Charge transfer complex

A charge-transfer complex (CT complex) or electron-donor-acceptor complex is an association of two or more molecule

Molecule

A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

s, or of different parts of one very large molecule, in which a fraction of electronic charge is transferred between the molecular entities. The resulting electrostatic

Electrostatics

Electrostatics is the branch of physics that deals with the phenomena and properties of stationary or slow-moving electric charges....

 attraction provides a stabilizing force for the molecular complex. The source molecule from which the charge is transferred is called the electron donor

Electron donor

An electron donor is a chemical entity that donates electrons to another compound. It is a reducing agent that, by virtue of its donating electrons, is itself oxidized in the process....

 and the receiving species is called the electron acceptor

Electron acceptor

An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound. It is an oxidizing agent that, by virtue of its accepting electrons, is itself reduced in the process....

.

The nature of the attraction in a charge-transfer complex is not a stable chemical bond, and is much weaker than covalent forces

Covalent bond

A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....

. The attraction is created by an electronic transition into an excited electronic state

Excited state

Excitation is an elevation in energy level above an arbitrary baseline energy state. In physics there is a specific technical definition for energy level which is often associated with an atom being excited to an excited state....

, and is best characterized as a weak electron resonance. The excitation energy of this resonance occurs very frequently in the visible region of the electro-magnetic spectrum, which produces the usually intense color characteristic for these complexes. These optical absorption bands

Absorption band

An absorption band is a range of wavelengths, frequencies or energies in the electromagnetic spectrum which are able to excite a particular transition in a substance...

 are often referred to as charge-transfer bands (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. A well-known example is the blue charge-transfer band exhibited by iodine when combined with starch.

In inorganic chemistry

Inorganic chemistry

Inorganic chemistry is the branch of chemistry concerned with the properties and behavior of inorganic compounds. This field covers all chemical compounds except the myriad organic compounds , which are the subjects of organic chemistry...

, most charge-transfer complexes involve electron transfer between metal atoms and ligand

Ligand

In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...

s. The charge-transfer bands in transition metal complexes result from shift of charge density between molecular orbitals (MO) that are predominantly metal in character and those that are predominantly ligand in character. If the transfer occurs from the MO with ligand-like character to the metal-like one, the complex is called a ligand-to-metal charge-transfer (LMCT) complex. If the electronic charge shifts from the MO with metal-like character to the ligand-like one, the complex is called a 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 Spectroscopy is also a powerful technique to assign and characterize charge-transfer bands in these complexes.

Donor-acceptor association equilibrium

Charge-transfer complexes are formed by weak association of molecules or molecular subgroups, one acting as an electron donor and another as an electron acceptor. The association does not constitute a strong covalent bond and is subject to significant temperature, concentration, and host, e.g., solvent, dependencies.

The charge-transfer association occurs in a chemical equilibrium with the independent donor (D) and acceptor (A) molecules:

Quantum mechanically, this is described as a resonance between the non-bonded state |D, A> and the dative state |D⁺...A^->. The formation of the dative state is an electronic transition giving rise to the colorful absorption bands.

The intensity of charge-transfer bands in the absorbance spectrum is strongly dependent upon the degree (equilibrium constant) of this association reaction. Methods have been developed to determine the equilibrium constant for these complexes in solution by measuring the intensity of absorption bands as a function of the concentration of donor and acceptor components in solution. The methods were first described for the association of iodine dissolved in aromatic hydrocarbons. The procedure is called the Benesi-Hildebrand method

Benesi-Hildebrand method

The Benesi-Hildebrand method is a mathematical approach used in physical chemistry for the determination of the equilibrium constant K and stoichiometry of non-bonding interactions...

, named after the authors of the study.

Charge-transfer transition energy

The color of charge-transfer bands, i.e., the charge-transfer transition energy, is characteristic of the specific type of donor and acceptor entities.

The electron donating power of a donor molecule is measured by its ionization potential

Ionization potential

The ionization energy of a chemical species, i.e. an atom or molecule, is the energy required to remove an electron from the species to a practically infinite distance. Large atoms or molecules have a low ionization energy, while small molecules tend to have higher ionization energies.The property...

 which is the energy required to remove an electron from the highest occupied molecular orbital

Molecular orbital

In chemistry, a molecular orbital is a mathematical function describing the wave-like behavior of an electron in a molecule. This function can be used to calculate chemical and physical properties such as the probability of finding an electron in any specific region. The term "orbital" was first...

. The electron accepting power of the electron acceptor is determined by its electron affinity

Electron affinity

The Electron affinity of an atom or molecule is defined as the amount of energy released when an electron is added to a neutral atom or molecule to form a negative ion....

 which is the energy released when filling the lowest unoccupied molecular orbital.

