![Q. 18 In [Mn(H2O)6]2+ (atomic number of Mn = 25), the d-d transitions according to crystal field theory (CFT) are Laporte forbidden and spin forbidden Laporte allowed and spin allowed Laporte forbidden and spin allowed Laporte allowed and spin forbidden](https://www.letstalkacademy.com/wp-content/uploads/2026/01/slide_18-2.png)
Q. 18 In [Mn(H2O)6]2+ (atomic number of Mn = 25), the d-d transitions according to crystal field theory (CFT) are
- Laporte forbidden and spin forbidden
- Laporte allowed and spin allowed
- Laporte forbidden and spin allowed
- Laporte allowed and spin forbidden
Mn(H2O)6 2+ d-d Transitions: Crystal Field Theory Selection Rules Explained
In [Mn(H2O)6]2+, d-d transitions are Laporte forbidden but spin allowed according to crystal field theory, making option C the correct answer. This octahedral high-spin d5 complex exhibits pale color due to weak absorption from these electronic transitions.
Correct Answer
Laporte forbidden and spin allowed
Mn (atomic number 25) in +2 oxidation state has d5 configuration. In octahedral field with weak H2O ligands, it remains high-spin with t2g3 eg2 electron arrangement and 6S ground state (all spins parallel).
Electron Configuration
Mn2+ features five unpaired electrons, yielding total spin S=5/2. Crystal field splitting produces t2g and eg orbitals, but d-d transitions occur within same parity (g → g), violating Laporte rule (Δl=±1 required).
Spin selection rule (ΔS=0) holds since transitions stay within high-spin 6A1g ground term to excited states of same multiplicity.
All Options Explained
| Option | Laporte | Spin | Why Correct/Incorrect |
|---|---|---|---|
| A | Forbidden | Forbidden | Incorrect: Spin allowed (ΔS=0), not forbidden |
| B | Allowed | Allowed | Incorrect: d-d inherently Laporte forbidden in Oh symmetry |
| C | Forbidden | Allowed | Correct: Matches high-spin d5 behavior |
| D | Allowed | Forbidden | Incorrect: Both rules violated |
Vibronic coupling weakly relaxes Laporte rule, enabling low-intensity bands (ε ≈ 0.1-10 M⁻¹cm⁻¹).
