Chemistry of C-trimethylsilyl-substituted heterocarboranes. 11. Synthetic, spectroscopic, structural, and bonding studies on the sodium, sodium lithium, and dilithium complexed "carbons adjacent" nido-carborane anions [2-(SiMe3)-3-(R)-2,3-C2B4H6-n]n- (R = SiMe3, Me, H; n = 1, 2). High-yield conversion to 1,2-dicarba-closo-hexaborane(6) derivatives: precursors to carbons apart dianionic carborane ligands