Chemistry of C-Trimethylsilyl-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-(SiMe3)2-2,4-C2B4H4 [L = 2,2'-C10H8N2, 2,2'-C8H6N4], closo-1-(Me2CH)-1-In-2,4-(SiMe3)2-2,4-C2B4H4, and 1-(Me2CH)-1-In(2,2'-C10H8N2)-2,4-(SiMe3)2-2,4-C2B4H4