Charge-transfer band

A charge transfer band describes a kind of light absorption associated with electron transfer within the chromophore.

Charge-transfer bands are identified by^[1]

Color: The color of CT complexes is indicative of the relative energy balance resulting from the transfer of electronic charge from donor to acceptor.
Solvatochromism: In solution, the transition energy and therefore the complex color varies with variation in solvent permittivity, indicating variations in shifts of electron density as a result of the transition. This distinguishes it from the π* ← π transitions on the ligand.
Intensity: CT absorptions bands are intense and often lie in the ultraviolet or visible portion of the spectrum. For inorganic complexes, the typical molar absorptivities, ε, are about 50000 L mol⁻¹ cm⁻¹, that are three orders of magnitude higher than typical ε of 20 L mol⁻¹ cm⁻¹ or lower, for d-d transitions (transition from t₂g to e_g). This is because the CT transitions are spin-allowed and Laporte-allowed. However, d-d transitions are only spin-allowed; they are Laporte-forbidden.

Inorganic charge-transfer complexes

Charge-transfer occurs often in inorganic ligand chemistry involving metals. Depending on the direction of charge transfer they are either classified as ligand-to-metal (LMCT) or metal-to-ligand (MLCT) charge transfer..

Ligand-to-metal charge transfer

LMCT complexes arise from transfer of electrons from MO with ligand like character to those with metal like character. This type of transfer is predominant if complexes have ligands with relatively high energy lone pairs (example S or Se) or if the metal has low lying empty orbitals. Many such complexes have metals in high oxidation states (even d⁰). These conditions imply that the acceptor level is available and low in energy.

Consider a d⁶ octahedral complex (example IrBr₆^3-). The t_2g levels are filled as shown in Figure 1. Consequently an intense absorption is observed around 250 nm corresponding to a transition from ligand σ MO to the empty e_g MO. However, in IrBr₆^2- that is a d⁵ complex two absorptions, one near 600 nm and another near 270 nm, are observed. This is because two transitions are possible, one to t_2g (that can now accommodate one more electron) and another to e_g. The 600 nm band corresponds to transition to the t_2g MO and the 270 nm band to the e_g MO.

Figure 1. MO diagram showing ligand to metal charge transfer for a d⁶ octahedral complex Another thing to note is that CT bands might also arise from transfer of electrons from nonbonding orbitals of the ligand to the e_g MO.

Trend of LMCT energies

Oxidation Number: +7 MnO₄^- < TcO₄^- < ReO₄^-; +6 CrO₄^2- < MoO₄^2- < WO₄^2-; +5 VO₄^3- < NbO₄^3- < TaO₄^3-

The energies of transitions correlate with the order of the electrochemical series. The metal ions that are most easily reduced correspond to the lowest energy transitions. The above trend is consistent with transfer of electrons from the ligand to the metal, thus resulting in a reduction of metal ions by the ligand.

Examples include:

MnO₄^- : The permanganate ion having tetrahedral geometry is intensely purple due to strong absorption involving charge transfer from MO derived primarily from filled oxygen p orbitals to empty MO derived from manganese(VII).
CdS: The color of artist’s pigment cadmium yellow is due to transition from Cd²⁺ (5s) ← S^2-(π).
HgS: it is red due to Hg²⁺ (6s) ← S^2-(π) transition.
Fe Oxides: they are red and yellow due to transition from Fe (3d) ← O^2-(π).

Metal-to-ligand charge transfer

Metal-to-ligand charge-transfer (MLCT) complexes arise from transfer of electrons from MO with metal like character to those with ligand like character.^[1]^[2] This is most commonly observed in complexes with ligands having low-lying π* orbitals especially aromatic ligands. The transition will occur at low energy if the metal ion has a low oxidation number for its d orbitals will relatively be high in energy.

