Pi interaction
In chemistry, π-effects or π-interactions are a type of non-covalent interaction that involves π systems. Just like in an electrostatic interaction where a region of negative charge interacts with a positive charge, the electron-rich π system can interact with a metal (cationic or neutral), an anion, another molecule and even another π system. Non-covalent interactions involving π systems are pivotal to biological events such as protein-ligand recognition.
The most common types of π-interactions involve:
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Metal π interactions play a major role in organometallics. Linear and cyclic π systems bond to metals allowing organic complexes to bond to metals.
Ethylene – π In the most simple linear π systems, bonding to metals takes place by two interactions. Electron density is donated directly to the metal like a sigma bond would be formed. Also, the metal can donate electron density back to the linear π system (ethylene) from the metal’s d orbital to the empty π* orbital of ethylene.
Allyl – π Allyl groups can bond to metals as trihapto or monohapto ligands. Monohapto ligands bind mostly sigma orbitals and trihapto ligands bind using delocalized π orbitals. In essence the monohapto ligand binds the metal as an allyl group and the trihapto ligand binds over all three carbons, where the lowest energy π orbital donates electron density and the highest energy π orbital accepts electron density. The allyl complex is diverse because it can alter the metal’s electron count by transferring between a monohapto (1 electron, η1) and trihapto ligand (3 electrons, η3). This fluctuation allows stability when a two-electron-donating group bonds or breaks from the metal.
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