Reaction of dilithiosilane R2SiLi2 and dilithiogermane R2GeLi2 (R = SiMetBu2) with MesBCl2 (Mes=2,4,6-trimethylphenyl):: Evidence for the formation of silaborene R2Si=BMes and germaborene R2Ge=BMes

1,1-Dilithiosilane and -germane, R2SiLi2 and R2GeLi2 (R = SiMe'Bu-2) reacted with MesBCl(2) to give the unexpected seven-membered ring products, [1,3,2]oxasila- and -gerrnaborepanes, through the intermediate formation of > Si=B- and > Ge=B- doubly bonded species.