Examples of such ligands taking part in MLCT include 2,2’-bipyridine (bipy), 1,10-phenanthroline (phen), CO, CN^- and SCN^-. Examples of these complexes include:

Tris(2,2’-bipyridyl)ruthenium(II) : This orange colored complex is being studied^[3] as the excited state resulting from this charge transfer has a lifetime of microseconds and the complex is a versatile photochemical redox reagent.
W(CO)₄(phen)
Fe(CO)₃(bipy)

Photoreactivity of MLCT excited states

The photoreactivity of MLCT complexes result from the nature of the oxidized metal and the reduced ligand. Though the states of traditional MLCT complexes like Ru(bipy)₃²⁺ and Re(bipy)(CO)₃Cl were intrinsically not reactive, several MLCT complexes have been synthesized that are characterized by reactive MLCT states.

Vogler and Kunkely^[4] considered the MLCT complex to be an isomer of the ground state which contains an oxidized metal and reduced ligand. Thus various reactions like electrophilic attack and radical reactions on the reduced ligand, oxidative addition at the metal center due to the reduced ligand, and outer sphere charge-transfer reactions can be attributed to states arising from MLCT transitions. MLCT states’ reactivity often depends on the oxidation of the metal. Subsequent processes include associative ligand substitution, exciplex formation and cleavage of metal---metal bonds.

Color of charge-transfer complexes

Many metal complexes are colored due to d-d electronic transitions. Visible light of the correct wavelength is absorbed, promoting a lower d-electron to a higher excited state. This absorption of light causes color. These colors are usually quite faint, though. This is because of two selection rules:

The spin rule: Δ S = 0

On promotion, the electron should not experience a change in spin. Electronic transitions which experience a change in spin are said to be spin forbidden.

Laporte's rule: Δ l = ± 1

d-d transitions for complexes which have a center of symmetry are forbidden - symmetry forbidden or Laporte forbidden.^[5]

Charge-transfer complexes do not experience d-d transitions. Thus, these rules do not apply and the absorptions are generally very intense.

For example, the classic example of a charge-transfer complex is that between iodine and starch to form an intense purple color. This has widespread use as a rough screen for counterfeit currency. Unlike most paper, the paper used in US currency is not sized with starch. Thus, formation of this purple color on application of an iodine solution indicates a counterfeit.

^ ^a ^b Cite error: The named reference shriver was invoked but never defined (see the help page).
^ Tarr, Donald A.; Miessler, Gary L. (1991). Inorganic chemistry (2nd ed.). Englewood Cliffs, N.J: Prentice Hall. ISBN 0-13-465659-8.{{cite book}}: CS1 maint: multiple names: authors list (link)
^ Kalyanasundaram, K. (1992). Photochemistry of polypyridine and porphyrin complexes. Boston: Academic Press. ISBN 0-12-394992-0.
^ Vogler, A.; Kunkely, H. (2000). "Photochemistry induced by metal-to-ligand charge transfer excitation". Coord. Chem. Rev. 208: 321. doi:10.1016/S0010-8545(99)00246-5.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^ Robert J. Lancashire. "Selection rules for Electronic Spectroscopy". University of the West Indies, Mona. Retrieved 2008-08-30.

[shriver-1] Cite error: The named reference shriver was invoked but never defined (see the help page).

[miessler-2] Tarr, Donald A.; Miessler, Gary L. (1991). Inorganic chemistry (2nd ed.). Englewood Cliffs, N.J: Prentice Hall. ISBN 0-13-465659-8.{{cite book}}: CS1 maint: multiple names: authors list (link)

[3] Kalyanasundaram, K. (1992). Photochemistry of polypyridine and porphyrin complexes. Boston: Academic Press. ISBN 0-12-394992-0.

[4] Vogler, A.; Kunkely, H. (2000). "Photochemistry induced by metal-to-ligand charge transfer excitation". Coord. Chem. Rev. 208: 321. doi:10.1016/S0010-8545(99)00246-5.{{cite journal}}: CS1 maint: multiple names: authors list (link)

[5] Robert J. Lancashire. "Selection rules for Electronic Spectroscopy". University of the West Indies, Mona. Retrieved 2008-08-30.

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