Matrix-isolated 1-silacyclopropenylidene (7) has been generated by pulsed flash pyrolysis of 2-ethynyl-1,1,1 -trimethyldisilane (4). By subsequent photolysis 7 can be isomerized into ethynylsilanediyl (5), vinylidenesilanediyl (8), and silacyclopropyne (9). The identification of the C2H2Si isomers and their C-13 and D isotopomers is based on the comparison of their experimental and calculated (MP2/6-31G**) infrared spectra. In case of 9, the first example of a "formal" cyclopropyne, the definitive assignment of the structure was only possible by studying C-13 isotopomers. Detailed ab initio calculations have been carried out in order to understand the unique bonding situation in 9. It is found that the Lewis structure 9B, where the dashed line indicates an electron pair which occupies a CC nonbonding orbital, is the best possible compromise to describe the electronic structure of 9 correctly. The value of 6.2 kcal mol(-1) for the barrier of 9 to the planar transition structure 13 represents the lowest inversion barrier of tetra-coordinated silicon described so far.