A novel and air-stable organo(hydro)diborane featuring a five-membered aryl ring supported bridging B–C–B three-centre–two-electron (3c–2e) bond has been reported. Pyrido[1,2-a]isoindole was found to undergo a stepwise BH3 addition reaction, during which a mono-BH3 adduct was formed from a electrophilic addition at the Cγ in pyrido[1,2-a]isoindole. A molecule of hydrogen was eliminated throughout the second step of addition reaction. DFT calculations indicate that the H2 evolution is concerted to the second BH3 addition rather than forming B=C before the second BH3 attack. [ABSTRACT FROM AUTHOR]