The overall energy balance (ΔE) is the energy gained in a spontaneous charge transfer. It is determined by the difference between the acceptor's electron affinity (E_A) and the donor's ionization potential (E_I), adjusted by the resulting electrostatic attraction (J) between donor and acceptor:

The positioning of the characteristic CT bands in the electromagnetic spectrum is directly related to this energy difference and the balance of resonance contributions of non-bonded and dative states in the resonance equilibrium.

Identification of CT bands

Charge-transfer complexes are identified by

• Color: The color of CT complexes is reflective 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
  Laporte rule
  The Laporte rule is a spectroscopic selection rule. It states that electronic transitions that conserve either symmetry or asymmetry with respect to an inversion center — i.e., g → g, or u → u respectively—are forbidden...
  
  -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:

1. 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).
2. CdS: The color of artist’s pigment cadmium yellow is due to transition from Cd²⁺ (5s) ← S^2-(π).
3. HgS: it is red due to Hg²⁺ (6s) ← S^2-(π) transition.
4. 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. 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

Carbon monoxide

Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...

, CN^-

Cyanide

A cyanide is a chemical compound that contains the cyano group, -C≡N, which consists of a carbon atom triple-bonded to a nitrogen atom. Cyanides most commonly refer to salts of the anion CN−. Most cyanides are highly toxic....

 and SCN^-

Thiocyanate

Thiocyanate is the anion [SCN]−. It is the conjugate base of thiocyanic acid. Common derivatives include the colourless salts potassium thiocyanate and sodium thiocyanate. Organic compounds containing the functional group SCN are also called thiocyanates...

. Examples of these complexes include:

1. Tris(2,2’-bipyridyl)ruthenium(II)
   Tris(bipyridine)ruthenium(II) chloride
   Trisruthenium dichloride is the coordination compound with the formula [Ru3]Cl2. This red crystalline salt is obtained as the hexahydrate, although all of the properties of interest are in the cation [Ru3]2+, which has received much attention because of its distinctive optical properties...
   
    : This orange colored complex is being studied as the excited state resulting from this charge transfer has a lifetime of microseconds and the complex is a versatile photochemical redox reagent.
2. W(CO)₄(phen)
3. 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 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 rule

Selection rule

In physics and chemistry a selection rule, or transition rule, formally constrains the possible transitions of a system from one state to another. Selection rules have been derived for electronic, vibrational, and rotational transitions...

s:

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.

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.

Other examples

Hexaphenylbenzene

Hexaphenylbenzene

Hexaphenylbenzene is an aromatic compound composed of a benzene ring substituted with six phenyl rings.It may be prepared through a Diels-Alder reaction by refluxing tetraphenylcyclopentadienone and diphenylacetylene in benzophenone or other high-temperature solvent.-Structure:Due to steric...

s like H (fig. 3) lend themselves extremely well to forming charge-transfer complexes. Cyclic voltammetry

Cyclic voltammetry

Cyclic voltammetry or CV is a type of potentiodynamic electrochemical measurement. In a cyclic voltammetry experiment the working electrode potential is ramped linearly versus time like linear sweep voltammetry. Cyclic voltammetry takes the experiment a step further than linear sweep voltammetry...

 for H displays 4 well separated maxima corresponding to H⁺ right up to H⁴⁺ with the first ionization at E_1/2 of only 0.51 eV

Electronvolt

In physics, the electron volt is a unit of energy equal to approximately joule . By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an electric potential difference of one volt...

. oxidation of these arene

Arene

Arene or Arênê or Arène may refer to:*an aromatic hydrocarbon*Arene , a genus of marine snails in the family Areneidae*Arene , the wife of Aphareus and mother of Idas and Lynceus in Greek mythology...

s by for instance dodecamethylcarboranyl

Carborane

A carborane is a cluster composed of boron and carbon atoms. Like many of the related boranes, these clusters are polyhedra and are similarly classified as closo-, nido-, arachno-, hypho-, etc...

 (B) to the blue crystal solid H⁺B^- complex is therefore easy.


Fig. 3 Synthesis of H⁺B^- complex: Alkyne trimerisation

Alkyne trimerisation

An alkyne trimerisation reaction is a 2+2+2 cyclization reaction in which three alkyne molecules react to form an aromatic compound. The reaction is 'pseudo' pericyclic since it has not been observed to occur without the assistance of metal catalysis; and the metal catalyst assembles the ring...

 of bisubstituted alkyne

Alkyne

Alkynes are hydrocarbons that have a triple bond between two carbon atoms, with the formula CnH2n-2. Alkynes are traditionally known as acetylenes, although the name acetylene also refers specifically to C2H2, known formally as ethyne using IUPAC nomenclature...

 with dicobalt octacarbonyl

Dicobalt octacarbonyl

Dicobalt octacarbonyl is the inorganic compound Co28. This metal carbonyl is a reagent and catalyst in organometallic chemistry and organic synthesis. It is used as a catalyst for hydroformylation, the conversion of alkenes to aldehydes....

