The reactivity of 2,6-lutidine/BR₃ and pyridine/BR₃ Lewis pairs (R = F, Me, C₆F₅) is investigated in detail by quantum chemical calculations. The observed reactivity difference of these pairs is interpreted in terms of the existence of a "frustrated complex" on the potential energy curve for coordination of Lewis acid and base, the profiles of local reactivity descriptors with respect to the bond distance between acid and base centers, and the thermodynamic/kinetic properties of the heterolytic dihydrogen cleavage reaction. The calculated results are shown to account well for the observed reactivity of these Lewis pairs.