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 C 4 B 8 H 9 ] and [(THF) 2 Mg(SiMe 3 ) 4 C 4 B 8 H 8 ] †