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Coordination Compounds Set-2

Practice Important MCQs for Coordination Compounds — Chemistry, Class 12.

Coordination compounds are a high-scoring area in CBSE and also a frequent topic in JEE/NEET due to their strong link with electronic structure (crystal field theory), magnetic properties, geometry, stereochemistry, and stability constants. Mastery of key ideas like splitting of d-orbitals in different geometries, spin-only magnetic moments, Jahn–Teller distortion, ligand effects, and formation constants directly helps solve both direct and assertion-reason based questions.

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Q1. The tetrahedral complex ion $[NiCl_4]^{2-}$ contains Ni(II) ( $d^8$ ). Using the spin‑only formula $\mu=\sqrt{n(n+2)}\,\mu_B$ where $n$ is the number of unpaired electrons, the expected spin‑only magnetic moment (in $\mu_B$ ) is:

(A)

$0.00\ \mu_B$

(B)

$1.73\ \mu_B$

(C)

$2.83\ \mu_B$

(D)

$3.87\ \mu_B$

Q2. For the equilibria $M+L\rightleftharpoons ML$ ( $K_1=10^4\ \mathrm{M^{-1}}$ ) and $ML+L\rightleftharpoons ML_2$ ( $K_2=10^3\ \mathrm{M^{-1}}$ ) the free ligand concentration is buffered at $[L]=10^{-3}\ \mathrm{M}$ . Using $\beta_2=K_1K_2$ and the relation $f_{ML_2}=\dfrac{\beta_2[L]^2}{1+K_1[L]+\beta_2[L]^2}$ , the fraction of total metal present as $ML_2$ (in %) is approximately:

(A)

$47.6\%$

(B)

$90.9\%$

(C)

$4.76\%$

(D)

$99.9\%$

Q3. Consider the octahedral complex ion $[Co(en)_2Cl_2]^+$ (en = ethylenediamine, a bidentate ligand). How many stereoisomers exist and which of them are optically active?

(A)

Two stereoisomers: cis and trans; neither is optically active

(B)

Four stereoisomers: two distinct cis enantiomeric pairs and two distinct trans forms; two are optically active

(C)

Two stereoisomers: only a pair of enantiomers (optically active); no trans isomer exists

(D)

Three stereoisomers: one trans isomer (achiral) and a pair of cis enantiomers (the cis forms are optically active)

Q4. Assertion (A): In octahedral $d^9$ complexes (for example $[Cu(H_2O)_6]^{2+}$ ) Jahn–Teller distortion commonly produces axial elongation and splits the degenerate $e_g$ orbitals.
Reason (R): Axial elongation reduces electrostatic repulsion along the z‑axis, lowering the energy of the orbital with lobes along $z$ (mainly $d_{z^2}$ ) relative to $d_{x^2-y^2}$ , thereby removing $e_g$ degeneracy.

(A)

A is true but R is false

(B)

Both A and R are true and R is the correct explanation of A

(C)

Both A and R are true but R is not the correct explanation of A

(D)

A is false but R is true

Q5. An octahedral Fe(II) ( $d^6$ ) complex shows a broad $d$ – $d$ band centered at $\lambda=600\ \mathrm{nm}$ assigned to the $t_{2g}\to e_g$ transition (so $\Delta_o\approx\dfrac{hcN_A}{\lambda}$ ). If the pairing energy is $P=150\ \mathrm{kJ\,mol^{-1}}$ , what is the likely spin state and the expected spin‑only magnetic moment (in $\mu_B$ ) for the ground state?

(A)

Low‑spin ( $\Delta_o>P$ ); spin‑only $\mu=0.00\ \mu_B$

(B)

High‑spin; spin‑only $\mu\approx4.90\ \mu_B$

(C)

High‑spin; spin‑only $\mu\approx5.92\ \mu_B$

(D)

Low‑spin; spin‑only $\mu\approx2.83\ \mu_B$

Q6. The complex ion $[Cr(NH_3)_4Cl_2]^+$ is formed with NH $_3$ neutral and Cl $^-$ monodentate. Determine the spin-only magnetic moment (in Bohr magneton, BM) of the complex. Use $\mu_{\text{spin}}=\sqrt{n(n+2)}\ \mathrm{BM}$ where $n$ is the number of unpaired electrons.

(A)

$1.73\ \mathrm{BM}$

(B)

$3.87\ \mathrm{BM}$

(C)

$4.90\ \mathrm{BM}$

(D)

$2.83\ \mathrm{BM}$

Q7. For the equilibria $M^{2+}+L\rightleftharpoons ML$ ( $K_1$ ) and $ML+L\rightleftharpoons ML_2$ ( $K_2$ ) with $K_1=1.0\times10^{4}$ and $K_2=1.0\times10^{3}$ , estimate the fraction of metal present as $ML_2$ when $[M]_{total}=1.0\times10^{-3}\ \mathrm{M}$ and free ligand concentration is $[L]=0.10\ \mathrm{M}$ (excess). (Neglect hydrolysis.)

(A)

$9.9\times10^{-3}\ (\,0.99\%\ )$

(B)

$0.50\ (\,50\%\ )$

(C)

$9.9\times10^{-2}\ (\,9.9\%\ )$

(D)

$0.990\ (\,99.0\%\ )$

Q8. How many stereoisomers (including optical isomers) are possible for the octahedral complex $[Co(en)_2Cl_2]^+$ , where en = ethylenediamine (bidentate)?

(A)

One

(B)

Two

(C)

Three

(D)

Four

Q9. Consider two octahedral $d^6$ metal ions M1 and M2. For ligand X, $\Delta_o(X)=180\ \mathrm{kJ\,mol^{-1}}$ and for ligand Y, $\Delta_o(Y)=420\ \mathrm{kJ\,mol^{-1}}$ . Pairing energies are $P_1=200\ \mathrm{kJ\,mol^{-1}}$ for M1 and $P_2=170\ \mathrm{kJ\,mol^{-1}}$ for M2. Predict the spin states (HS = high-spin, LS = low-spin) of the four complexes M1X, M1Y, M2X and M2Y.

(A)

M1X = HS, M1Y = LS, M2X = LS, M2Y = LS

(B)

M1X = HS, M1Y = HS, M2X = LS, M2Y = LS

(C)

M1X = LS, M1Y = LS, M2X = HS, M2Y = HS

(D)

M1X = LS, M1Y = HS, M2X = HS, M2Y = LS

Q10. Given the standard reduction potential for the free-ion couple $Fe^{3+}/Fe^{2+}$ is $E^\circ=+0.77\ \mathrm{V}$ . At 298 K the formation constants for hexacyanoferrate complexes are $\beta_{4}=1.0\times10^{14}$ for $Fe^{2+}+6CN^-\rightleftharpoons[Fe(CN)_6]^{4-}$ and $\beta_{3}=1.0\times10^{2}$ for $Fe^{3+}+6CN^-\rightleftharpoons[Fe(CN)_6]^{3-}$ . Using $E^\circ' = E^\circ + \dfrac{RT}{nF}\ln\!\dfrac{\beta_{4}}{\beta_{3}}$ with $T=298\ \mathrm{K}$ and $n=1$ , the standard potential $E^\circ'$ for the couple $[Fe(CN)_6]^{3-}/[Fe(CN)_6]^{4-}$ (in V) is closest to:

(A)

$1.06\ \mathrm{V}$

(B)

$1.48\ \mathrm{V}$

(C)

$0.77\ \mathrm{V}$

(D)

$0.91\ \mathrm{V}$

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