Question

Among the ligands $N{H_3},en,C{N^ - }$ and $CO$ the correct order of their increasing field strength, is
A.$CO < N{H_3} < en < C{N^ - }$
B.$N{H_3} < en < C{N^ - } < CO$
C.$C{N^ - } < N{H_3} < CO < en$
D.$en < C{N^ - } < N{H_3} < CO$

Answer 1

Hint:Strong field ligands are the ligands that produce large splitting. Weak field ligands are the ligands that produce small splitting. Ligands are ranked according to the energy difference between ${{t}_{2g}}$ and ${{e}_{g}}$ in the spectrochemical series.

Complete step by step answer:
In the molecular orbital diagram, the antibonding interaction between the ${{e}_{g}}$ metal orbitals and the ligands are reflected by the splitting between the electron levels. Therefore, we expect field strength to co-relate with metal ligand orbital overlap. These ligands are arranged in spectrochemical series on the basis of weak and strong ligands. A spectrochemical series consists of ligands in the order of ligand strength. The left side of the spectrochemical series contains weak ligands and the right side contains the strong ligands. The weak ligands cannot be paired with the electrons within the $3d$ level and therefore, it shows high spin ligands. On the other hand, strong ligands pair up with the electrons within the $3d$ level and thus show low spin ligands.
Ligand field strength is directly proportional to the splitting strength of $d$ orbital.
Splitting strength of $d$ orbital means more the $d$ orbital splits, more will be field strength.
Spectrochemical series is written as follows:
${I^ - } < B{r^ - } < {S^{2 - }} < SC{N^ - } < C{l^ - } < {F^ - } < O{H^ - } < {C_2}{O_4}^{2 - } < {O^{2 - }} < {H_2}O < NC{S^ - } < N{H_3} < en < N{O_2}^ - < C{N^ - } < CO$
In spectrochemical series, ${{I}^{-}}$ is the weakest field ligand and $CO$ is the strongest field ligand. In spectrochemical series, when moving from left to right, field strength of the ligands keeps on increasing. And therefore, on the basis of this spectrochemical series, the correct option is (B) $N{{H}_{3}} < en < C{{N}^{-}} < CO$

Note: Crystal field theory gives us a bonding interaction between ligands and transition metals. It basically tells us about the attraction of positive charge on a metal cation and negative charge on the ligand. When the ligands come closer to the central metal ion, the degeneracy of the $d$ orbitals are broken due to electric field produced by surrounding charge distribution.
The factors that affect splitting are
-The nature of the metal ion
-The nature of ligand
-Oxidation state of a metal