Question

In the given complexes:
${{[Co{{(N{{H}_{3}})}_{5}}({{H}_{2}}O)]}^{3+}}$……… (i)
${{[Co{{(N{{H}_{3}})}_{5}}Cl]}^{2+}}$……….. (ii)
${{[Co{{(N{{H}_{3}})}_{6}}]}^{3+}}$………… (iii)
The decreasing order of $\lambda$ absorbed is:
(a)- iii > ii > i
(b)- iii > i > ii
(c)- ii > i > iii
(d)- ii > iii > i

Answer 1

All the compounds given in the options are coordination compounds because the central metal atom in all the compounds is cobalt and three different types of ligands are present with it. All the ligands are based on the strength as strong field ligand and weak field ligand, so this strength of the ligand is inversely proportional to the absorption of the wavelength.

Complete answer:
We know that in coordination compounds, there are two types of elements, i.e., central metal atom and ligands, as the central metal atom has vacant orbitals while the ligands have unbounded pairs of electrons which are donated to the central metal atom forming a coordinate bond.
As the central metal ion has electrons that either get paired up or remain as such when the ligands approach the central metal ion. This depends on the strength of the ligands. If the ligand has high strength then it is a strong field ligand and if the ligand has low strength then it is a weak strong field ligand. So, the amount of the wavelength absorbed by the coordination compound depends on the strength of the ligands, i.e., the wavelength absorbed is inversely proportional to the strength of the ligand.
All the compounds given in the options are coordination compounds because the central metal atom in all the compounds are cobalt and three different types of ligands are present with it. These are:
${{[Co{{(N{{H}_{3}})}_{5}}({{H}_{2}}O)]}^{3+}}$, ${{[Co{{(N{{H}_{3}})}_{5}}Cl]}^{2+}}$, ${{[Co{{(N{{H}_{3}})}_{6}}]}^{3+}}$
So, the ligands are:
$N{{H}_{3}},{{H}_{2}}O,C{{l}^{-}}$
The decreasing order of the strength of the ligands is:
$N{{H}_{3}}>{{H}_{2}}O>C{{l}^{-}}$
So, the absorption of wavelength ($\lambda$) will be in the order of:
${{[Co{{(N{{H}_{3}})}_{5}}Cl]}^{2+}}$ > ${{[Co{{(N{{H}_{3}})}_{5}}({{H}_{2}}O)]}^{3+}}$ > ${{[Co{{(N{{H}_{3}})}_{6}}]}^{3+}}$

Therefore, the correct answer is option (c)- ii > i > iii

Note:
If the ligands in the complex are strong field ligands then the crystal field splitting energy or crystal field stabilization energy is also high. The highest CFSE (crystal field splitting energy) is for the CO group.