CHEMISTRY OF C-TRIMETHYLSILYL-SUBSTITUTED HETEROCARBORANES .22. SYNTHETIC, SPECTROSCOPIC, STRUCTURAL, AND BONDING STUDIES ON HALF-SANDWICH AND FULL-SANDWICH GALLACARBORANES OF 2,3-C2B4 AND 2,4-C2B4 CARBORANE LIGAND SYSTEMS

The reaction of the TMEDA-solvated group 1 salts of the carborane dian ions [2,n-(SiMe3)(2)-2,n-C2B4H4](2-) (n = 3, 4) with GaCl3 in a 2:1 ca rborane-to-CaCl3 molar ratio gave the corresponding full-sandwich gall acarboranes n'-(SiMe3)(4)-1,1'-commo-Ga(1,2,n-GaC2B4H4)(2)](-) (n = 3 (I), 4 (II) in 36% and 38% yields, respectively. Lowering the carboran e-to-GaCl3 molar ratio to 1:1 produced the half-sandwich chlorogallaca rboranes 1-(TMEDA)-l-Cl-2,n-(SiMe3)(2)-1,2,n-GaC2B4H4 (n = 3 (III), 4 (IV)) in 51% and 41% yields, respectively. Compounds I-IV were charact erized on the basis of H-1, B-11, and C-13 NMR spectra, IR spectra, an d single-crystal X-ray analyses. The structures of the gallacarboranes show that the gallium atoms are not equally bonded to the atoms in th e C2B3 faces of the carboranes but are slipped toward the more boron r ich sides of the faces. Slippages in I and II are such that maximum di fferences in the Ga-C2B3 distances were 0.34 Angstrom in I and 0.45 An gstrom in II. The distortions were much larger in III and TV such that the carboranes are better described as being eta(3)- and eta(2)-bonde d, respectively.