Crystal field splitting by various ligands Metal complexes show different colours due to d-d transitions. The complex absorbs light of specific wavelength to promote the electron from t2g to eg level. The colour of the complex is due to the transmitted light, which is complementary of the colour absorbed. The wave number of light absorbed by different complexes of Cr ion are given below: Complex Wavenumber of light absorbed (cm-1) Energy of light absorbed (kJ/mol) --- --- --- [CrA_6]³- 13640 163 [CrB_6]³+ 17830 213 [CrC_6]³+ 21680 259 [CrD_6]³- 26280 314 Answer the following questions: (a) Out of the ligands "A", "B", "C" and "D", which ligand causes maximum crystal field splitting? Why? (a) Which of the two, "A" or "D" will be a weak field ligand? Why? (b) Which of the complexes will be violet in colour? [CrA_6]³- or [CrB_6]³+ and why? (Given: If 560-570 nm of light is absorbed, the colour of the complex observed is violet.) (c) If the ligands attached to Cr³+ ion in the complexes given in the table above are water, cyanide ion, chloride ion, and ammonia (not in this order) Identify the ligand, write the formula and IUPAC name of the following: (i) [CrA_6]³- (ii) [CrC_6]³+

ChemistryCBSE Class 12 · Sample paper, session 2023-24 · 4 marks · Case-based

CBSE Class 12 Chemistry sample paper 2023-24: question 29

From the official CBSE sample question paper for 2023-24, section D, question 29, for 4 marks. Try it first; the hints and our step-by-step solution are below.

Q29 · 4 marks · Case-basedCompetency-basedCoordination Compounds

Crystal field splitting by various ligands
Metal complexes show different colours due to d-d transitions. The complex absorbs light of specific wavelength to promote the electron from t2g to eg level. The colour of the complex is due to the transmitted light, which is complementary of the colour absorbed.
The wave number of light absorbed by different complexes of Cr ion are given below:

Complex	Wavenumber of light absorbed (cm-1)	Energy of light absorbed (kJ/mol)
$[Cr A_{6}]^{3 -}$	13640	163
$[Cr B_{6}]^{3 +}$	17830	213
$[Cr C_{6}]^{3 +}$	21680	259
$[Cr D_{6}]^{3 -}$	26280	314

Answer the following questions:

(a)
Out of the ligands "A", "B", "C" and "D", which ligand causes maximum crystal field splitting? Why?
1 mark
OR
(a)
Which of the two, "A" or "D" will be a weak field ligand? Why?
1 mark
(b)
Which of the complexes will be violet in colour? $[Cr A_{6}]^{3 -}$ or $[Cr B_{6}]^{3 +}$ and why? (Given: If 560-570 nm of light is absorbed, the colour of the complex observed is violet.)
1 mark
(c)
If the ligands attached to $C r^{3 +}$ ion in the complexes given in the table above are water, cyanide ion, chloride ion, and ammonia (not in this order)
Identify the ligand, write the formula and IUPAC name of the following:
(i) $[Cr A_{6}]^{3 -}$ (ii) $[Cr C_{6}]^{3 +}$
2 marks

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2023-24.

Hints

Open one hint at a time and try again before the next.

Hint 1

Energy absorbed equals the crystal field splitting; a bigger wavenumber means bigger energy.

Hint 2

Convert wavenumber (cm⁻¹) to wavelength with λ = 1/ν̄, then change cm to nm.

Hint 3

Match the ligands by the spectrochemical series and by the charge on each complex (3– needs anionic ligands).

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

(a) Ligand D causes maximum splitting. $[Cr D_{6}]^{3 -}$ absorbs the highest wavenumber (26280 cm⁻¹), i.e. the highest energy (314 kJ/mol), since $E \propto \overset{ν}{ˉ}$ . So the t₂g → e_g gap ( $Δ_{o}$ ) is largest.
1 mark
(a) OR: A is the weak field ligand. $[Cr A_{6}]^{3 -}$ absorbs the lowest energy (13640 cm⁻¹, 163 kJ/mol), so the splitting it causes is least. (Alternative part, 1 mark)
(b) $λ = 1/ \overset{ν}{ˉ}$ . For $[Cr B_{6}]^{3 +}$ : $λ = \frac{1}{17830}$ cm $= 5.61 \times 1 0^{- 5}$ cm ≈ 560 nm, which lies in 560–570 nm, so it appears violet. For $[Cr A_{6}]^{3 -}$ : $λ = \frac{1}{13640}$ cm ≈ 733 nm, so it is not violet.
1 mark
(c) Spectrochemical order: Cl⁻ < H₂O < NH₃ < CN⁻. A (weakest, and anionic since the complex is 3–) = Cl⁻; B = H₂O; C = NH₃; D = CN⁻. (i) $[CrC l_{6}]^{3 -}$ : hexachloridochromate(III) ion.
1 mark
(ii) $[Cr (N H_{3})_{6}]^{3 +}$ : hexaamminechromium(III) ion.
1 mark

Answer: (a) D (highest energy absorbed); OR A is weak field; (b) [CrB₆]³⁺ (absorbs ≈560 nm); (c) (i) [CrCl₆]³⁻, hexachloridochromate(III) ion; (ii) [Cr(NH₃)₆]³⁺, hexaamminechromium(III) ion

Where the marks are

(a) D, with highest energy/wavenumber meaning largest Δo (½ + ½ mark); OR A weak field as it absorbs least energy (½ + ½ mark)
(b) [CrB₆]³⁺, λ = 1/17830 cm ≈ 560 nm vs 733 nm for [CrA₆]³⁻ (½ + ½ mark)
(c)(i) [CrCl₆]³⁻, hexachloridochromate(III) (1 mark)
(c)(ii) [Cr(NH₃)₆]³⁺, hexaamminechromium(III) (1 mark)

Revise the chapter

Chapter: Coordination Compounds (notes, formulas, where marks are lost and every past question)
Topic: Crystal Field Theory

Practise this chapterBoard-style questions from Coordination Compounds, one at a time, marked against the answer key and marking points.Back to the full paperSample paper 2023-24: every question, by section.

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CBSE Class 12 Chemistry sample paper 2023-24: question 29

Hints

Step-by-step solution

Revise the chapter

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