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MCQEasyJEE 2026Crystal Field Theory

JEE Chemistry 2026 Question with Solution

Given below are two statements:

Reaction scheme or figure associated with the two statements as shown in the source image, included before the statement text.

Statement I: [CoBr4]2−[CoBr_4]^{2-} ion will absorb light of lower energy than [CoCl4]2−[CoCl_4]^{2-} ion.

Statement II: In [CoBr4]2−[CoBr_4]^{2-} ion, the energy separation between the two set of dd-orbitals is more than [CoCl4]2−[CoCl_4]^{2-} ion.

In the light of the above statements, choose the correct answer from the options given below :

  • A

    Both Statement I and Statement II are true

  • B

    Statement I is false but Statement II is true

  • C

    Statement I is true but Statement II is false

  • D

    Both Statement I and Statement II are false

Answer

Correct answer:C

Step-by-step solution

Standard Method

Given: Two statements compare [CoBr4]2−[CoBr_4]^{2-} and [CoCl4]2−[CoCl_4]^{2-}.

Find: Which statement is true.

Crystal field splitting for tetrahedral complexes is represented by Δt\Delta_t. The absorbed light energy is equal to the splitting energy.

Eabsorbed=hν=ΔE_{\text{absorbed}} = h\nu = \Delta

From the spectrochemical order given in the solution,

I−<Br−<Cl−<F−I^- < Br^- < Cl^- < F^-

So, Cl−Cl^- is a stronger field ligand than Br−Br^-. Therefore,

Δt([CoCl4]2−)>Δt([CoBr4]2−)\Delta_t\big([CoCl_4]^{2-}\big) > \Delta_t\big([CoBr_4]^{2-}\big)

Hence [CoBr4]2−[CoBr_4]^{2-} has smaller splitting and will absorb lower energy light. So Statement I is true.

Also, because Br−Br^- is the weaker field ligand, the energy separation in [CoBr4]2−[CoBr_4]^{2-} is less, not more, than in [CoCl4]2−[CoCl_4]^{2-}. So Statement II is false.

Therefore, the correct option is C.

Concept-Based Explanation

Given: Tetrahedral cobalt complexes [CoBr4]2−[CoBr_4]^{2-} and [CoCl4]2−[CoCl_4]^{2-}.

Find: Truth value of the two statements.

Step 1: Crystal field splitting depends on ligand field strength.

Step 2: The solution states the ligand field strength order as

I−<Br−<Cl−<F−I^- < Br^- < Cl^- < F^-

This means Cl−Cl^- produces greater splitting than Br−Br^-.

Step 3: Therefore,

Δt([CoCl4]2−)>Δt([CoBr4]2−)\Delta_t([CoCl_4]^{2-}) > \Delta_t([CoBr_4]^{2-})

Step 4: Since the absorbed energy matches the splitting,

E=ΔE = \Delta

smaller splitting means lower absorbed energy. Thus [CoBr4]2−[CoBr_4]^{2-} absorbs lower energy light than [CoCl4]2−[CoCl_4]^{2-}.

So Statement I is true.

Step 5: Statement II says the separation in [CoBr4]2−[CoBr_4]^{2-} is more than in [CoCl4]2−[CoCl_4]^{2-}, but the relation above shows the opposite.

So Statement II is false.

Therefore, the correct option is C, that is, Statement I is true but Statement II is false.

Common mistakes

  • Assuming Br−Br^- is a stronger field ligand than Cl−Cl^-. This is wrong because the spectrochemical order given in the solution shows Br−<Cl−Br^- < Cl^-. Use ligand field strength order before comparing splitting energies.

  • Confusing lower absorbed energy with greater crystal field splitting. This is wrong because absorbed energy equals the splitting energy. Smaller Δ\Delta means lower energy absorption.

  • Treating Statement II as true because both complexes have the same metal ion. This is wrong because ligand identity changes the splitting significantly. Compare ligands, not only the metal center.

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