Question

Answer the following with respect to electronic transitions;

1.      Name the two selection rules governing electronic transitions, resulting in transition being either ‘allowed’ or ‘forbidden’. Give an example of each type, depicting each and naming the ground and excited states for each of your examples.

2.      Depict a transition for each of the two complexes. As above, name each state and propose a value for the extinction coefficient.

2.1   [Co(NH₃)₆]³⁺ and [CoF₆]^3-

2.2   [NiBr₄]^2-

3       State for each of the complexes below, what you expect the trend in absorption intensity will be, noting the rules governing electronic transitions. Provide a motivation for your answer.                                                                                               

3.1   [VBr₆]^3- (bit of a challenge)

3.2   [Mn (NH₃)₆]²⁺

3.3 [Cr (H₂O)₆]³⁺

Answer 1

'Laporte rule' and 'spin rule' are the two selection rules that govern electronic transitions, resulting in transition being either ‘allowed’ or ‘forbidden’.

Laporte rule: l = ±1, gerage (g) gerade (g) and ungerade (u) ungerade (u) are allowed, whereas gerade (g) ungerade (u) is forbidden.

s-s, p-p transitions are forbidden.

But d-d transitions (vibronic transitions) are allowed due to asymmetric vibration of the molecules.

Examples:

Laporte allowed transition: T_2g E_g

Laporte Forbidden transition: T_2g T_1u

Spin rule: The transitions taking place between the spin states having the same spin multiplicity are only allowed, i.e. s = 0

Examples:

Spin allowed transition: ³T_2g³E_g

Spin forbidden transition: ³T_2g²E_g