Question

Two complexes [Cr(H₂O)₆]Cl₃ (a) and  [Cr(NH₃)₆]Cl₃ (b) are violet and yellow coloured, respectively. The incorrect statement regarding them is
1. ∆₀ value for (a) is less than that of (b).
2. ∆₀ values of (a) and (b) are calculated from the energies of violet and yellow light, respectively
3. Both absorb energies corresponding to their complementary colours. 
4. Both are paramagnetic with three unpaired electrons

Answer 1

Correct Answer - Option 1 : ∆₀ value for (a) is less than that of (b).

Concept:

When ligands attach to a transition metal to form a coordination complex, electrons in the d orbital split into high energy and low energy orbitals.

The difference in energy of the two levels is denoted as ∆, and it is a characteristic property both of the metal and the ligands, the "o" subscript on the ∆ indicates that the complex has octahedral geometry.

• ‘A’ [Cr(H₂O)₆]Cl₃ Hexaaquachromium (III) chloride absorbs yellow light of less energy and emits the violet light of high energy with complementary colour because  H₂O a is weak field ligand.
• But when we consider a case ‘B’ [Cr(NH₃)₆]Cl₃ Hexaamminechromium (III) chloride, due to presence of strong field ligand NH₃, it absorbs high energy violet light and emits low energy complementary colour.

The energy difference, Δ_o, determines the color of the coordination complex. According to the spectrochemical series, the high spin ligands are considered “weak field”, and absorb longer wavelengths of light (weak Δ_o), while complexes with low spin ligands absorb light of greater frequency (high Δo). ∆₀ order will be compared by spectro chemical series not by energies of violet & yellow light so Δo order is

[Cr(H₂O)₆] Cl₃<[Cr(NH₃)₆ ] Cl₃