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 the product of the reaction of 1 with benzil. The new compounds are characterized by NMR (1H, 13C, 29Si) spectroscopy and by mass spectrometry and microanalytical data. The X-ray crystal structure analysis of 2 is presented.

In the reaction of the nucleophilic decamethylsilicocene, ((Me5C5)2Si (1), with the electrophilic heterocumulenes CO2, COS, CS2, and RNCS, multistep processes are observed, which in most cases proceed via several highly reactive intermediates. In all reactions, the formal oxidation state of the silicon atom changes from +2 in 1 to +4, and the hapticity of the pentamethylcyclopentadienyl ligands changes from η5 to η1. In the reaction of 1 with CO2, COS, or RNCS, double-bond species of the type (Me5C5)2SiX (X = O, S) are formed, which are stabilized via different routes. Thus, reaction of 1 with CO2 in toluene as solvent finally leads to the spiro compound [(Me5C5)2SiO2]2C (4), whereas in pyridine as solvent, the eight-membered heterocycle [(Me5C5)2SiO2CO]2 (5) is formed. The dithiadisiletane [(Me5C5)2SiS]2 (9) results from the reaction of 1 with COS. Dithiasiletane derivates of the type (Me5C5)2SiS2CNR (10, 11) are the final products from the reaction of 1 with RNCS (R = CH3, C6H5). Multistep rearrangement processes have to be postulated to explain the formation of the dithiadisiletane derivate 15 in the surprising reaction of 1 with CS2; ligands that initially are bonded to the silicon (Me5C5) and to the carbon atom (S) have to be completely exchanged to build up the final product. The silaheterocycles 4, 5, 10, and 15 and also some of the basic molecular framework have been unknown in the literature. The crystal strucutres of 4, 5, 10, and 15 were determined by X-ray crystallography and are presented.

The first real silylene complex was obtained by reaction of decamethylsilicocene 1 with carbonylgold(I) chloride. To reduce the electron deficit at the silicon in 2, one of the two cyctopentadienyl ligands remains π‐bonded. The deep‐red silylene complex 2 resembles the Fischer carbene complexes. Addition of pyridine or tert‐butyl isocyanide leads to the donor‐stabilized silylene complexes of the type (Me5C5)2Si(donor)–AuCl, in which both cyclopentadienyl ligands are σ‐bound.

Completely different products are formed upon reaction of decamethylsilicocene 2 with the structurally related substrates CO2 and CS2. [2 + 1] Cycloaddition results in initial formation of reactive three‐membered ring compounds of the type , which then react further to give the novel silacycles 1 and 3, respectvely (RC5Me5)

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