1,2,3-Trisila- and 1,3-disila-2-germabicyclo[1.1.0] butane derivatives 7a, b were synthesized in high yields by the reduction of the appropriate precursors, 1,3-dichloro[1,2,3] trisiletane and 1,3-dichloro [ 1,3,2] disilagermetane, with KC8. The X-ray analysis of 7a revealed an exceedingly short Si-Si bridging bond of 2.2664(9) angstrom, which is much shorter than the normal Si-Si single bond of 2.34 angstrom. The nature of such an unusual bridging bond is discussed in terms of both electronic and steric effects of the methylene bridge. The thermolysis of bicyclo[1.1.0]butanes 7a, b unexpectedly resulted in the formation of their valence isomers, alkyl-substituted 1H-trisilirene 8a and 1H-disilagermirene 8b, representing a new type of E-4 valence isomer (E = group 14 element). The germanium versions, 1H-siladigermirene 8c and 1H-trigermirene 8d, were also prepared.