beta-Lactams were synthesized by the carbonylative ring expansion of aziridines catalyzed by dicobalt octacarbonyl under CO pressure. The active catalyst, cobalt tetracarbonyl anion, induces nucleophilic ring opening of the heterocycle, resulting in inversion of configuration. The regio- and stereospecificity of this reaction resulted in the synthesis of the first highly strained trans-7-azabicyclo[4-2-0]octan-8-one derivatives.