Question

Among the statements(a)-(d) the incorrect ones are :
(a) Octahedral CO(III) complexes with strong fields ligands have very high magnetic moments.
(b) When \( \Delta \)₀ < P, the d-electron configuration of Co(III) in an octahedral complex is \( t_{eg}^4e_g^2 \)
(c) Wavelength of light absorbed by [Co(en)₃]³⁺ is lower than that of [CoF₆]^3-
(d) If the \( \Delta \)₀ for an octahedral complex of CO(III) is 18,000 cm^-1, the \( \Delta \)_t for its tetrahedral complex with the same ligand be 16,000 cm^-1

• (a) and (b) only
• (b) and (c) only
• (c) and (d) only
• (a) and (d) only

Hint:

Strong-field Co(III) is low-spin \(t_{2g}^6\) (diamagnetic); \(\Delta_t=\tfrac49\Delta_0\); stronger ligand gives larger \(\Delta_0\) and absorption at shorter wavelength.

Answer 1

The Correct Option is D

Step 1: (a) Co(III) is d⁶. With strong-field ligands it is low spin, \(t_{2g}^6e_g^0\), with no unpaired electrons, so it is diamagnetic, not highly paramagnetic. (a) is incorrect.
Step 2: (b) When \(\Delta_0\lt P\) the electrons occupy \(e_g\) orbitals before pairing: high spin \(t_{2g}^4e_g^2\). (b) is correct.
Step 3: (c) en is a stronger ligand than F^-, so \(\Delta_0\) of [Co(en)₃]³⁺ is larger, it absorbs higher-energy light, i.e. lower wavelength. (c) is correct.
Step 4: (d) \(\Delta_t=\tfrac49\Delta_0=\tfrac49\times18000=8000\) cm^-1, not 16000. (d) is incorrect.
The incorrect statements are (a) and (d).
Answer: (D) (a) and (d) only