An adduct formed by electron-pair donation from a $\ce{\uppi\!\mbox{-}orbital}$ into a $\ce{\upsigma\!\mbox{-}orbital}$, or from a $\ce{\upsigma\!\mbox{-}orbital}$ into a $\ce{\uppi\!\mbox{-}orbital}$, or from a $\ce{\uppi\!\mbox{-}orbital}$ into a $\ce{\uppi\!\mbox{-}orbital}$. For example: Such an adduct has commonly been known as a '$\ce{\uppi\!\mbox{-}complex}$', but, as the bonding is not necessarily weak, it is better to avoid the term complex.