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Colour and magnetic properties: NEET MCQs with answers

90 questions on this topic of Coordination Compounds (NEET 2027 syllabus). Here are 12; tap one to see its answer and explanation.

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easy

1. A metal complex absorbs light in the blue-green region (around 490-500 nm) of the visible spectrum. The transmitted colour exhibited by the complex is:

  1. Yellow
  2. Red
  3. Violet
  4. Green
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Answer: (B) Red

According to the colour wheel, the complementary colour of blue-green is red. Therefore, when a coordination complex absorbs light in the blue-green region, it transmits and appears red.

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2. An octahedral complex of a 3d divalent metal ion M²⁺ with weak-field ligands has a magnetic moment of 3.87 BM. The metal ion M²⁺ is:

  1. Mn²⁺
  2. Fe²⁺
  3. Ni²⁺
  4. Co²⁺
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Answer: (D) Co²⁺

A spin-only magnetic moment of 3.87 BM corresponds to 3 unpaired electrons (μ = √(3(5)) = √15 ≈ 3.87 BM). For high-spin octahedral complexes: Mn²⁺ (d⁵) has 5, Fe²⁺ (d⁶) has 4, Co²⁺ (d⁷) has configuration t₂g⁵ eg² with 3 unpaired electrons, and Ni²⁺ (d⁸) has 2 unpaired electrons. Hence, M²⁺ is Co²⁺.

hard

3. Arrange the following complexes in increasing order of the energy of light absorbed for d-d transitions: I. [CrCl₆]³⁻ II. [Cr(CN)₆]³⁻ III. [Cr(H₂O)₆]³⁺ IV. [Cr(NH₃)₆]³⁺

  1. I < IV < III < II
  2. II < IV < III < I
  3. I < III < IV < II
  4. III < I < IV < II
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Answer: (C) I < III < IV < II

All complexes contain the Cr³⁺ central ion. The energy of light absorbed (E = hν = Δ₀) directly correlates with the crystal field splitting parameter. According to the spectrochemical series of ligands: Cl⁻ < H₂O < NH₃ < CN⁻. Therefore, the absorbed energy increases in the order [CrCl₆]³⁻ < [Cr(H₂O)₆]³⁺ < [Cr(NH₃)₆]³⁺ < [Cr(CN)₆]³⁻.

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4. Which of the following complex species has a spin-only magnetic moment of 4.90 BM?

  1. [Co(C₂O₄)₃]³⁻
  2. [Fe(CN)₆]⁴⁻
  3. [Mn(CN)₆]³⁻
  4. [Cr(H₂O)₆]²⁺
Show answer and explanation

Answer: (D) [Cr(H₂O)₆]²⁺

A magnetic moment of 4.90 BM indicates n = 4 unpaired electrons (√(4(4+2)) = √24 ≈ 4.90 BM). Cr²⁺ has a 3d⁴ configuration; with weak field ligand H₂O, it remains high-spin (t₂g³ eg¹), giving 4 unpaired electrons. [Co(C₂O₄)₃]³⁻ is low spin d⁶ (n = 0), [Fe(CN)₆]⁴⁻ is low spin d⁶ (n = 0), and [Mn(CN)₆]³⁻ is low spin d⁴ (n = 2).

easy

5. Given below are two statements: Statement I: Zinc(II) complexes such as [Zn(H₂O)₆]²⁺ are colourless and diamagnetic. Statement II: The Zn²⁺ ion has a completely filled 3d¹⁰ electronic configuration, which forbids d-d electronic transitions. In the light of the above statements, choose the most appropriate answer from the options given below:

  1. Both Statement I and Statement II are correct.
  2. Statement I is correct but Statement II is incorrect.
  3. Statement I is incorrect but Statement II is correct.
  4. Both Statement I and Statement II are incorrect.
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Answer: (A) Both Statement I and Statement II are correct.

Zn²⁺ has an [Ar] 3d¹⁰ configuration. Because all 3d orbitals are completely filled, there are no unpaired electrons (diamagnetic) and no vacant d-orbitals available for d-d electronic transitions, making its complexes colourless.

hard

6. A metal ion M²⁺ with 3d⁸ configuration forms a tetrahedral complex [MCl₄]²⁻. The number of unpaired electrons and its crystal field configuration are respectively:

  1. 2 unpaired electrons, t₂⁴ e⁴
  2. 0 unpaired electrons, e⁴ t₂⁴
  3. 2 unpaired electrons, e⁴ t₂⁴
  4. 4 unpaired electrons, e³ t₂⁵
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Answer: (C) 2 unpaired electrons, e⁴ t₂⁴

In a tetrahedral crystal field, the d-orbitals split into a lower doublet 'e' and higher triplet 't₂'. Since Δₜ is small, tetrahedral complexes are high-spin. For a d⁸ ion, electrons fill as e⁴ t₂⁴, leaving 2 unpaired electrons in the t₂ set.

easy

7. Which of the following homoleptic octahedral complexes of cobalt is diamagnetic?

  1. [Co(NH₃)₆]³⁺
  2. [Co(H₂O)₆]²⁺
  3. [CoF₆]³⁻
  4. [CoCl₆]³⁻
Show answer and explanation

Answer: (A) [Co(NH₃)₆]³⁺

In [Co(NH₃)₆]³⁺, cobalt is in the +3 oxidation state (3d⁶). NH₃ acts as a strong field ligand for Co³⁺, causing pairing of electrons to give a low-spin t₂g⁶ eg⁰ configuration with zero unpaired electrons, making it diamagnetic. In [CoF₆]³⁻, F⁻ is a weak field ligand yielding high-spin t₂g⁴ eg² (4 unpaired electrons).

