Chemistry of C-trimethylsilyl-substituted main-group heterocarboranes. 4. Reactivity of stannacarboranes toward monodentate and bis(bidentate) Lewis bases. Syntheses and structures of 1- or 1,1'-(L)-[closo-1-Sn-2-(SiMe3)-3-(R)-2,3-C2B4H4]n {n = 1, L = [(.eta.5-C5H5)Fe(.eta.5-C5H4CH2(CH3)2N)]; n = 2, L = 2,2'-C8H6N4; R = SiMe3, Me, or H}