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Coordination Compounds Set-4 MCQ Online Practice Test | Class 12 | Quiz Tweek

Coordination Compounds Set-4

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

Coordination compounds form a major and scoring chapter because they link core concepts like oxidation state, geometry, crystal field splitting, magnetism, stereochemistry, and stability constants. CBSE and competitive exams commonly test these ideas through calculation-based MCQs and conceptual HSAB/trans-effect/Jahn–Teller questions, so strong command here directly boosts both accuracy and speed.

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Q1. Consider the complex ion $[Co(NH_3)_4Cl_2]^+$ . Determine the oxidation state of Co and the number of $d$ ‑electrons on Co.

(A)

Oxidation state $+2$ , configuration $d^7$

(B)

Oxidation state $+3$ , configuration $d^6$

(C)

Oxidation state $+1$ , configuration $d^8$

(D)

Oxidation state $+4$ , configuration $d^5$

Q2. For the equilibrium $M^{2+}+4L\rightleftharpoons[ML_4]^{2+}$ the overall formation constant is $\beta_4=1.0\times10^{12}$ . If initial concentrations are $[M^{2+}]_0=1.0\times10^{-3}\ \mathrm{M}$ and $[L]_0=1.0\times10^{-2}\ \mathrm{M}$ (ligand in large excess so $[L]\approx1.0\times10^{-2}\ \mathrm{M}$ ), estimate the fraction of metal present as $[ML_4]^{2+}$ at equilibrium.

(A)

Approximately $50\%$

(B)

Approximately $90\%$

(C)

Approximately $99\%$

(D)

Approximately $99.99\%$

Q3. Which complex has the larger spin‑only magnetic moment: $[Fe(H_2O)_6]^{2+}$ or $[Fe(CN)_6]^{4-}$ ? Use $\mu=\sqrt{n(n+2)}\ \mu_B$ where $n$ is the number of unpaired electrons.

(A)

$[Fe(H_2O)_6]^{2+}$ has larger $\mu$ ; $\mu([Fe(H_2O)_6]^{2+})\approx4.90\ \mu_B$ , $\mu([Fe(CN)_6]^{4-})\approx0\ \mu_B$

(B)

$[Fe(CN)_6]^{4-}$ has larger $\mu$ ; $\mu([Fe(CN)_6]^{4-})\approx4.90\ \mu_B$ , $\mu([Fe(H_2O)_6]^{2+})\approx0\ \mu_B$

(C)

Both have similar $\mu\approx2.83\ \mu_B$

(D)

$[Fe(H_2O)_6]^{2+}$ $\mu\approx1.73\ \mu_B$ , $[Fe(CN)_6]^{4-}$ $\mu\approx4.90\ \mu_B$

Q4. Consider octahedral Fe(II) (d $^6$ ) complexes A and B with $\Delta_o(\text{A})=2.0\times10^{4}\ \mathrm{cm^{-1}}$ and $\Delta_o(\text{B})=1.2\times10^{4}\ \mathrm{cm^{-1}}$ . Take pairing energy $P=1.8\times10^{4}\ \mathrm{cm^{-1}}$ . Neglect other contributions. For a d $^6$ ion the energy difference (low‑spin minus high‑spin) can be written as $\Delta E=-2\Delta_o+2P$ . Which statement is correct?

(A)

Both A and B are low‑spin; $\Delta E(\text{A})=-4.0\times10^{3}\ \mathrm{cm^{-1}}$ , $\Delta E(\text{B})=-1.2\times10^{4}\ \mathrm{cm^{-1}}$

(B)

Both A and B are high‑spin; $\Delta E(\text{A})=+4.0\times10^{3}\ \mathrm{cm^{-1}}$ , $\Delta E(\text{B})=+1.2\times10^{4}\ \mathrm{cm^{-1}}$

(C)

A is low‑spin, B is high‑spin; $\Delta E(\text{A})=-4.0\times10^{3}\ \mathrm{cm^{-1}}$ , $\Delta E(\text{B})=+1.2\times10^{4}\ \mathrm{cm^{-1}}$

(D)

A is high‑spin, B is low‑spin; $\Delta E(\text{A})=+4.0\times10^{3}\ \mathrm{cm^{-1}}$ , $\Delta E(\text{B})=-1.2\times10^{4}\ \mathrm{cm^{-1}}$

Q5. Assertion (A): The complex $[Co(en)_2Cl_2]^+$ exhibits optical isomerism. Reason (R): The coordination number of Co in $[Co(en)_2Cl_2]^+$ is six.

(A)

A is true, R is true and R is a correct explanation of A

(B)

A is true, R is true but R is NOT a correct explanation of A

(C)

A is true, R is false

(D)

A is false, R is true

Q6. (Calculate the spin-only magnetic moment (in Bohr magneton, BM) of the metal ion in the complex $[\mathrm{Ti}(\mathrm{H}_2\mathrm{O})_6]^{3+}$ . Use $\mu_s=\sqrt{n(n+2)}$ where $n$ is the number of unpaired electrons.)

(A)

$0.00\ \mathrm{BM}$

(B)

$3.87\ \mathrm{BM}$

(C)

$1.73\ \mathrm{BM}$

(D)

$2.83\ \mathrm{BM}$

Q7. (A $0.010\ \mathrm{M}$ solution of $\mathrm{Ag}^+$ is mixed with $0.100\ \mathrm{M}$ $\mathrm{NH}_3$ . The formation constant for $[\mathrm{Ag}(\mathrm{NH}_3)_2]^+$ is $K_f=1.7\times10^7$ . Calculate the equilibrium concentration of free $\mathrm{Ag}^+$ (in M). Assume ideal behaviour.)

