[Mn(H₂O)₆]²⁺ high spin complex due to

GUJARAT SET 2022

In [Mn(H₂O)₆]²⁺ high spin complex, all electronic transitions have observed low molar extinction coefficient values, due to:

• (A) d-d transitions are Laporte allowed and spin forbidden
• (B) d-d transitions are Laporte forbidden and spin allowed
• (C) d-d transitions are Laporte allowed and spin allowed
• (D) d-d transitions are Laporte forbidden and spin forbidden

Solution & Explanation

The correct answer is (D).

1. Laporte Selection Rule: This complex has an octahedral geometry which is centrosymmetric. According to the Laporte rule, transitions between states of the same parity (such as g to g) are forbidden. Since d-orbitals are centrosymmetric (gerade), d-d transitions are Laporte forbidden.

2. Spin Selection Rule: In a high-spin d⁵ complex like Mn²⁺, all five d-orbitals are singly occupied with parallel spins (S = 5/2). Any electronic transition would require an electron to flip its spin to pair up, which is spin-forbidden (ΔS ≠ 0).

Because these transitions break both rules, they result in extremely pale colors and very low molar extinction coefficients (ε ≈ 0.01-0.1 L mol^-1 cm^-1).