Some Organometallic Chemistry of Tetracyanoethene: CN-displacement and Cycloaddition Reactions with Alkynyl-Transition Metal Complexes and Related Chemistry

The highly electron-deficient cyanocarbons tetracyanoethene (tcne) and, to a lesser extent, tetracyanoquinodimethane (tcnq), display a fascinating chemistry with transition metal substrates. In particular, the [2 + 2]-cycloadditions of the cyanocarbons with alkynyl- or poly-ynyl-metal complexes have been extensively studied by the author's group. These reactions proceed via polar adducts to give sigma-cyclobutenyl complexes, which then undergo facile ring-opening (retroelectrocyclic) reactions to form the corresponding butadienyl derivatives. In some cases, further reactions can occur by displacement of weakly bound ligands from the metal centre. The subsequent chemistry of these derivatives has been only cursorily investigated, while related studies of organic analogues have produced molecules with interesting electronic and optical properties.