Treatment of the labile μ-methylene species, Cp 2 Ru 2 (μ-CH 2 )(CO) 2 (MeCN) 1, with diazoalkanes results in C-C coupling of the alkylidene units. The reaction of monosubstituted diazoalkanes (N 2 CHR) and 4-t-butyldiazocyclohexane gives the μ-alkenyl-μ-hydride complexes, Cp 2 Ru 2 (μ-CH CR 2 )(μ-H)(CO) 2 2, as sole products, whereas the reaction of disubstituted aryldiazoalkanes (N 2 CR 1 R 2 ) produces olefins (CH 2 CR 1 R 2 ) together with mixtures of unidentified organometallic compounds. Labeling experiments using 1-d 2 (μ-CD 2 ) and 1- 1 3 C (μ- 1 3 CH 2 ) reveal that the methylene bridge in 1 is converted to the α-CH and μ-H parts in 2 and the alkylidene part of diazoalkane is incorporated into the β-CR 2 part of the alkenyl bridge. The olefinic products should come from a η 2 -olefin intermediate, Cp 2 Ru 2 (η 2 CH 2 CHR)(CO) 2 7, resulting from coupling of the two alkylidene fragments within the coordination sphere of the diruthenium core. It is notable that C-H oxidative addition in 7 takes place prior to free rotation of the η 2 -olefin ligand as judged by the regiospecificity of the labeling experiments. Thus specific coupling of alkylidene units proceeds under mild conditions and this C-C coupling reaction serves as a model system for the carbon chain propagation step of surface-catalyzed reactions such as catalytic CO hydrogenation.