Question

Which among the following is the strongest ligand?
A. $En$
B. $NH_3$
C. $CO$
D. $CN^-$

Answer 1

Hint: In the given question we have to find out the strongest ligand by taking help of spectrochemical series. This series helps to find out which is the strong and weak ligand.

Complete answer:
Spectrochemical series states that ligand that binds with $C$ is known as strong field ligand and those bind with electronegative atoms like $O$ and halogens are weak field ligand.
In easy words we can say that ligands in which donor atoms are oxygen, sulphur and halogens are weak field ligands. On other hand ligands in which donor atoms are carbon, phosphorus and sulphur are known to be strong field ligands.
Spectrochemical series is as follows
Weak field $I^- < Br^- < Cl^- < {NO_3}^- < F^- < OH^- < H_2O < Pyridine < NH_3 < NO_2^- < CN^- < CO $ Strong field
According to this series $CO$ is the strongest ligand among the following because carbon is donor in this, it has double bond $(C = O)$ and is positively charged.

Hence, the correct answer is option d. CO

Note:
The strength of any ligand is determined by the amount of crystal field energy (CFT). As $CO$ causes more amount of crystal field splitting because it has more crystal field energy, thus is a strong field ligand. Also it depends upon frontier orbital; CO is isoelectronic and has high orbital energy therefore has lower HOMO energy which means it is poor $\sigma $ donor and better $\pi $ acceptor. There is a strong field ligand.