Question: 87: The order of energy absorbed which is responsible for the color of complexes
(A) \left[\mathrm{Ni}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}(\mathrm{en})_{2}\right]^{2+}
(B) \left[\mathrm{Ni}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}(\mathrm{en})\right]^{2+} and
(C) \left[\mathrm{Ni}(\mathrm{en})_{3}\right]^{2+}
is
(1) (C) > (A) > (B)
(2) (B) > (A) > (C)
(3) (A) > (B) > (C)
(4) (C) > (B) > (A)
Answer: Option (1)
Explanation:
The color of coordination complexes arises due to absorption of energy corresponding to the crystal field splitting energy \Delta,
which depends on the nature and strength of ligands present around the metal ion.
The ligand ethylenediamine \mathrm{en} is a stronger field ligand than water \mathrm{H_2O}.
Stronger ligands produce a larger crystal field splitting energy \Delta,
and hence more energy is absorbed.
In complex (C), \left[\mathrm{Ni}(\mathrm{en})_{3}\right]^{2+},
all ligands are \mathrm{en},
so the crystal field splitting is maximum and the energy absorbed is highest.
In complex (A), \left[\mathrm{Ni}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}(\mathrm{en})_{2}\right]^{2+},
two ligands are \mathrm{en} and two are \mathrm{H_2O},
so the crystal field splitting energy is intermediate.
In complex (B), \left[\mathrm{Ni}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}(\mathrm{en})\right]^{2+}, only one ligand is \mathrm{en}
and the remaining are weak field \mathrm{H_2O},
so the crystal field splitting energy is the least.
Therefore, the order of energy absorbed is (C) > (A) > (B).