(A)

$9.2\times10^{-8}\ \mathrm{M}$

(B)

$1.2\times10^{-4}\ \mathrm{M}$

(C)

$5.8\times10^{-6}\ \mathrm{M}$

(D)

$1.0\times10^{-10}\ \mathrm{M}$

Q8. (For an octahedral $d^6$ metal ion (Fe $^{2+}$ ) the pairing energy is $P=120\ \mathrm{kJ\,mol^{-1}}$ and the octahedral splitting $\Delta_o=140\ \mathrm{kJ\,mol^{-1}}$ . Which spin-state is predicted and why?)

(A)

High-spin; because $\Delta_o < P$

(B)

Low-spin; because $\Delta_o > 2P$

(C)

High-spin; because additional pairing energy $2P$ (240 kJ,mol $^{-1}$ ) exceeds CFSE gain $2\Delta_o$ (280 kJ,mol $^{-1}$ )

(D)

Low-spin; because CFSE gain upon low-spin formation ( $2\Delta_o=280\ \mathrm{kJ\,mol^{-1}}$ ) is greater than extra pairing energy ( $2P=240\ \mathrm{kJ\,mol^{-1}}$ )

Q9. (Compare $[\mathrm{Fe}(\mathrm{CN})_6]^{3-}$ (Fe $^{3+}$ ) and $[\mathrm{Fe}(\mathrm{CN})_6]^{4-}$ (Fe $^{2+}$ ). Which statement about Fe–C bond length and CN stretching frequency $\nu_{\mathrm{CN}}$ is correct?)

(A)

Fe–C bond shorter in $[\mathrm{Fe}(\mathrm{CN})_6]^{3-}$ ; $\nu_{\mathrm{CN}}$ higher in $[\mathrm{Fe}(\mathrm{CN})_6]^{4-}$

(B)

Fe–C bond shorter in $[\mathrm{Fe}(\mathrm{CN})_6]^{4-}$ ; $\nu_{\mathrm{CN}}$ lower in $[\mathrm{Fe}(\mathrm{CN})_6]^{4-}$

(C)

Fe–C bond lengths equal in both complexes; $\nu_{\mathrm{CN}}$ identical

(D)

Fe–C bond shorter in $[\mathrm{Fe}(\mathrm{CN})_6]^{4-}$ ; $\nu_{\mathrm{CN}}$ higher in $[\mathrm{Fe}(\mathrm{CN})_6]^{4-}$

Q10. (Thiocyanate ion SCN $^-$ is ambidentate. Statement A: In complexes of Al $^{3+}$ with SCN $^-$ , the ligand binds preferentially through nitrogen (isothiocyanato). Statement R: According to HSAB principle, hard acids prefer hard bases; Al $^{3+}$ being a hard acid therefore prefers the harder donor site N over the softer S of SCN $^-$ .)

(A)

A and R are true, and R correctly explains A

(B)

A and R are true, but R is not the correct explanation for A

(C)

A is false but R is true

(D)

A is true but R is false

Q11. Calculate the spin-only magnetic moment (in Bohr magneton, BM) for the high-spin octahedral complex $[Fe(H_2O)_6]^{2+}$ . Use $\mu_{\text{spin}}=\sqrt{n(n+2)}$ where $n$ is the number of unpaired electrons.

(A)

3.87 BM

(B)

4.90 BM

(C)

0.00 BM

(D)

2.83 BM

Q12. How many stereoisomers (counting enantiomers separately) exist for the complex $[Co(en)_2Cl_2]^+$ ?

(A)

(B)

(C)

(D)

Q13. $Ni^{2+}$ is $d^8$ . Experimentally $[Ni(CN)_4]^{2-}$ is square-planar and diamagnetic, while $[NiCl_4]^{2-}$ is tetrahedral and paramagnetic (two unpaired electrons). Which option best explains this contrast?

(A)

Because $CN^-$ has a larger ionic radius that forces square-planar coordination.

(B)

Because $CN^-$ is a strong $\pi$ -donor which increases pairing energy and forces electron pairing.

(C)

Because $Cl^-$ is a stronger-field ligand than $CN^-$ , giving a larger splitting and pairing.

(D)

Because $CN^-$ is a strong-field (often $\pi$ -acceptor) ligand producing large crystal-field splitting that favours electron pairing and square-planar arrangement for $d^8$ , whereas $Cl^-$ is weak-field giving small splitting and a tetrahedral arrangement with two unpaired electrons.

Q14. Assertion (A): In square-planar $Pt(II)$ complexes the ligand trans to a strong trans-effect ligand is displaced more rapidly than when trans to a weak trans-effect ligand.
Reason (R): Strong trans-effect ligands weaken the metal–ligand bond trans to them by donating electron density or stabilizing the transition state, thereby lowering the activation energy for substitution.

(A)

Both A and R are true but R does not explain A.

(B)

A is true but R is false.

(C)

Both A and R are true and R correctly explains A.

(D)

A is false but R is true.

Q15. Assertion (A): Octahedral complexes of $Cu^{2+}$ ( $d^9$ ) typically exhibit a pronounced Jahn–Teller distortion (usually elongation of two opposite bonds).
Reason (R): Uneven occupancy of the $e_g$ set ( $d_{z^2},\,d_{x^2-y^2}$ ) in a $d^9$ configuration creates electronic degeneracy that is relieved by a tetragonal distortion, lowering the overall electronic energy.

(A)

Both A and R are true but R does not explain A.

(B)

Both A and R are true and R correctly explains A.

(C)

A is true but R is false.

(D)

A is false but R is true.

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