Question

Find the number of strong field ligands or ligands from the following?
$N{H_3}$ , $en$ , $C{N^ - }$ , $C{l^ - },C{H_3}CO{O^ - }$

Answer 1

To proceed with the answer of this question, first we need to understand what ligands are. Ligands are the ions or the atoms or neutral molecules that bond with the central atom mostly in coordination compounds.

Complete answer:
Ligands basically act as a Lewis base, that is they donate their lone pair of electrons to the central atom. Whereas the central metal atom acts as the Lewis acid that is, it accepts lone pairs of electrons from the ligands.
Ligands must contain at least one donor atom to form covalent bonds with the central atom.
For categorising the ligands into the category of strong field ligand and weak field ligand, let’s understand them.
So, a weak field ligand causes a transition metal to have a small crystal field splitting. Whereas, on the other hand the strong field ligand causes a transition metal to have a large crystal field splitting.
Also, weak field ligands have high spin whereas strong field ligands have low spin.
So, among the given options and categorising them into weak field ligands and strong field ligands, we have:
Strong field ligands: $N{H_3}$ , $en$ , $C{N^ - }$
Weak field ligands: $C{l^ - },C{H_3}CO{O^ - }$
Thus, from the above categorisation, we can see that there are three strong field ligands.
Therefore, the correct answer is $3$ .

Note:
Crystal field splitting is a theory given to explain the interaction between the ligands and the metal ion. Actually Crystal field splitting theory explains the difference in energy between the d- orbitals of the ligands. It also explains the change in colour between the two metal ligand complexes.