, delocalization is favored with activating group

Activating group

In organic chemistry, a functional group is called an activating group if a benzene molecule to which it is attached more readily participates in electrophilic substitution reactions...

s 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

Radical ion

A radical ion is a free radical species that carries a charge. Radical ions are encountered in organic chemistry as reactive intermediates and in mass spectrometry as gas phase ions...

 is therefore through space delocalised through the 6 benzene rings in the shape of a toroid

Toroid

Toroid may refer to*Toroid , a doughnut-like solid whose surface is a torus.*Toroidal inductors and transformers which have wire windings on circular ring shaped magnetic cores.*Vortex ring, a toroidal flow in fluid mechanics....

. The complex has 5 absorption bands in the near infrared region which can be assigned to specific electronic transition

Molecular electronic transition

Molecular electronic transitions take place when electrons in a molecule are excited from one energy level to a higher energy level. The energy change associated with this transition provides information on the structure of a molecule and determines many molecular properties such as color...

s with the aid of deconvolution

Deconvolution

In mathematics, deconvolution is an algorithm-based process used to reverse the effects of convolution on recorded data. The concept of deconvolution is widely used in the techniques of signal processing and image processing...

 and the Mulliken-Hush theory.

Electrical conductivity

In 1954 researchers at Bell Laboratories and elsewhere reported charge-transfer complexes with resistivities as low as 8 ohms·cm in combinations of perylene

Perylene

Perylene or perilene is a polycyclic aromatic hydrocarbon with the chemical formula C20H12, occurring as a brown solid. It or its derivatives may be carcinogenic, and it is considered to be a hazardous pollutant. In cell membrane cytochemistry, perylene is used as a fluorescent lipid probe...

 with iodine

Iodine

Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....

 or bromine

Bromine

Bromine ") is a chemical element with the symbol Br, an atomic number of 35, and an atomic mass of 79.904. It is in the halogen element group. The element was isolated independently by two chemists, Carl Jacob Löwig and Antoine Jerome Balard, in 1825–1826...

. In 1962, the well-known acceptor tetracyanoquinodimethane

TCNQ

Tetracyanoquinodimethane is the organic compound with the formula 2CC6H4C2. This relative of para-quinone is an electron-acceptor that is used to prepare charge transfer salts, which are of interest in the area of molecular electronics....

 (TCNQ) was reported. Tetrathiafulvalene

Tetrathiafulvalene

Tetrathiafulvalene is a organosulfur compound with the formula 2. Studies on this heterocyclic compound contributed to the development of molecular electronics. TTF is related to the hydrocarbon fulvalene, 2, by replacement of four CH groups with sulfur atoms...

 (TTF) was synthesized in 1970 and found to be a strong electron donor. In 1973 it was discovered that a combination of these components formed a strong charge-transfer complex, henceforth referred to as TTF-TCNQ. The complex is formed in solution and may be crystallized into a well-formed crystalline solid. The solid shows almost metallic electrical conductance and was the first discovered purely organic conductor. In a TTF-TCNQ crystal, TTF and TCNQ molecules are arranged independently in separate parallel-aligned stacks and an electron transfer occurs from donor (TTF) to acceptor (TCNQ) stacks. Hence, electrons and electron hole

Electron hole

An electron hole is the conceptual and mathematical opposite of an electron, useful in the study of physics, chemistry, and electrical engineering. The concept describes the lack of an electron at a position where one could exist in an atom or atomic lattice...

s are separated and concentrated in the stacks and can traverse in a one-dimensional direction along the TCNQ and TTF columns, respectively, when an electric potential is applied to the ends of a crystal in the stack direction.

The first organic molecule that forms a superconductor

Superconductivity

Superconductivity is a phenomenon of exactly zero electrical resistance occurring in certain materials below a characteristic temperature. It was discovered by Heike Kamerlingh Onnes on April 8, 1911 in Leiden. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum...

 was discovered in 1980. Tetramethyl-tetraselenafulvalene-phosphorus hexafloride (TMTSF₂PF₆), a semi-conductor at ambient conditions, shows superconductivity at low temperature

Temperature

Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

 (critical temperature) and high pressure

Pressure

Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

: 0.9 K

Kelvin

The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

 and 12 kbar

Bar (unit)

The bar is a unit of pressure equal to 100 kilopascals, and roughly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the megabar , kilobar , decibar , centibar , and millibar...

. 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.