The unique structures and reactivities of metallabicyclo[1.1.0]-butanes containing heavy main group elements have been the focus of experimental and theoretical studies. 1-3 As revealed by the theoretical calculations, one of the most interesting features of metallabicyclo[1.1.0]butanes 3 as well as bicyclo[1.1.0]butanes 4 should be the existence of the bond-stretch isomers, which differ primarily in the distance between the bridge head atoms. As shown in Chart 1, the short-bond (SB) isomers are characterized by not only the short-bridge bond distance (r), but also the small interflap angle (φ) and the large R-M-M angle (θ), while the long-bond (LB) isomers have long r, large φ, and small θ. The bridge bond in an SB isomer is a normal bent σ bond, while the bond in an LB isomer should be ascribed to an inverted σ bond with significant singlet biradical character.Although theoretical calculations have predicted that the LB isomers are more stable than the SB isomers in 1,3-disilabicyclo-[1.1.0]butane as well as tetrasilabicyclo[1.1.0]butane, 3 no experimental evidence for the LB isomers with the bridge Si-Si bond has been reported so far. We wish herein to report the first synthesis of a 1,3-disilabicyclo[1.1.0]butane derivative, which has characteristics of the LB isomer.Disilabicyclo[1.1.0]butane 1 was obtained by using a formal double sila-Peterson reaction 5 of tetrakis(tert-butyldimethylsilyl)-dilithiodisilane 2 6 (eq 1). Adamantanone (64 mg, 0.43 mmol) and 2‚6THF (200 mg, 0.20 mmol), which was prepared by the reaction of the corresponding tetrasilyldisilene 7 with lithium metal, was placed in a Schlenk tube under vacuum, and dry benzene (10 mL) was introduced by distillation to the mixture. The mixture was stirred overnight at room temperature to give a bright-yellow solution; the complete conversion was confirmed by 1 H NMR. After quenching the mixture by excess Me 3 SiCl, the solvent and the resulting t-BuMe 2 SiOSiMe 3 and lithium chloride were removed. Pure 1 was obtained by crystallization from hexane in 70.1% yield. 8 The structure of 1 was determined by 1 H, 13 C, and 29 Si NMR, MS, and X-ray crystallography. 10,11 Compound 1 is oxygen-and moisture-sensitive, but thermally very stable; 1 melts at 175.2-177.3°C without decomposition.X-ray single-crystal analysis of 1 has shown that 1 has the crystallographical mirror plane which includes eight carbon atoms of adamantane moieties and bisects the bridge Si-Si bond ( Figure 1). Two three-membered Si 2 C rings are isosceles. The following four important characteristics were found in the structure of 1: (i) The distance of the bridge Si1-Si1* bond (r) is 2.412(1) Å, (1) For reviews on small ring compounds including group 14 elements, see: (a) Weidenbruch, M. Comments Inorg. A number of metallabicyclo[1.1.0]butanes have been reported. Tetrasilabicyclo[1.1.0]butanes: (a) Masamune, S.; Kabe, Y.; Collins, S.; Williams, D. -disilabicyclo[1.1.0]-butanes: (l) Driess, M.; Fanta, A. D.; Powell, D. R.; West, R. Angew. Chem., Int. Ed. Engl. 1989, 28, 1038. (m) Driess...