Question

According to the crystal field theory, d–d transition observed in [Ti(H$_2$O)$_6$]$^{3}+$ is

• A. Laporte forbidden and spin forbidden
• B. Laporte allowed and spin forbidden
• C. Laporte allowed and spin allowed
• D. Laporte forbidden and spin allowed

Hint:

In octahedral complexes, $d$–$d$ transitions are Laporte forbidden but can occur weakly due to vibronic coupling. Spin-allowed transitions have higher intensity than spin-forbidden ones.

Answer 1

The Correct Option is D

Step 1: Electronic configuration.
In [Ti(H$_2$O)$_6$]$^{3+}$, Ti$^{3+}$ = 3d$^1$ configuration.
Hence, there is only one possible transition: $t_{2g}^1 \rightarrow e_g^1$.
Step 2: Selection rules.
(a) Laporte rule: Transitions within the same parity (g → g or u → u) are forbidden.
As this is an octahedral complex (center of symmetry present), the $d$–$d$ transition is Laporte forbidden.
(b) Spin selection rule: Transitions that do not change spin multiplicity are spin allowed.
Here, only one unpaired electron is present (no spin change occurs). Hence, spin allowed.
Step 3: Conclusion.
The transition is Laporte forbidden but spin allowed.