ELECTRON-ACCEPTOR BEHAVIOR OF 2,4,7,9-TETRACARBA-NIDO-DODECABORANE(12) WITH GROUP-1 AND GROUP-2 METALS - SYNTHESES AND CRYSTAL-STRUCTURES OF [(THF)(4)LI][(SIME(3))(4)C4B8H9] AND [(THF)(2)MG(SIME(3))(4)C4B8H8]

The room-temperature reaction between a, THF solution of 2,4,7,9-tetra carba-nido-dodecaborane-(12) (SiMe(3))(4)C4B8H8 and finely cut excess lithium or magnesium metal, in the absence of naphthalene and aromatic solvents, produced either {[(THF)(4)Li][SiMe(3))(4)C4B8H9]} (1) as an EPR-silent transparent crystalline solid in 41% yield or [(THF)(2)Mg( SiMe(3))(4)C4B8H8] (2) as an off-white crystalline solid in 88% yield. While the solid-state structure of I showed a single anionic [(SiMe(3 ))(4)C4B8H9](-) unit that was well-separated from a discrete [Li(THF)( 4)](+) cation, the structure of 2 is that of a novel magnesacarborane in which the carborane cage incorporates both electron-precise and ele ctron-deficient carbon and boron atoms.