Olefin complexes
Zeise's salt, K[PtCl3(C2H4)], is the oldest known organometallic compound and was synthesized and analyzed in ca. 1825 by Zeise, although its coordination structure was assumed only in 1954 and confirmed by the neutron diffraction in 1975. The mode of coordination of an olefin to a transition metal is described by the Dewar-Chatt-Duncanson model and the bond between the metal and olefin is stabilized by the contribution of dπ-pπ* back donation. An olefin is a 2-electron ligand and there are many olefin complexes in which the central metal is in a relatively low oxidation state. Dienes or trienes with two or more double bonds coordinate to a metal as 4-electron or 6-electron ligands. Fe(CO)3(C4H6) and Ni(cod)2, in which a butadiene or cyclooctadienes (cod) are coordinated to the metal, are well known examples. Since cyclooctadienes are easily eliminated from Ni(cod)2, it is conveniently used for generating atomic, zero valent nickel. This complex is sometimes called naked nickel.