Tetrel-substituted pyramidanes as electron donors in noncovalent bonds

The apex C atom of square pyramidane C(C₄H₄) is replaced by heavier tetrel T atoms Si, Ge, Sn, and Pb. The molecular electrostatic potential surrounding the substituted T-pyramidane features minima at both the apex and the square base. Quantum chemical calculations are conducted to examine the ability of these two sites to act as electron donors in H-bonds, halogen bonds, or osme bonds to group 8 metals Fe, Ru, and Os. Osme bonds are strongest, with some approaching 40 kcal mol^-1, with a heavy covalent contribution, followed by halogen and then by hydrogen bonding. The latter are fairly weak, with interaction energies between 0.6 and 3.3 kcal mol^-1. The apex T site is favored over the square base for osme bonds, but the base is preferred for H-bonds. There is a more delicate balance for halogen bonds, with Si preferred over the pyramid base, but the latter favored for Sn. The primary electron donor site at the apex is a pseudo-lone pair on T, while the π-electron system serves this function when the electrophile attacks the base of the pyramid.