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).
Related Question:
(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.
Related Question:
Which of the
Complex of the following pairs has the highest value of CFSE?
Colour of
Complexes due to charge transfer:
Why violet
colour of [Ti(H2O)6]Cl3 disapear (colourless) on heating heating ?
Why [Ni(CN)4]-2
is colourless while [Ni(H2O)4]-2 although both have +2 oxidation state and 3d*8
configuration ?
Why [FeF6]3– is
colourless whereas [CoF6]3– is coloured ?
Why Fe(CO)5 is
colourless while Fe(bipy)(CO)3 is intensely purple in colour ?
Why all the
tetrahedral Complexes are high spin Complexes ?
Colour of Complexes due to charge transfer:
Why violet colour of [Ti(H2O)6]Cl3 disapear (colourless) on heating heating ?
Why [Ni(CN)4]-2 is colourless while [Ni(H2O)4]-2 although both have +2 oxidation state and 3d*8 configuration ?
Why [FeF6]3– is colourless whereas [CoF6]3– is coloured ?
Why Fe(CO)5 is colourless while Fe(bipy)(CO)3 is intensely purple in colour ?
Why all the tetrahedral Complexes are high spin Complexes ?
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