Charge - transfer transition

Charge transfer transition are associated transfer of an electron from one atom to another. Charge transfer band in the spectra of the complex ions are so named because the transition is responsible for them are between levels which correspond to different electron distribution among the metal and the ligand atoms in the ground and excited states.

A d-d transition involves redistribution of electrons among orbitals that are mainly localized on the metal atom. There are also electronic transitions in which electron moves from an essentially ligand-based orbital to an essentially metal based orbital or vice versa. When this happens charge is transferred from one part of the coordination sphere to another. The resulting spectroscopic features and the electronic transitions from which they arise are called, charge-transfer bands and charge transfer transitions.

In the process of transferring charge to or fro from the central ion that gains or losses electron so that charge transfer spectra have also been called redox spectra.

Type of charge - transfer spectra:

1) Intra ligand transition: The ligand such as SCN- as internal charge transfer transition usually located in the UV region of the spectrum. Corresponding transition occur in the coordinated ligand.

2) Ligand to metal (reduction) charge transfer transition (LMCT):
Transition may occur from molecular orbitals located primarily on the ligands, the metal ligand bonding sigma or pi molecular orbital located primarily on the metal atom. Such transition are termed ligand to metal charge transfer transitions.

3) Metal to ligand (oxidation) charge transfer transitions (MLCT):
Transition which involve electron being exited from non-bonding or anti-bonding orbitals located primarily on the metal atom to anti-bonding orbitals located primarily on the ligands are termed metal to ligand charge transfer transition.
The charge transfer bands generally occur in the UV region but occasionally are found tailing off N2 or even peaking in the visible region.
Charge transfer requires the energy levels on the 2 different atoms are fairly closed. Charge transfer always produce intense color since the restriction of the laposte & spin selection rules don't apply to transition between atom.