Charge transfer complex

Metal to ligand charge transfer complexes

Metal to ligand charge transfer (MLCT) complexes experience a partial transfer of electrons from the metal to the ligand. This usually occurs for metals with well-filled d orbitals which can donate electrons into the antibonding orbitals of the ligand. Of course, the usual rules apply - the non-bonding d orbitals should match the antibonding orbitals in terms of size, shape, and symmetry.^{[citation needed]}

Ligand to metal charge transfer complexes

Ligand to metal charge transfer (LMCT) complexes are the reverse of MLCT complexes - they experience a partial transfer of electrons from the ligand to the metal. This is common when the metal is at a high oxidation state; an example is permanganate (MnO₄^-), where the manganese atom has an oxidation state of +7.

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:

1. 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.

2. Laporte's rule: Δ l = ± 1

   d-d transitions for complexes which have a center of symmetry are forbidden - symmetry forbidden or Laporte forbidden.^[2]

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 ^{[3] [4]}. In 1962, the well-known acceptor, tetracyanoquinodimethane (TCNQ) was reported. Similarly, the classic donor, holes can transfer in the TCNQ and TTF columns, respectively.

In 1980, the first organic molecule that was also a 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.

Examples

Hexaphenylbenzenes like H (fig. 1) lend themselves extremely well to forming charge transfer complexes. dodecamethylcarboranyl (B) to the blue crystal solid H⁺B^- complex is therefore easy.^[6]

Fig. 1 Synthesis of H⁺B^- complex: 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 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. 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

1. ^ IUPAC Compendium of Chemical Terminology, Charge transfer complex
2. ^ Robert J. Lancashire, Selection rules for Electronic Spectroscopy, accessed 01 October 2006
3. ^ Y. Okamoto and W. Brenner Organic Semiconductors, Rheinhold (1964)
4. ^ H. Akamatsu,H.Inokuchi, and Y.Matsunaga, Nature 173 (1954) 168
5. ^ P. W. Anderson, P. A. Lee, M. Saitoh, Solid State Communications, 13 (1973) 595-598
6. ^ Through-Space (Cofacial) -Delocalization among Multiple Aromatic Centers: Toroidal Conjugation in Hexaphenylbenzene-like Radical Cations Duoli Sun, Sergiy V. Rosokha, Jay K. Kochi Angewandte Chemie International Edition Volume 44, Issue 32 , Pages 5133 - 5136 2005 Abstract