Charge transfer complex

A charge-transfer complex (or CT complex, electron-donor-acceptor-complex) is a chemical association of two or more molecules, or of different parts of one very large molecule, in which the attraction between the molecules (or parts) is created by an electronic transition into an excited electronic state, such that a fraction of electronic charge is transferred between the molecules. The resulting electrostatic attraction provides a stabilizing force for the molecular complex. The source molecule from which the charge is transferred is called the electron donor, and the receiving molecule is called the electron acceptor, hence the alternate name, electron-donor-acceptor-complex.

The nature of the attraction in a charge-transfer complex is not a stable chemical bond and is much weaker than covalent forces, rather it is better characterized as a weak electron resonance. As a result, the excitation energy of this resonance occurs very frequently in the visible region of the electro-magnetic spectrum. This produces the usually intense colors characteristic for these complexes. These optical absorption bands are often referred to as charge-transfer bands, or CT bands. Optical spectroscopy is a powerful technique to characterize charge-transfer bands.

Charge-transfer complexes exist in many types of molecules, inorganic as well as organic, and in all phases of matter, i.e. in solids, liquids, and even gases.

In inorganic chemistry, most charge-transfer complexes involve electron transfer between metal atoms and ligands. The charge-transfer bands in transition metal complexes result from movement of electrons between molecular orbitals (MO) that are predominantly metal in character and those that are predominantly ligand in character. If the electron moves from the MO with ligand like character to the metal like one, the complex is called ligand-to-metal charge-transfer (LMCT) complex. If the electron moves from the MO with metal like character to the ligand-like one, the complexes is called metal-to-ligand charge-transfer (MLCT) complex. Thus, a MLCT results in oxidation of the metal center whereas a LMCT results in the reduction of the metal center. Resonance Raman Spectroscopycite book |author=Atkins, P. J.; Shriver, D. F. |title=Inorganic chemistry |publisher=W.H. Freeman and CO |location=New York |year=1999 |edition = 3rd ed. |isbn=0-7167-3624-1 |oclc= |doi= |accessdate=] is also a powerful technique to assign and characterize charge transfer bands in these complexes.

= Identification of CT bands =

Charge transfer complexes are identified by

1) Intensity: CT absorptions bands are highly intense and often lie in the ultraviolet or visible portion of the spectrum. The typical molar absorptivities, ε, of charge transfer complexes 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 not spin or Laporte forbidden as d-d transitions.

2) Solvatochromism: The transition frequency varies with variation in solvent permittivity, indicating a large shift in electron density as a result of the transition. This distinguishes it from the π* ← π transitions on the ligand.

Ligand to metal charge transfer complexes

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 t₂g 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 t₂g (that can now accommodate one more electron) and another to e_g. The 600 nm band corresponds to transition to the t₂g 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 complexAnother 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 Complex (MLCT)cite book |author=Tarr, Donald A.; Miessler, Gary L. |title=Inorganic chemistry |publisher=Prentice Hall |location=Englewood Cliffs, N.J |year=1991 |pages= |isbn=0-13-465659-8 | edition = 2nd ed.] =

MLCT complexes arise from transfer of electrons from MO with metal like character to those with ligand like character. 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 [cite book |author=Kalyanasundaram, K. |title=Photochemistry of polypyridine and porphyrin complexes |publisher=Academic Press |location=Boston |year=1992 |pages= |isbn=0-12-394992-0 |oclc= |doi= |accessdate=] 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 [cite journal | author = Vogler, A.; Kunkely, H. | journal = Coord. Chem. Rev. | year = 2000 | volume = 208 | pages = 321 | doi = 10.1016/S0010-8545(99)00246-5 | title = Photochemistry induced by metal-to-ligand charge transfer excitation] considered the MLCT complex to be an isomer of the ground state which contains an oxidized metal and reduced ligand. Thus various reactions like electrophillic 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.

