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: Metal–π interactions: involves interaction of a metal and the face of a π system, the metal can be a cation (known as cation–π interactions) or neutral Polar–π interactions: involves interaction of a polar molecule and quadrupole moment a π system. Aromatic–aromatic interactions (π stacking): involves interactions of aromatic molecules with each other. Arene–perfluoroarene interaction: electron-rich benzene ring interacts with electron-poor hexafluorobenzene. π donor–acceptor interactions: interaction between low energy empty orbital (acceptor) and a high-energy filled orbital (donor). Anion–π interactions: interaction of anion with π system Cation–π interactions: interaction of a cation with a π system C–H–π interactions: interaction of C-H with π system: These interactions are well studied using experimental as well as computational techniques. 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.