Q2 / 75 · JEE Main 2026 (23 January Shift 1) · Chemistry
Given below are two statements :
Statement I: $\quad\left[\mathrm{CoBr}_{4}\right]^{2-}$ ion will absorb light of lower energy than $\left[\mathrm{CoCl}_{4}\right]^{2-}$ ion.
Statement II : In $\left[\mathrm{CoI}_{4}\right]^{2-}$ ion, the energy separation between the two set of d-orbitals is more than $\left[\mathrm{CoCl}_{4}\right]^{2-}$ ion.
In the light of the above statements, choose the correct answer from the options given below :
Statement I: $\quad\left[\mathrm{CoBr}_{4}\right]^{2-}$ ion will absorb light of lower energy than $\left[\mathrm{CoCl}_{4}\right]^{2-}$ ion.
Statement II : In $\left[\mathrm{CoI}_{4}\right]^{2-}$ ion, the energy separation between the two set of d-orbitals is more than $\left[\mathrm{CoCl}_{4}\right]^{2-}$ ion.
In the light of the above statements, choose the correct answer from the options given below :
- ABoth Statement I and Statement II are false
- BBoth Statement I and Statement II are true
- CStatement I is false but Statement II is true
- Correct answerDStatement I is true but Statement II is false
Solution
Crystal field theory explains d-d transitions in complexes based on ligand field strength.
Statement I: Ligand field strength order is I⁻ < Br⁻ < Cl⁻ < F⁻. Since Br⁻ is weaker than Cl⁻, [CoBr₄]²⁻ has smaller crystal field splitting (Δ) than [CoCl₄]²⁻. The d-d absorption energy equals the splitting energy, so [CoBr₄]²⁻ absorbs lower energy light. TRUE
Statement II: I⁻ is a much weaker field ligand than Cl⁻. Therefore, [CoI₄]²⁻ has smaller crystal field splitting than [CoCl₄]²⁻, not larger. The energy separation in [CoI₄]²⁻ is less than in [CoCl₄]²⁻. FALSE
Statement I is true but Statement II is false.
Statement I: Ligand field strength order is I⁻ < Br⁻ < Cl⁻ < F⁻. Since Br⁻ is weaker than Cl⁻, [CoBr₄]²⁻ has smaller crystal field splitting (Δ) than [CoCl₄]²⁻. The d-d absorption energy equals the splitting energy, so [CoBr₄]²⁻ absorbs lower energy light. TRUE
Statement II: I⁻ is a much weaker field ligand than Cl⁻. Therefore, [CoI₄]²⁻ has smaller crystal field splitting than [CoCl₄]²⁻, not larger. The energy separation in [CoI₄]²⁻ is less than in [CoCl₄]²⁻. FALSE
Statement I is true but Statement II is false.
Hence, the correct answer is option: 'D'.