field splitting. “For octahedral complexes, crystal field splitting is denoted by ${ \Delta }_{ o }$ (or ${ \Delta }_{ oct }$ )”. The ${ d }_{ z^{ 2 } }$ and ${ d }_{ x^{ 2 }-{ y }^{ 2 } }$ orbitals are raised in energy while the ${ d }_{ xy }$ , ${ d }_{ xz }$ and ${ d }_{ yz }$ orbitals are lowered in energy with respect to the initial energy level possessed by the degenerate d orbitals. This leads to more stability.
Strong field ligands will cause more splitting and hence the complex will be more stable. Among the options, Co is the strongest ligand.

Hence the correct answer is (d) CO

Note: CFT is applicable for all geometries as long as the metal cation possesses the d electrons but it also has its own limitations. It does not take into account the covalent character in metal-ligand bonding in the complexes. It fails to explain the ${ \Pi }$ bonding and back-bonding between the metal atom and the ligand and its effects on CFSE. It does not take into account the s and p orbitals of the central metal atom. It does not take into account the orbitals of the ligands.