Question

Which one of the following is a strong field ligand?
\[
  A)C{N^ - } \\
  B)NO_2^ - \\
  C)en \\
  D)N{H_3} \\
 \]

Answer 1

Hint: Strong field ligands produce a large magnitude of splitting, while weak field ligands produce a small magnitude of splitting. We can find the strong field ligand by taking the help of the spectrochemical series.

Complete step by step answer:
Spectro chemical series are the order in which ligands are arranged in the increasing order of the magnitude of splitting they produce.
The order is:
${I^ - } < B{r^ - } < C{l^ - } < NO_3^ - < {F^ - } < O{H^ - } < {H_2}O < Pyridine < N{H_3} < NO_2^ - < C{N^ - } < CO$
Strong field ligands are present on the right hand side of the series while weak field ligands are present on the left hand side of the series.
Strong field ligand causes higher magnitude of splitting in the d-orbitals as compared to the weak field ligands.
The strong field ligand cyanide ion has a more electropositive carbon atom, which allows for greater orbital overlap and electron pair sharing.
We all know that the carbon atom, rather than the nitrogen atom, is used to coordinate metal ions in the cyanide ion.
Since it is a pseudo halide ion, the cyanide ion is a strong field ligand.
Pseudo halide ions are more powerful coordinating ligands, with the ability to form bonds (from the pseudohalide to the metal) and bonds (from the metal to the pseudohalide).

Therefore, option $A)C{N^ - }$ is the correct answer.

Note: The Crystal Field Theory (CFT) is a model for transition metal and ligand bonding interactions. It explains the effect of the ligand's non-bonding electrons being attracted by the positive charge of the metal cation and the negative charge of the metal cation.
Weak field ligands bind through electronegative atoms like $O$ and halogens, whereas strong field ligands bind through $C$ or $P$ . The power of ligands that bind through $N$ is intermediate.