Facile pyramidal inversion at phosphorus in [R3EP7W(CO)3]2− type complexes: An EXSY NMR study

A series of [R3EP7W(CO)3]2− complexes where (E = Si, Ge, Sn, Pb; R = alkyl, phenyl) were prepared from [P7W(CO)3]3− and R3EX reagents (X = Cl, Br) in dmf or CH3CN solutions. The Pb derivatives were prepared at −50 °C and are not thermally stable. The compounds were characterized by 31P NMR spectroscopy and selected ESI-MS studies. All compounds undergo rapid inversion at the ER3-bound phosphorus atom. The barriers to inversion were measured by way of 2D 31P EXSY experiments at various temperatures. The analysis showed very low barriers to pyramidal inversion (ΔG‡ 10.3–13.5 kcal/mol) that were essentially enthalpic in origin. The activation barriers generally increased with increasing electronegativity of the E atom and the steric bulk of the ER3 substituents. The latter was interpreted by way of a non-linear transition state.