Chemistry of C-trimethysilyl-substituted heterocarboranes. 18. Synthetic, spectroscopic, reactivity, and bonding studies on the group 13 element metallacarboranes: Crystal structures of 1-(CMe3)-1-Ga(2,2'-C10H8N2)- 2,3-(SiMe3)2-2,3-C2B4H4, 1-(CMe3)-1-Ga(L)- 2,4-(Si-Me3)2-2,4-C2B4H4 [L=2,2'-C10H8N2, 2,2'-C8H6N4], closo-1-(Me2CH)-1-In-2,4-(SiMe3)2-2,4-C2B4H$ -4$/, and 1-(Me2CH)-1-In(2,2'-C10H8N2)- 2,4-(SiMe3)2-2,4-C2B4H4