Question

Match the List-I with List-II and choose the correct option

	List-I (Complexes)		List-II (Colour)
(P)	$[Co(NH_3)_6]Cl_3$	(1)	Green
(Q)	$[Co(NH_3)_5(H_2O)]^{3+}$	(2)	Violet
(R)	Cis $[Co(NH_3)_4Cl_2]Cl$	(3)	Yellow
(S)	Trans $[Co(NH_3)_4Cl_2]Cl$	(4)	Red
		(5)	Blue

• A. P-1, Q-2, R-3, S-4
• B. P-3, Q-4, R-1, S-2
• C. P-2, Q-1, R-4, S-3
• D. P-4, Q-3, R-2, S-1

Answer 1

Answer: (B)

P-3, Q-4, R-1, S-2

Answer 2

The colour of coordination complexes arises from d-d electronic transitions, which are dependent on the crystal field splitting energy ($\Delta_o$). $\Delta_o$ is influenced by the nature of ligands (spectrochemical series) and the complex's geometry. The observed colour is complementary to the absorbed light.

Spectrochemical Series: $Cl^- < H_2O < NH_3$ (Weak < Medium < Strong field ligands).

Complexes Analysis:

• (P) $[Co(NH_3)_6]Cl_3$: $Co^{3+}$ ($d^6$) with six strong field $NH_3$ ligands. This leads to a large $\Delta_o$. The complex absorbs blue-green light and appears Yellow.
• (Q) $[Co(NH_3)_5(H_2O)]^{3+}$: $Co^{3+}$ ($d^6$) with five strong field $NH_3$ and one medium field $H_2O$. This results in a slightly smaller $\Delta_o$ than the hexammine complex. It absorbs green light and appears Red.
• (R) Cis $[Co(NH_3)_4Cl_2]Cl$: $Co^{3+}$ ($d^6$) with four strong field $NH_3$ and two weak field $Cl^-$ ligands in a cis arrangement. This results in a significantly smaller $\Delta_o$. It absorbs red-orange light and appears Green.
• (S) Trans $[Co(NH_3)_4Cl_2]Cl$: $Co^{3+}$ ($d^6$) with four strong field $NH_3$ and two weak field $Cl^-$ ligands in a trans arrangement. This results in a smaller $\Delta_o$ than the cis isomer. It absorbs yellow-green light and appears Violet.

Matching:

• (P) $\rightarrow$ (3) Yellow
• (Q) $\rightarrow$ (4) Red
• (R) $\rightarrow$ (1) Green
• (S) $\rightarrow$ (2) Violet