Ambiphilic Hydrogen in Trisubstituted Silanes: Substituent- Driven Polarity Flip Confirmed by NMR

Atomic partial charges are local, model-dependent descriptors that often fail to capture the global electrostatic environment governing noncovalent interactions and reactivity. Here we show that the molecular electrostatic potential (ESP) at the Si–H hydrogen in trisubstituted silanes is a decisive predictor of electrophilic versus nucleophilic behavior, whereas local charges alone are misleading. Using PBE0-D3/def2-TZVPP calculations, we evaluated atomic charges and ESP extrema in the gas phase and in two solvents, benzene and o-dichlorobenzene. Electron-donating groups (EDGs) generate hydridic hydrogens with a negative ESP near H (nucleophilic), while electron-withdrawing groups (EWGs) generally retain a negative local charge on H but induce a positive ESP region along the Si–H axis (electrophilic). These effects are solvent dependent: with increasing dielectric constant, V _s,max at H becomes more negative for EDG-substituted silanes and more positive for EWG-substituted silanes. The same solvent influence is mirrored in ¹H NMR chemical shifts, producing upfield shifts for EDG- and downfield shifts for EWG-substituted silanes. Targeted experimental NMR measurements validate these predictions. The positive ESP region near H in EWG-substituted silanes is σ-hole-like in directionality, but unlike classical σ-holes arising from lone-pair depletion (e.g., halogens, chalcogens), it reflects a collective molecular ESP effect.