Question

Explain the origin of d-d, metal-to-ligand charge transfer, and ligand-to-metal charge transfer transitions for octahedral metal complexes with appropriate molecular orbital diagrams. Also, comment on the allowance of these transitions using appropriate selection rules.

Answer 1

The origin of d - d Spectra -

For, Octahedral complex, the d-d transition occur due to incompletely or partially filled d orbital.

Selection rule -

(a) Spin selection rule - For forbidden transition∆S ≠ 0 ; For allowed transition

∆S=0; i.e. the transition are permitted between the state of same spin multiplicity.

Spin multiplicity (S) = 2s+1

(b) Laporte or orbital selection rule -

Only those transition are allowed for which ∆l = +- 1, where, l= azimuthal quantum no, other than these all are forbidden.

s- gerade(g) containing centrosymmetry. g-g transition forbidden

p- ungerade(u) no centrosymmetry u-u transition forbidden

d- gerade(g) containing centrosymmetry. g-u transition allowed

u-g transition allowed

The origin of MLCT Spectra -

Metal to ligand charge transfer is common among he Octahedral complexes when pi accepter type ligands are present. The electron travels from a bonding pi(t_{​​​​​​2g} ) or antibonding sigma^* (e_{​​​​​​g} ) orbital into a sigma^* or pi^* orbital.

Here, metal is highly reducing and the ligands are oxidizing. There occurs transfer of charge from reducing partner to oxidizing partner. Transition is possible only when the ligand possess low lying empty pi^* - orbitals and the metal ion has filled orbitals at a higher level than HOMO of ligands.

The origin of LMCT Spectra -

Ligand to metal charge transfer is common among the Octahedral complexes when pi-donor type ligands are present. The electron travels from a bonding pi or non-binding pi orbital into a sigma^* orbital.

Here metal ion is oxidizing and ligands becomes reducing becomes reducing. The L- M CT bands are common in the Spectra of complexes in which the ligands have filled pi^* orbitals and the meal ions have low lying empty orbitals.

The charge transfer transition are Laporte allowed but spin forbidden.

d-d transition are Laporte forbidden.