Stereoinversion of tetrahedral p-block element hydrides.

The potential energy surfaces of 15 tetrahedral p-block element hydrides were screened on the multireference level. It was addressed whether stereoinversion competes against other reactions, such as reductive H 2 -elimination or hydride loss, and if so, along which pathway the stereomutation occurs. Importantly, stereoinversion transition structures for the ammonium cation (C 4v ) and the tetrahydridoborate anion (C s ) were identified for the first time. Revisiting methane's C s symmetric inversion transition structure with the mHEAT+ protocol revealed an activation enthalpy for stereoinversion, in contrast to all earlier studies, which is 5 kJ mol -1 below the C-H bond dissociation enthalpy. Square planar structures were identified lowest in energy only for the inversion of AlH 4 - , but a novel stepwise C s -inversion was discovered for SiH 4 or PH 4 + . Overall, the present contribution delineates essentials of the potential energy surfaces of p-block element hydrides, while structure-energy relations offer design principles for the synthetically emerging field of structurally constrained compounds.