In chemistry, p-effects or p-interactions are a type of non-covalent interaction that involves p systems. Just like in an electrostatic interaction where a region of negative charge interacts with a positive charge, the electron-rich p system can interact with a metal (cationic or neutral), an anion, another molecule and even another p system. Non-covalent interactions involving p systems are pivotal to biological events such as protein-ligand recognition.