The potential energy surface of Si2B2H4, explored ab initio at the MP2/6-31G* and density functional theory, Becke3LYP/6-311+G** levels, is rich in detail. Twelve minima were found out of the 39 stationary points located. 1,3-Disiladiboretene (1a), with a puckered four-membered ring, is predicted to be the global minimum. Its calculated inversion barrier (1.2 kcal/mol), however, is much lower than that of the analogous 13-diboretene (17.0 kcal/mol). The relative energies of two other Si2B2H4 isomers (5 and 7) with B-H-Si bridges are only 2.0 kcal/mol above 1a. The relative energies of 9 and 15a, both with planar pentacoordinate borons, are 7.3 and 27.1 kcal/mol. The acyclic (18) and linear (22a,b) isomers are noncompetitive energetically. Many other Si2B2H4 silaboranes, possessing similar structures and bonding as the corresponding carboranes, are calculated to be saddle points. In general, hydrogen bridging and cyclic pi-delocalization stabilize Si2B2H4 isomers, while structures with multiply bonded silicon are unfavorable.