Group 14 and 16 hybrid heavy bicyclo[1.1.0]butanes ((Bu2MeSi)-Bu-t)(4)Si3E (E = S, Se, and Te) 2a-c have been prepared by the [1+2] cycloaddition reaction of trisilirene 1 and the corresponding chalcogen. Bicyclo[1.1.0]butanes 2 have exceedingly short bridging Si-Si bonds (2.2616(19) angstrom for 2b and 2.2771(13) angstrom for 2c), a phenomenon explained by the important contribution of the trisilirene-chalcogen pi-complex character to the overall bonding of 2. Photolysis of 2a and 2b produced their valence isomers, the heavy cyclobutenes; 3a and 3b, featuring flat four-membered Si3E rings and a planar geometry of the Si=Si double bond. The mechanism of such isomerization was studied using deuterium-labeled 2a-d(6) to ascertain the preference of the pathway, involving the direct concerted symmetry-allowed transformation of bicyclo[1.1.0] butane 2 to cyclobutene 3.