Reactions of decamethylsilicocene, (Me5C5)2Si (1), with representative organic carbonyl compounds are described. They proceed via [2 + 1] cycloaddition products of the oxasilirane type as reactive intermediates to give different types of compounds. The formal oxidation state of silicon is changed from +II in the substrate to +IV in the final products; at the same time the hapticity of the pentamethylcyclopentadienyl ligands changes from η5 to η1. In the reaction of 1 with aldehydes such as benzaldehyde and trans-cinnamaldehyde as well as with acetone, formation of the respective dioxasilolane derivatives 2, 3, and 5 takes place. Here, C−C bond formation is observed for the first time in the reaction of a divalent silicon compound; reaction with the aldehydes is stereospecific. In the reaction of 1 with acetophenone or benzophenone, the bicyclic ring systems 6 and 7, respectively, containing an oxasilacyclopentene unit are formed by rearrangement of the transient oxasiliranes. The 1,3,2-dioxasilole 9 is th...