Theoretical Group 14 Chemistry. 4. Cyclotriplumbanes: Relativistic and Substituents Effects.

We report a study on the first newly synthesized homonuclear lead ring system, the cyclotriplumbane Pb3R6. Its geometrical features can be best reproduced using perturbation theory (MP2) together with the Stuttgart-Dresden basis set and ECP for lead. The experimentally observed tilting of the groups R in the cyclotriplumbanes is attributed to the bonding situation:  the lead-lead contacts, formed from weak interactions of plumbylene lone pairs with empty p orbitals of neighboring lead atoms, try to maximize overlap. Surprisingly and in contrast to the inert pair effect, hybridization of the former plumbylene lone pair orbitals in the cyclotriplumbane Pb3R6 is observed, depending on the substituent. Hybrid orbitals with a 6s orbital contribution of only about 40% are found. Hydrogen atoms and methyl groups promote this effect, while for phenyl substitution the expected 6s lone pair orbital is identified as the bond-forming orbital.