Charge Transfer Spectra

Charge Transfer Spectra or Band

A charge transfer spectra or band may be defined as the peak arising from the transition in which an electron is transferred from one atom or group of atoms in the molecule to another one.
In other words, the transition occurs between molecular orbitals that are centered on different atoms or groups. Unlike d-d transitions, charge transfer transitions are Laporte and spin allowed and therefore, show very intense (strong) absorption. The charge transfer bands have extinction coefficients in the 500-2000 unit range (for d-d transitions, the coefficients are below 1000 units).
These bands mostly occurs near the ultraviolet region. When these transitions occurs in visible region, the compound show ntense color.
In the spectra of some metal complexes, an overlap between the end of the charge transfer band and d-d absorption occurs. This makes clearly the full d-d spectrum of the complex impossible.
A charge transfer transitions are intramolecular transitions so it requires much more energy than d-d transitions.
A charge transfer transition may be regarded as an internal redox process as the charge distribution in the excited state differs considerably from the ground state.

Nature of Charge Transfer Spectra

1. These are Laporte and spin allowed transitions.
Δl = ± 1 and ΔS = 0
2. Charge transfer transition mostly occur in UV region and visible region.
3. Charge transfer transitions only in visible region are responsible for the color of the complexes.
4. It is observed almost in all complexes.

Types of Charge Transfer Spectra

Charge transfer spectra may be classified in four ways-
1. Ligand to Metal Charge Transfer spectra (LMCT)
2. Metal to Ligand Charge Transfer spectra (MLCT)
3. Metal to Metal (Inter valence) Charge Transfer spectra (MMCT)
4. Ligand to Ligand (Intra Ligand) Charge Transfer spectra (LLCT)

1. Ligand to Metal charge transfer spectra (LMCT)

If the transfer of electron takes place from the ligand to metal is called ligand to metal charge transfer(LMCT). These transitions take place with lower energy as the metal becomes more easily reducible and the ligand gets readily oxidizable.

Conditions for LMCT

A. Metals should be in high oxidation state.
B. Metals should have high ionization energy so that it would have empty orbitals at fairly low energies.
C. Ligands should have lone pair of electrons of relatively high energy and low electron affinity.
A majority of charge transfer complexes of transition metals involves L → M transfer as expected from the availability of nonbonding or antibonding orbitals on the metal.

2. Metal to Ligand charge transfer spectra (MLCT)

If the transfer of electron takes place from the metal to ligand is called metal to ligand charge transfer(MLCT).
Charge transfer processes in the opposite direction, from metal to ligand, are favored in complexes that have occupied metal-centered orbitals and vacant low lying ligand centered orbitals.

Conditions for MLCT

A. Metals have low oxidation state.
B. Metal d-orbitals are filled.
C. Metal d-orbitals are of relatively high in energy.
D. Ligands have empty π-antibonding orbitals.
MLCT mainly occurs with ligands having antibonding π-orbitals such as CO, NO, CN⁻, SCN⁻, pyridine, bipyridine, pyrazine, dithioline, o-phenanthroline etc.
In octahedral complexes, when t_2g and e^*_g orbitals both belong to metal are occupied, then two MLCT bands π^* ← t_2g and π^* ← e^*_g are observed. (π^* is the vacant ligand orbital). If either t_2g or e^*_g orbitals are occupied, then only one charge transfer band π^* ← t_2g or π^* ← e^*_g is observed.

3. Metal to Metal charge transfer spectra (Intervalence Transition)

In this transitions and electron gets excited from the valence shell of one atom to valence shell of the other atom. Electron transfer takes place from an atom of lower oxidation state to an another atom of higher oxidation state.
Compounds which contain metal-metal bonds are intensely colored. The color is due to σ → σ^*, π → π^* and δ → δ^* transitions. The metal carbonyls containg M-M single bond are generally intense in color due to σ → σ^* transitions. Color of compounds having quadruple bond is due to δ → δ^* transitions.
Mn₂(CO)₁₀ is bright yellow, [Re₂Cl₈]⁻² is deep blue, Co₂(CO)₈ is purple-black, Mo₂Cl₈]^–4 is cherry red, and Fe₂(CO)₉ is gold; the observed transitions for these compounds are of the types σ → σ^*, π → π^* and δ → δ^* and are fully allowed.
Prussian blue KFe[Fe(CN)₆] shows intense blue color because of transfer of an electron from Fe⁺² to Fe⁺³. In prussian blue, Fe⁺² ion is octahedrally coordinated with carbon atom of cyanide ion ligands and Fe⁺³ is octahedrally coordinated with nitrogen atom of cyanide ion ligands. Thus an electron transfer takes place through bridging cyanide ligands.
Red lead (Pb₃O₄) contains Pb(II) and Pb(IV). Due to electron transfer from Pb(II) Tto Pb(IV), it gives intense red color.

4. Intra Ligand charge transfer

Some organic ligand behave itself as a chromophore and still another type of absorption band, an intraligand band, may be observed. There are four electronic transitions σ → σ^*, π → π^*, n → π^* and n → δ^* within a chromophore. When such a ligand is coordinated with metal or ions, energy of absorption changes.

Color of octahedral complexes are less intense than tetrahedral complexes

Extinction coefficient is high and color of the tetrahedral complex is more intense relative to octahedral complex due to d-p mixing in tetrahedral complexe.

Due to mixing, some p-character is introduced in t₂ of d-orbital and hence symmetry rule is relaxed to some extent and extinction coefficient value is higher than that of octahedral. Further gerade property is lost in tetrahedral geometry.
We kow that- Δ_tet = (4/9)Δ_oct
So, the d-d transition in tetrahedral complexes occurs at lower energy due to small gap betwee t₂ and e orbitals which often fall in visible region. Thats why color of octahedral complexes are less intense than tetrahedral complexes.

Which of the following has lowest frequency

A. [Co(NH₃)₅F]⁺²
B. [Co(NH₃)₅Cl]⁺²
C. [Co(NH₃)₅Br]⁺²
D. [Co(NH₃)₅I]⁺²
Answer: D. [Co(NH₃)₅I]⁺²