Electron-deficient bridge-bonded main structures

The purpose of this review is to summarize the recent information available on the structures of derivatives of the main group elements that contain electron-deficient bridge bonds and to focus attention on the relationships between the structures and the bonding in these molecules. 

J. P. Oliver, Structures of main group organometallic compounds containing electron-deficient bridge bonds. Adv. Organomet. Chem. 15, 235-271 (1977). 

Structures of Main Group Organometallic Compounds Containing Electron-Deficient Bridge Bonds, 15, 235... 

According to theoretical studies, the HOMO of benzocyclopropene is mainly located at the bridging rr-bond and it therefore would be predicted that it would participate in cycloadditions with electron-deficient dienes. Stable adducts 1 with the norcaradiene structure were obtained upon heating benzocyclopropene with 4,5-dibromo-l,2-benzoquinone, 4,5-dichloro-l,2-ben-zoquinone and tetrachlorobenzoquinone. ... 

---