The cyclic diazoketone diazodimedon ( 2) reacts with the dinuclear rhodium complex [(η 5-C 5Me 5)Rh(μ-CO)] 2 ( 1) in boiling tetrahydrofuran with elimination of dinitrogen to yield the unusual cycloadduct 3 which arises from addition of the ketocarbene moiety derived from the diazo precursor 2 to the bridging metal carbonyl frame of the metalmetal doubly bonded organometallic metallic species 1 (X-ray diffraction). One interesting structural aspect is the formation of a ketene unit resulting from carboncarbon coupling.