Metal-containing macromolecules with extraordinary properties have attracted a great deal of interest owing to their potential application in the development of functional materials.

Treatment of the osmium hydrido alkenylcarbyne complex [OsH{CC(PPh 3 )CHPh}(PPh 3 ) 2 Cl 2 ]BF 4 (1) with allenoates (CH 2 CCHCOOR, R = Me, Et) in the presence of excess AgBF 4 leads to the insertion products 2/3. The reactions of complex 1 with substituted allenoates, i.e., diethyl 2-vinylidenesuccinate (CH 2 CC(CH 2 COOEt)-(COOEt)) and ethyl penta-2,3-dienoate (CH(CH 3 )C CH(COOEt)), result in the formation of [3 + 2] annulation products 4 and 6, respectively. Mechanisms of the reactions have been proposed with the isolation of the key intermediates. The results reveal that the divergent pathways for these reactions are mainly of steric origin.