Inorganic chemistry,negative hyperconjugation is the donation ofelectron density from a filled π- or p-orbital to a neighboringσ*-orbital.[1] This phenomenon, a type ofresonance, can stabilize the molecule ortransition state.[2] It also causes an elongation of theσ-bond by adding electron density to itsantibonding orbital.[1]
Negative hyperconjugation is rarely observed, though it can be most commonly observed when the σ*-orbital is located on certain C–F or C–O bonds.[3][4]
In negative hyperconjugation, the electron density flows in theopposite direction (from a π- or p-orbital to an empty σ*-orbital) than it does in the more commonhyperconjugation (from a σ-orbital to an empty p-orbital).