DFT studies of the sigma-donor/pi-acceptor properties of [SnCB10H11](-) and its relationship to [SnCl3](-), CO, PF3, [SnB11H11](2-), SnC2B9H11, and related SnC2BnHn+2 compounds

The 1,2-carbastanna-closo-dodecaborate, [SnCB10H11](-), was found via DFT calculations to have intermediate sigma-donor/pi-acceptor properties between those of [SnB11H11](2-) and 3,1,2-SnC2B9H11, and quite similar HOMO and LUMO energies and shapes to those of [SnCl3](-), which is a stronger sigma-donor and weaker pi-acceptor than CO or PF3. The non-carbon containing cluster [SnB11H11](2-) is shown to be a very strong donor, whereas the dicarbon cluster 3,1,2-SnC2B9H11 is a poor sigma-donor but good Lewis acid, consistent with experimental results. Thus, these systems can be tuned across a very wide range via isolobal replacement of [BH](-) for CH vertices. The higher negative charge in the non-carbon-containing systems, as well as the fact that boron is more electropositive than carbon, contribute to the increased energies of both HOMO and LUMO in the stannaboranes compared to the stannacarboranes, explaining their relative donor and acceptor properties.