The ruthenium(II)-catalyzed tandem cycloaddition of 1,6-heptadiynes with bicyclic alkenes, such as bicyclo[3.2.1]heptenones and norbornene derivatives, furnishes the 1:2 adducts between the diynes and two molecules of the bicycloalkenes together with common [2 + 2 + 2] cyclocotrimerization products. The structure of a representative tandem 1:2 adduct between dimethyl dipropargylmalonate and 2,4-dimethylbicyclo[3.2.1]oct-6-en-3-one was unequivocally determined by X-ray analysis and was concluded to involve an unusual 1,2-dicyclopropylcyclopentene skeleton. On the basis of the spectroscopic analogy, the previously communicated structures of the tandem cycloadducts between the diynes and norbornene derivatives were corrected. The formation of the tandem double-cyclopropanation products from the diynes is chemical evidence of a bis-carbenoid hybrid structure, 1,3,5-metallacyclopentatriene, of the corresponding 2,4-metallacyclopentadiene intermediates. The selectivity for the formation of the tandem cyclopropanation adducts was increased in the order of (eta(5)-C9H7)Ru(PPh3)(2)Cl > CpRu(cod)CI > Cp*Ru(cod)Cl, indicative of the eta(5) - eta(3) ring slippage of the cyclopentadienyl type ligands playing a key role in the tandem cyclopropanation. On the other hand, the normal [2 + 2 + 2] cyclocotrimerization between 1,6-heptadiynes and alkenes was selectively catalyzed by Cp*Ru(cod)Cl, in the case of cyclic or linear alkenes possessing heteroatoms at the allylic position. The latter heteroatom-assisted cyclocotrimerization was also catalyzed by a paramagnetic dinuclear ruthenium(III) complex, [Cp*RuCl2](2), at lower temperature.