A [2.2.1] aluminium metallobicycle is capable of reversibly inserting CO to form a [2.2.2] metallobicycle at 100 °C. Computational studies reveal a highly asynchronous, but concerted, transition state for CO insertion. The coordination of CO to aluminium precedes C–C bond formation. The reversible migratory insertion of CO at aluminium thus mimics well-established transition-metal reactivity.