Question

The correct order of increasing intensity (molar absorptivity) of the UV-visible absorption bands for the ions [Ti(H₂O)₆]³⁺, [Mn(H₂O)₆]²⁺, [CrO₄]^2– , and [NiCl₄]^2– is: 

1. [Ti(H₂O)₆] ³⁺ < [Mn(H₂O)₆] ²⁺ < [CrO₄] ^2– < [NiCl₄] ^2– 
2. [Mn(H₂O)6] ²⁺ < [Ti(H₂O)₆] ³⁺ < [NiCl₄] ^2– < [CrO₄] ^2–
3. [NiCl₄] ^2– < [Ti(H₂O)₆] ³⁺ < [Mn(H₂O)₆] ²⁺ < [CrO₄] ^2– 
4. [Ti(H₂O)6] ³⁺ < [NiCl₄] ^2– < [CrO₄] ^2– < [Mn(H₂O)₆] ²⁺

Answer 1

Correct Answer - Option 2 : [Mn(H₂O)6] ²⁺ < [Ti(H₂O)₆] ³⁺ < [NiCl₄] ^2– < [CrO₄] ^2–

Concept:

The selection rules for electronic transition for spectra are:

• La Porte Or orbital selection Rule: In a centrosymmetric molecule, the transitions are allowed between orbitals having different symmetries with respect to inversion. Gerade to Ungerade are allowed and vice versa but 'g' to 'g' and 'u' to 'u' are not allowed.
• In other words, we can say that the transitions are allowed for Δ L = +/- 1.
• If a complex has no centre of inversion, for example, a tetrahedral molecule, then the La Porte Selection Rule does not apply and in those complexes, the La Porte allowed transitions are high in intensity. The intensity of tetrahedral complexes is nearly 100 times as compared to similar octahedral complexes.
• Spin selection Rule: The spin selection rule states that the transitions which involve a change in the spin multiplicity of the complex are forbidden. The electronic transition will occur only when the change in spin state is zero.
• Spin selection rule and La Porte is relaxed in many instances.
• The La Porte selection rule is allowed in many cases where the centre of inversion is relaxed. For example, when mixing of p and d orbitals occurs due to distortion in the molecule, transitions become allowed.

Explanation:

Let us check the molecules for allowed transitions:

Complex	Spin allowed	La Porte Allowed	Typical transition
[Ti(H2O)6]3+	Spin allowed, with no change in the multiplicity	La Porte forbidden	d → d with change in the multiplicity
[Mn(H2O)6]2+	Spin forbidden	La Porte forbidden	d → d with no change in the multiplicity
[CrO4]2–	Spin allowed, with no change in the multiplicity	La Porte allowed	Charge Transfer
[NiCl4]2–	Spin allowed	La Porte 'partially allowed'	d → dp, no change of multiplicity

• The intensity order follows: Charge transfer > Non- centrosymmetric molecules > centrosymmetric molecules > Molecules with Spin and La Porte are forbidden.
• Hence, the intensity oedder of the complexes will be [Mn(H₂O)₆] ²⁺ < [Ti(H₂O)₆] ³⁺ < [NiCl₄] ^2– < [CrO₄] ^2–.