Frequently Asked Question

Here's a breakdown of the Orgel diagram and the spectral analysis:
1. Orgel Diagram for Cr(III) in an Octahedral Field:

Cr(III) Electronic Configuration: [Ar] 3d³
Octahedral Field Splitting: In an octahedral complex, the d orbitals split into two sets: the lower energy t₂g orbitals (dxy, dyz, dxz) and the higher energy eg orbitals (dz², dx²-y²).

Orgel Diagram: The Orgel diagram for a d³ system (like Cr(III)) shows three possible spin-allowed transitions:

⁴T₂g ← ⁴A₂g: This is the lowest energy transition and corresponds to the absorption at 575 nm (red-orange light).

⁴T₁g(F) ← ⁴A₂g: This is the next higher energy transition and corresponds to the absorption at 408 nm (violet light).

⁴T₁g(P) ← ⁴A₂g: This is the highest energy transition and corresponds to the absorption at 265 nm (UV light).
2. Spectroscopic Properties of [Cr(H₂O)₆]Cl₃:

The complex exhibits the three spin-allowed bands observed at 265, 408, and 575 nm. These correspond to the transitions described in the Orgel diagram. The colors we see are the complementary colors to the absorbed wavelengths.
3. Comparing with CoCl₄²⁻ (Tetrahedral Complex):

Tetrahedral Field Splitting: In a tetrahedral complex, the d orbitals split in the opposite way to an octahedral complex, with the e orbitals lower in energy and the t₂ orbitals higher. The energy difference (Δt) is also smaller than in an octahedral field.
Spectral Shifts: This means that the d-d transitions in CoCl₄²⁻ (which has a tetrahedral geometry) will be lower in energy than those in [Cr(H₂O)₆]Cl₃. Therefore, the bands in the spectrum of CoCl₄²⁻ would be at longer wavelengths.
Additional Notes:

The actual colors observed depend on the specific ligands and the energy differences between the d orbitals.
The intensity of the bands can vary depending on the nature of the complex and the symmetry of the ligand field.