Abstract

This study investigates, through quantum-chemical calculations, how replacing nitrogen with phosphorus in ansa-type frustrated Lewis pairs reshapes both the FLP-CLA equilibrium and H2 activation thermodynamics. Energy decomposition analysis shows that the stabilization of ansa-phosphinoborane adducts arises mainly from steric relief, which compensates for weaker donor-acceptor interactions. For H2 activation, the energetic effect of Lewis base substitution reaches up to 35.9 kcal mol-1 and correlates directly with the proton affinity differences between the corresponding amines and phosphines. This correlation identifies proton affinity as a predictive descriptor of reactivity. By establishing how N -> P substitution redefines the steric-electronic balance controlling adduct stability and H2 cleavage, this work provides conceptual design principles for tailoring frustrated Lewis pairs. These insights advance the molecular-level understanding of main-group systems and support the rational development of next-generation metal-free hydrogenation catalysts under sustainable conditions.