Solvent stabilization of the edge inversion transition state in tetrahedral molecules

The edge inversion process in phosphoric acid, PO(OH)3 has been studied by large scale ab initio molecular orbital theory. Edge inversion of the tetrahedral ground state via a square-planar transition state requires 69.5 kcal/mol (MP-2). Addition of two NH3 solvent molecules to the vacant NLUMO stabilizes the transition state by 45 kcal/mol (MP-2). The value for ΔH(300K) for the reaction 2NH3 + PO(OH)3 PO(OH)3 · (NH3)2 is 24.1 kcal/mol (MP-2). The complex with two NH3 molecules is an intermediate. Addition of one NH3 to PO(OH)3 leads to an energy lowering of the planar form of 31 kcal/mol. This structure is now a transition state. The value for ΔH(300K) for the reaction NH3 + PO(OH)3 PO(OH)3 · NH3 is 38.6 kcal/mol (MP-2). The complex of PF3O with two NH3 molecules was studied and is an intermediate. The value for ΔH(300K) for the reaction 2NH3 + POF3 POF3 · (NH3)2 is only 3.3 kcal/mol (MP-2). Electron-donor solvents clearly will stabilize the edge inversion transition state for tetracoordinate main group compounds.