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Coordination Compounds Set-5

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

Coordination compounds form the foundation of modern inorganic chemistry and appear frequently in CBSE boards as well as competitive exams like JEE/NEET. Mastery of concepts such as oxidation state, stereochemistry of coordination complexes, spin states (high-spin/low-spin), magnetic properties, and Jahn–Teller effects is essential because questions often combine multiple ideas in one problem and require careful application of rules.

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Q1. (An octahedral complex of $\mathrm{Mn}^{2+}$ shows a magnetic moment of $5.92\ \mu_B$ at room temperature. Using the spin-only formula $\mu_{SO}=\sqrt{n(n+2)}\,\mu_B$ , determine the number of unpaired electrons and whether the complex is high-spin or low-spin.)

(A)

(Three unpaired electrons; high-spin)

(B)

(Four unpaired electrons; high-spin)

(C)

(Five unpaired electrons; high-spin)

(D)

(One unpaired electron; low-spin)

Q2. (For the octahedral complex $\mathrm{[Co(en)_2Cl_2]^+}$ (en = ethylenediamine, bidentate), how many stereoisomers (geometrical + optical) are possible and how many of them are optically active?)

(A)

(Three stereoisomers in total; two are optically active (a cis enantiomeric pair) and one (trans) is achiral)

(B)

(Two stereoisomers in total; neither is optically active)

(C)

(Four stereoisomers in total; all four are optically active)

(D)

(One stereoisomer only; it is optically active)

Q3. (For an octahedral $d^6$ metal ion the pairing energy is $P=150\ \text{kJ mol}^{-1}$ . Ligand $L_1$ gives $\Delta_o=220\ \text{kJ mol}^{-1}$ while ligand $L_2$ gives $\Delta_o=100\ \text{kJ mol}^{-1}$ . Predict the most likely spin states for $[M L_1]_6$ and $[M L_2]_6$ , respectively.)

(A)

(Both $[M L_1]_6$ and $[M L_2]_6$ will be low-spin)

(B)

(Both $[M L_1]_6$ and $[M L_2]_6$ will be high-spin)

(C)

( $[M L_1]_6$ high-spin; $[M L_2]_6$ low-spin)

(D)

( $[M L_1]_6$ low-spin; $[M L_2]_6$ high-spin)

Q4. (When excess $CN^-$ is added to an aqueous solution of $\mathrm{cis\text{-}[PtCl_2(NH_3)_2]}$ , the major isolated product is $\mathrm{trans\text{-}[Pt(CN)_2(NH_3)_2]}$ . Which of the following explanations best accounts for formation of the trans product?)

(A)

(The trans isomer uniquely maximizes crystal-field stabilization energy for Pt(II), so it is thermodynamically preferred)

(B)

( $CN^-$ has a stronger trans-effect than $Cl^-$ or $NH_3$ ; after the first substitution the ligand trans to the incoming $CN^-$ is labilized, promoting sequential replacement that yields the trans product)

(C)

(A Jahn–Teller distortion in the cis isomer forces rearrangement to the trans form)

(D)

(Steric repulsion between small $Cl^-$ ligands forces $CN^-$ to bind trans to $NH_3$ , directing formation of the trans isomer)

Q5. (Arrange the octahedral aqua complexes in order of decreasing expected Jahn–Teller distortion intensity: $\mathrm{[Cr(H_2O)_6]^{3+}}$ ( $d^3$ ), $\mathrm{[Mn(H_2O)_6]^{3+}}$ ( $d^4$ , high-spin), $\mathrm{[Co(H_2O)_6]^{2+}}$ ( $d^7$ , high-spin), $\mathrm{[Cu(H_2O)_6]^{2+}}$ ( $d^9$ ).)

(A)

( $\mathrm{[Mn(H_2O)_6]^{3+}} > \mathrm{[Cu(H_2O)_6]^{2+}} > \mathrm{[Co(H_2O)_6]^{2+}} > \mathrm{[Cr(H_2O)_6]^{3+}}$ )

(B)

( $\mathrm{[Cr(H_2O)_6]^{3+}} > \mathrm{[Co(H_2O)_6]^{2+}} > \mathrm{[Mn(H_2O)_6]^{3+}} > \mathrm{[Cu(H_2O)_6]^{2+}}$ )

(C)

( $\mathrm{[Cu(H_2O)_6]^{2+}} > \mathrm{[Mn(H_2O)_6]^{3+}} > \mathrm{[Co(H_2O)_6]^{2+}} > \mathrm{[Cr(H_2O)_6]^{3+}}$ )

(D)

( $\mathrm{[Co(H_2O)_6]^{2+}} > \mathrm{[Cu(H_2O)_6]^{2+}} > \mathrm{[Mn(H_2O)_6]^{3+}} > \mathrm{[Cr(H_2O)_6]^{3+}}$ )

Q6. For the high-spin complex $[\mathrm{Mn}(H_2O)_6]^{2+}$ , calculate the spin-only magnetic moment $\mu_s$ in Bohr magneton. Use $\mu=\sqrt{n(n+2)}\,\mu_B$ , where $n$ is the number of unpaired electrons.

