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COLOURS OF COMPLEXES AND SELECTION RULE:

Electronic transitions in a complex are governed by Selection rules A selection rule is a quantum mechanical rule that describes the types of quantum mechanical transitions that are permitted.they reflect the restrictions imposed on the state changes for an atom or molecule during an electronic transition. Transitions not permitted by selection rules are said forbidden, which means that theoretically they must not occur (but in practice may occur with very low probabilities).
(1) Laporte Selection Rule:
Laporte Selection Rule is given by Otto Laporte a German American Physicist
According to Laporte selection rule only allowed transitions are those occurring with a change in parity (flip in the sign of one spatial coordinate.) OR During an electronic transition the azimuthal quantum number can change only by ± 1 (Δ l = ±1) The Laporte selection rule reflects the fact that for light to interact with a molecule and be absorbed, there should be a change in dipole moment.

Practical meaning of the Laporte rule:

Laporte allowed transitions: are those which occur between gerade to ungerade or ungerade to gerade orbitals.
Laporte forbidden transitions: are those which occur between gerade to gerade or ungerade to ungerade orbitals.

Gerade = symmetric with respect to centre of inversion i.e. atomic or molecular orbital with center of symmetry or number of nodal plane = 0, 2, 4 (even number)
Ungerade = anti symmetric with respect to centre of inversion i.e. atomic or molecular orbital without center of symmetry or number of nodal plane = 1, 3, 5, (odd numbers)

Important Note: 

This rule affects Octahedral and Square planar complexes as they have center of symmetry. Tetrahedral complexes do not have center of symmetry therefore this rule does not apply

(2) Spin Selection Rule:
Spin selection rule states that transitions that involve a change in spin multiplicity as compare to ground state  are forbidden. 

(1) According to this rule, any transition for which Δ S = 0 (it means no change in spin multiplicity after d-d transition) is allowed. 

(2) If Δ S ≠ 0 ( change in spin multiplicity after transition) then it is  forbidden (transition not allowed)

ILLUSTRATIVE EXAMPLE: [Mn(H2O)6]2+ and [FeF6]3-  both have a d5 configuration and high-spin complexes. Electronic transitions are not only Laporte-forbidden, but also spin-forbidden. The dilute solutions of Mn2+ and Fe +3 complexes are therefore colorless

Important Note:
For first transition series d5 system, weak ligand field, and coordination number six (6) Complexes are found to be colourless due to violation of selection rule.

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