ethylenecoordinate

Ethylenecoordinate is a term used in coordination chemistry to describe the binding of an ethylene molecule (C2H4) to a metal center as a ligand. In typical cases the binding is described as η2-ethylene coordination, in which the two carbon atoms of the C=C bond share interaction with the metal. The ligand acts as a two-electron donor, with σ-donation from the ethylene π-bond to the metal and π-back donation from metal d-orbitals into the antibonding π* orbital of ethylene, which weakens the C=C bond and stabilizes the complex.

Mononuclear complexes such as M(η2-C2H4)(L)n (where M is a transition metal and L are ancillary ligands) are common, as are dinuclear or polynuclear species in which an ethylene bridge coordinates to more than one metal center (η2:η2-ethylene). Ethylenecoordinate motifs are typically labile, allowing rapid exchange with free ethylene gas, a property exploited in catalytic cycles including olefin polymerization and hydrofunctionalization.

Examples of metals that form well-studied ethylene complexes include nickel, palladium, rhodium, iron, and ruthenium carbonyl systems. Classic synthetic models include Ni(CO)3(η2-C2H4) and Cr(CO)5(η2-C2H4). The study of these complexes provides insight into olefin activation, back-donation, and the role of electron-rich metals in facilitating alkene coordination.

Terminology: while "ethylenecoordinate" is encountered in some texts, the more common way to describe such systems is simply as η2-ethylene coordination or an ethylene complex. The term is part of the broader concept of alkene coordination in organometallic chemistry.