Dithiolene

A 1,2-dithiolene is a bidentate ligand consisting of two sulfur atoms connected via an unsaturated carbon-carbon backbone. These ligands readily form complexes by co-ordinating to metal centres via the sulfur atoms; such complexes are often referred to as metallodithiolenes or "dithiolene complexes".

Dithiolenes have been intensely studied since the 1960's. A large number of such complexes have been described with regards to their structural, spectroscopic, and electrochemical properties. Research on dithiolenes intensified with the discovery that most molybedenum-containing and tungsten-containing proteins contain dithiolene ligands, the so-called molybdopterin cofactor.

Background and nomenclature
Early studies on dithiolene ligands, although not called by that name until the the 1960's, focused on the quinoxaline-2,3-dithiolates and 3,4-toluenedithiolates, which form brightly colored precipitates with several metal centres. Such species were originally of interest in analytical chemistry. Dithiolenes lacking benzene backbones represented an importan development of the area, especially maleonitrile-2,3-ditholate ("mnt"), (NC)2C2S22-, and ethylenedithiolene, H2C2S22-.

Electronic structure
The electronic structure of dithiolene complexes has been the subject of intense study and the source of much hyperbole as well. Dithiolene ligands are often thought to exist in two oxidation states separated by a two-electron redox; the dianionic ene-1,2-dithiolate and the neutral 1,2-dithioketone, which possess different electronic characteristics. In reality, 1,2-dithiones have not been characterized crystallographically. Dithioketones, more so than thioketones, are expected to be unstable with respect to self-condensation to give oligomers.

The two substituents on the backbone of the dithiolene ligand, R and R', affect the properties of the resulting metal complex in the expected way. Long chains confer solubility in less polar solvents and electron acceptors (e.g. CN, CO2Me) stabilized reduced complexes. Derivatives are known where the substituents are the same, symmetrical dithiolenes (R = R') are more common than unsymmetrical.

Due to the delocalized nature of dithiolenes, metal dithiolenes often exist in multiple oxidation states. In oxidized dithiolene complexes have relatively more 1,2-dithioketone character. In reduced complexes, the ligand assumes more ene-1,2-dithiolate character. These ambiguous descriptions are based on subtle differences in C-C and C-S bond distances.



In reality, though, these limiting structures do not represent a true description of the actual structure of the complex, they are merely two extremes of which the true structure lies somewhere between. Reflecting the impossiblility to provide an unequivocal description of the structure, McCleverty introduced the term 'dithiolene' to give a general name for the ligand that does not specify a particular oxidations state. This suggestion was generally accepted, and 'dithiolene' is now a universally accepted term.

From alkenedithiolates
Most dithiolene complexes are prepared by reaction of alkali metal salts of 1,2-alkenedithiolates with metal halides. A common alkenedithiolate is mnt2-, which forms a very stable Ni(II) complex:
 * Ni2+ +  2 (NC)2C2S22-  → Ni[S2C2(CN)2]22-

This red-colored complex is often used as a standard for electrochemical studies.

For more electron-rich alkenedithiolates, the dianion is generated in situ, treated with the metal salt, and the product is often air-oxidized:
 * cis-H2C2(SCH2Ph)2 +  4 Na  →  cis-H2C2(SNa)2  +  2 NaCH2Ph
 * NiCl2 +  2 cis-H2C2(SNa)2  →  Na2[Ni(S2C2H2)2]  +  2 NaCl
 * [Ni(S2C2H2)2]2- →  Ni(S2C2H2)2  +  2 e-

From acyloins
An early and still powerful method for the synthesis of dithiolenes entails the reaction of α-hydroxyketones, acyloins, with P4S10 followed by hydrolysis and treatment of the ill-defined mixture with metal salts. This method is used to prepare Ni[S2C2Ar2]2 (Ar = aryl).

From dithietes
Although 1,2-dithiones are extremely rare and thus not useful precursors, their valence isomer, the dithietes are occasionally used. Dithietes have even been characterized crystallographically. Probably the most important dithiete is (CF3)2C2S2, prepared from the reaction of elemental sulfur and hexafluoro-2-butyne. This electrophilic reagent oxidatively adds to many low valent metals to give bis- and tris(dithiolene) complexes.
 * Mo(CO)6 +  3 1,2-(CF3)2C2S2  → [(CF3)2C2S2]3Mo  +  6 CO
 * Ni(CO)4 +  2 1,2-(CF3)2C2S2  → [(CF3)2C2S2]2Ni  +  4 CO

By reactions of metal sulfides with alkynes
Species of the type Ni[S2C2Ar2]2 were first prepared by reactions of nickel sulfides with diphenylacetylene. More modern versions of this method entail the reaction of electrophilic acetylenes with well defined polysulfido complexes.

Structural Characteristics
Dithiolene complexes can be broadly divided into two types; complexes where the metal centre is coordinated by only dithiolene ligands, and complexes where the metal is coordinated by at least one dithiolene ligand and other ligands besides (often referred to as mixed ligand dithiolene complexes.

Depending on the coordination number of the metal, and other surrounding groups thereof, a metal can be coordinated by one, two, or three, dithiolenes, and are referred to as mono-, bis-, and tris- dithiolene complexes, respectively. Complexes where the metal is coordinated by only one dithiolene unit and the rest of the coordination sites are filled by other species are