Silicon(IV) .pi.-complexes of C-(trimethylsilyl)-substituted carboranes. Syntheses of [(Me3Si) (R)C2B4H4]2Si and [(Me3Si)(Me)C2B4H4]SiH(R') [R = Me3Si, Me, or H; R' = H, or Cl]. Crystal structure of 2,2'-bis(trimethylsilyl)-3,3'-dimethyl-[1,1'-commo-bis(2,3-dicarba-1-sila-closo-heptaborane)]-(12)