Charge transfer complexes and color

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". [cite web | author = Robert J. Lancashire | title = Selection rules for Electronic Spectroscopy | accessdate = 2008-08-30 | url = http://wwwchem.uwimona.edu.jm/courses/selrules.html | publisher = University of the West Indies, Mona]

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 wide-spread 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.

History

In 1954 researchers at Bell Labs and elsewhere reported charge-transfer complexes with resistivities as low as 8 ohms/cm. [Y. Okamoto and W. Brenner "Organic Semiconductors", Rheinhold (1964)] [cite journal | author = H. Akamatsu, H. Inokuchi, and Y.Matsunaga | journal = Nature | volume = 173 | year = 1954 | pages = 168 | doi = 10.1038/173168a0 | title = Electrical Conductivity of the Perylene–Bromine Complex] In 1962, the well-known acceptor, tetracyanoquinodimethane (TCNQ) was reported. Similarly, the classic donor, tetrathiafulvalene (TTF), was synthesized in 1970. A CT complex composed of the TTF and TCNQ was discovered in 1973. [cite journal | author = P. W. Anderson, P. A. Lee, M. Saitoh | journal = Solid State Communications | volume = 13 | year = 1973 | pages = 595–598 | doi = 10.1016/S0038-1098(73)80020-1 | title = Remarks on giant conductivity in TTF-TCNQ] This was the first organic conductor to show almost metallic conductance. In a crystal of TTF-TCNQ, the TTF and TCNQ are stacked independently and an electron transfer from donor (TTF) to acceptor (TCNQ) occurs. Hence, electrons and holes can transfer in the TCNQ and TTF columns, respectively.

In 1980, the first organic molecule that was also a superconductor was discovered. Tetramethyl-tetraselenafulvalene-phosphorus hexafloride TMTSF₂PF₆ shows superconductivity at low temperature (critical temperature) and high pressure: 0.9 K and 12 kbar. Since 1980, many organic superconductors have been synthesized, and the critical temperature has been raised to over 100 K as of 2001. Unfortunately, critical current densities in these complexes are very small.

CT complexes have many useful applications and more properties are expected to be discovered.

Other examples

Hexaphenylbenzenes like H (fig. 3) lend themselves extremely well to forming charge transfer complexes. Cyclic voltammetry for H displays 4 well separed maxima corresponding to H⁺ right up to H⁴⁺ with the first ionization at E_1/2 of only 0.51 eV. oxidation of these arenes by for instance dodecamethylcarboranyl (B) to the blue crystal solid H⁺B^- complex is therefore easy. [cite journal | title = Through-Space (Cofacial) -Delocalization among Multiple Aromatic Centers: Toroidal Conjugation in Hexaphenylbenzene-like Radical Cations | author = Duoli Sun, Sergiy V. Rosokha, Jay K. Kochi | journal = Angew. Chem. Int. Ed. | volume = 44 | issue = 32 | pages = 5133–5136 | year = 2005 | doi = 10.1002/anie.200501005 ]

Fig. 3 Synthesis of H⁺B^- complex: Alkyne trimerisation of bisubstituted alkyne with dicobalt octacarbonyl, delocalization is favored with activating groups such as a di(ethylamino) group
The phenyl groups are all positioned in an angle of around 45° with respect to the central aromatic ring and the positive charge in the radical cation is therefore through space delocalised through the 6 benzene rings in the shape of a toroid. The complex has 5 absorption bands in the near infrared region which can be assigned to specific electronic transitions with the aid of deconvolution and the Mulliken-Hush theory.

Charge transfer complexes and disease

In humans, elevated systemic levels of transition-series metals, electron-donors, etc. [http://www.drproctor.com/rev/crcpap2.htm are associated] with specific disease symptoms. These include psychosis, movement disorders, pigmentary abnormalities, and deafness. This may involve charge-transfer complexes with the Melanin in the midbrain, skin, and the stria vascularis of the inner ear.

See also

* Organic semiconductor
* Organic superconductor

References