Dithiolene

Metal dithiolene complexes are complexes containing dithiolene ligands. Dithiolene ligands are unsaturated bidentate ligand wherein the two donor atoms are sulfur. Dithiolenes are often referred to as "metallodithiolenes" or "dithiolene complexes". Most molybdenum- and tungsten-containing proteins have dithiolene-like moieties at their active sites, which feature the so-called molybdopterin cofactor bound to the Mo or W.

Metal dithiolenes have been studied since the 1960s when they were first popularized by Gerhard N. Schrauzer, who prepared Ni(S₂C₂Ph₂)₂ by the reaction of nickel sulfide and diphenylacetylene. The structural, spectroscopic, and electrochemical properties of many related complexes have been described.

Dithiolene complexes can be found where the metal centre is coordinated by one, two, or three dithiolene ligands. The tris(dithiolene) complexes were the first examples of trigonal prismatic geometry in coordination chemistry. One example is Mo(S₂C₂Ph₂)₃. Similar structures have been observed for several other metals.

Because of the unusual redox and intense optical properties of dithiolenes, the electronic structure of dithiolene complexes has been the subject of intense study. Dithiolene ligands can exist in three oxidation states: the dianionic "ene-1,2-dithiolate", the neutral "1,2-dithioketone," and a monoanionic radical intermediate between these two. When the latter two are complexed to a metal centre, the oxidation state of the ligand (and therefore the metal centre) cannot be easily defined. Such ligands are therefore referred to as non-innocent. The 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. Electron acceptors (e.g. CN, CO₂Me) stabilize reduced and anionic complexes. Derivatives are known where the substituents are the same, symmetrical dithiolenes (R = R') are more common than unsymmetrical.

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