(A)

$1.73\ \mu_B$

(B)

$3.87\ \mu_B$

(C)

$5.92\ \mu_B$

(D)

$6.93\ \mu_B$

Q7. Consider the octahedral complex ion $[\text{Co}(\text{en})_2\text{Cl}_2]^+$ (en = ethylenediamine, bidentate). How many stereoisomers are possible for this complex and how many of them are optically active?

(A)

Three stereoisomers; two optically active

(B)

Two stereoisomers; both optically active

(C)

Three stereoisomers; none optically active

(D)

Four stereoisomers; two optically active

Q8. A $d^6$ metal ion in an octahedral field has pairing energy $P=200\ \mathrm{kJ\,mol^{-1}}$ and octahedral splitting $10Dq=\Delta_o=240\ \mathrm{kJ\,mol^{-1}}$ . Determine whether the complex will be high-spin or low-spin and calculate its crystal field stabilization energy (CFSE) in kJ·mol $^{-1}$ (use CFSE for low-spin $d^6=-2.4\Delta_o$ and for high-spin $d^6=-0.4\Delta_o$ ).

(A)

High-spin; CFSE = $-96\ \mathrm{kJ\,mol^{-1}}$

(B)

Low-spin; CFSE = $-576\ \mathrm{kJ\,mol^{-1}}$

(C)

High-spin; CFSE = $-240\ \mathrm{kJ\,mol^{-1}}$

(D)

Low-spin; CFSE = $-240\ \mathrm{kJ\,mol^{-1}}$

Q9. Assertion (A): Octahedral Cu(II) complexes ( $d^9$ ) commonly undergo Jahn–Teller distortion that results in elongation of two axial bonds.
Reason (R): In a $d^9$ octahedral configuration the $e_g$ orbitals ( $d_{x^2-y^2}, d_{z^2}$ ) are unevenly occupied, creating electronic degeneracy which is relieved by axial elongation.

(A)

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

(B)

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

(C)

A is true but R is false

(D)

A is false but R is true

Q10. Which one of the following complexes will exhibit optical isomerism (enantiomers) but will not display any geometrical (cis–trans) isomerism?

(A)

$[\text{Co}(\text{en})_2\text{Cl}_2]^+$

(B)

$[\text{PtCl}_2(\text{NH}_3)_2]$

(C)

$[\text{Fe}(\text{CN})_6]^{4-}$

(D)

$[\text{Co}(\text{en})_3]^{3+}$

Q11. For the complex $[Co(NH_3)_5Cl]Cl_2$ , what is the oxidation state of cobalt and the number of $d$ -electrons on the metal?

(A)

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

(B)

Oxidation state $+3$ , $d^7$

(C)

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

(D)

Oxidation state $+2$ , $d^8$

Q12. Which of the following compounds will give a precipitate on addition of $AgNO_3$ due to the presence of free chloride ions in solution?

(A)

$[Co(NH_3)_5Cl]SO_4$

(B)

$[Co(NH_3)_5Cl]Cl_2$

(C)

$K_2[PtCl_6]$

(D)

$[CoCl_3(NH_3)_3]$

Q13. Consider $[Fe(CN)_6]^{3-}$ and $[FeF_6]^{3-}$ (both contain $Fe^{3+}$ , $d^5$ ). Which of the following statements is correct regarding their magnetic moments and crystal field stabilization energy (CFSE)?

(A)

$[Fe(CN)_6]^{3-}$ has a lower magnetic moment and a larger (more negative) CFSE than $[FeF_6]^{3-}$

(B)

$[Fe(CN)_6]^{3-}$ has a higher magnetic moment and a larger CFSE than $[FeF_6]^{3-}$

(C)

Both complexes have the same magnetic moment but different CFSE values

(D)

$[Fe(CN)_6]^{3-}$ has a lower magnetic moment but a smaller (less negative) CFSE than $[FeF_6]^{3-}$

Q14. Assertion (A): Square‑planar $d^8$ complexes are paramagnetic.
Reason (R): In square‑planar geometry the ligand field splitting is large and arranges the $d$ ‑levels so that the lower orbitals can accommodate paired electrons, favouring diamagnetism.

(A)

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

(B)

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

(C)

A is true but R is false.

(D)

A is false but R is true.

Q15. Metal $M$ forms successive complexes with ligand $L$ with stepwise formation constants $K_1=10^4\ \mathrm{M^{-1}}$ and $K_2=10^3\ \mathrm{M^{-1}}$ . If total metal $[M]_{tot}=1.0\times10^{-3}\ \mathrm{M}$ and free ligand concentration $[L]=0.10\ \mathrm{M}$ (large excess), the fraction of metal present as $ML_2$ at equilibrium is approximately:

(A)

$90.0\%$

(B)

$9.09\%$

(C)

$99.01\%$

(D)

$0.99\%$

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