Question

Given below are two statements:
Statement I: [CoBr₄]²⁻ ion will absorb light of lower energy than [CoCl₄]²⁻ ion.
Statement II: In [CoBr₄]²⁻ ion, the energy separation between the two set of d-orbitals is more than [CoCl₄]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

Hint:

Weak field ligands result in smaller $\Delta$, meaning they absorb light in the lower energy (longer wavelength) part of the spectrum.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Crystal Field Splitting ($\Delta_t$ for tetrahedral complexes) depends on the position of the ligand in the Spectrochemical Series. The energy of light absorbed ($E = h\nu = \Delta$) is directly proportional to the field strength of the ligand.

Step 2: Key Formula or Approach:
1. Ligand field strength order: $I^-<Br^-<Cl^-<F^-$.
2. $\Delta \propto$ Ligand field strength.
3. $E_{absorbed} = \Delta$.
Step 3: Detailed Explanation:
Statement I is true: $Cl^-$ is a stronger field ligand than $Br^-$. Therefore, the crystal field splitting ($\Delta_t$) for $[CoCl_4]^{2-}$ is greater than for $[CoBr_4]^{2-}$. Since the energy of absorbed light matches the splitting energy, $[CoBr_4]^{2-}$ absorbs light of lower energy.
Statement II is false: As $Br^-$ is a weaker field ligand than $Cl^-$, the energy separation (splitting) in $[CoBr_4]^{2-}$ is actually less than in $[CoCl_4]^{2-}$.
Step 4: Final Answer:
Statement I is true but Statement II is false.