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8. Given below are two statements: Statement I: The complex [Cr(NH₃)₆]³⁺ is paramagnetic with a spin-only magnetic moment corresponding to three unpaired electrons. Statement II: Ammonia is a strong-field ligand and forces all three 3d electrons of Cr³⁺ to pair up in a single t₂g orbital. In the light of the above statements, choose the most appropriate answer from the options given below:

  1. Both Statement I and Statement II are correct
  2. Statement I is correct but Statement II is incorrect
  3. Statement I is incorrect but Statement II is correct
  4. Both Statement I and Statement II are incorrect
Show answer and explanation

Answer: (B) Statement I is correct but Statement II is incorrect

Statement I is correct: Cr³⁺ has a 3d³ configuration, giving t₂g³ eg⁰ with 3 unpaired electrons (μ = 3.87 BM). Statement II is incorrect: the three degenerate t₂g orbitals can each accommodate one electron according to Hund's rule without violating Aufbau principle; electrons do not pair up because empty t₂g orbitals are available at the same energy level.

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9. A complex [FeF₆]³⁻ shows a spin-only magnetic moment of 5.92 BM, while [Fe(CN)₆]³⁻ shows 1.73 BM. Which of the following statements correctly explains this difference?

  1. F⁻ is a strong field ligand with Δₒ > P, whereas CN⁻ is a weak field ligand with Δₒ < P.
  2. F⁻ is a weak field ligand causing high-spin configuration (t₂g³ eg²), whereas CN⁻ is a strong field ligand causing low-spin configuration (t₂g⁵ eg⁰).
  3. F⁻ causes pairing of d-electrons while CN⁻ does not.
  4. The oxidation state of iron is +2 in [FeF₆]³⁻ and +3 in [Fe(CN)₆]³⁻.
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Answer: (B) F⁻ is a weak field ligand causing high-spin configuration (t₂g³ eg²), whereas CN⁻ is a strong field ligand causing low-spin configuration (t₂g⁵ eg⁰).

In both complexes Fe is in the +3 oxidation state (3d⁵). F⁻ is a weak field ligand (Δₒ < P), leading to high-spin t₂g³ eg² (5 unpaired electrons, μ = 5.92 BM). CN⁻ is a strong field ligand (Δₒ > P), leading to low-spin t₂g⁵ eg⁰ (1 unpaired electron, μ = 1.73 BM).

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10. Three complexes of Co(III) absorb radiation in the visible region at wavelengths λ₁, λ₂, and λ₃ respectively: (X) [Co(CN)₆]³⁻ (λ₁) (Y) [Co(NH₃)₆]³⁺ (λ₂) (Z) [Co(H₂O)₆]³⁺ (λ₃) The correct order of absorbed wavelengths is:

  1. λ₃ < λ₂ < λ₁
  2. λ₁ < λ₂ < λ₃
  3. λ₂ < λ₁ < λ₃
  4. λ₁ < λ₃ < λ₂
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Answer: (B) λ₁ < λ₂ < λ₃

According to the spectrochemical series, ligand strength decreases in the order: CN⁻ > NH₃ > H₂O. Thus, the crystal field splitting energy follows Δ₀(X) > Δ₀(Y) > Δ₀(Z). Since energy is inversely proportional to wavelength (Δ₀ = hc/λ), the absorbed wavelength follows the reverse order: λ₁ < λ₂ < λ₃.

easy

11. Given below are two statements: Statement I: Tetrahedral complexes of transition metals are generally high-spin and rarely show low-spin configurations. Statement II: The crystal field splitting in tetrahedral complexes (Δₜ) is small and roughly equal to (4/9)Δ₀, which is almost always smaller than the pairing energy (P). In the light of the above statements, choose the most appropriate answer from the options given below:

  1. Both Statement I and Statement II are correct
  2. Statement I is correct but Statement II is incorrect
  3. Statement I is incorrect but Statement II is correct
  4. Both Statement I and Statement II are incorrect
Show answer and explanation

Answer: (A) Both Statement I and Statement II are correct

Statement I is correct because tetrahedral complexes rarely pair up electrons into low-spin complexes. Statement II is the correct explanation: because Δₜ ≈ (4/9)Δ₀, the splitting is smaller than the electron pairing energy (Δₜ < P), making orbital splitting insufficient to force pairing.

easy

12. Which of the following octahedral complexes has the highest value of spin-only magnetic moment?

  1. [CoF₆]³⁻
  2. [Mn(H₂O)₆]²⁺
  3. [Cr(H₂O)₆]²⁺
  4. [Fe(H₂O)₆]²⁺
Show answer and explanation

Answer: (B) [Mn(H₂O)₆]²⁺

Mn²⁺ has a 3d⁵ configuration. Since H₂O is a weak field ligand, it forms a high-spin complex with t₂g³ eg² configuration possessing 5 unpaired electrons, which gives the maximum spin-only magnetic moment (5.92 BM) among the given options.

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