Pi complex
Pi complex (π complex) is a term from complex chemistry . This refers to coordination compounds that often only occur as unstable intermediate stages. They are based on interactions between electron pairs in a double bond and free coordination sites, often those of a transition metal .
Conceptual problem
The definition often refers to the interaction of an occupied π orbital of the ligand with a free coordination site (usually an unoccupied σ * orbital) of the central atom. However, also occur in normal electron pair - donors often called (σ donors) backbonds therefore the interaction to, a π-electron pair of the central atom with an unoccupied σ * orbital of the ligand, but which are not referred to as pi-complex. For this reason, the IUPAC advises against the use of the term and prefers the term pi adduct for a directional interaction between a π orbital and a π or σ orbital, regardless of whether it is a central atom or a ligand.
meaning
Π complexes are most frequently found in organic chemistry , where they are of great importance in explaining reaction mechanisms . In this context, they are often mentioned for electrophilic addition and substitution reactions . They are known, for example, in reactions between halogens such as bromine or iodine with a double bond or an aromatic system ( benzene ) or as important intermediate stages in allylic substitution and olefin metathesis . Π-complexes also play a role in the Sonogashira coupling and similar transition metal-catalyzed coupling reactions. In the copper cycle, in which the copper acetylide is generated in situ , a π complex is formed between the π electrons of the triple bond of the terminal alkyne and the copper atom of the copper halide required as a co-catalyst.
See also
literature
- Reinhard Brückner: reaction mechanisms . 3rd edition, Spektrum Akademischer Verlag, Munich 2004 , ISBN 3-8274-1579-9 .