The results are summarized in Figure , with detailed structural data in Table of the Supporting Information. The strength of the B-N bond in the X3B-LB Lewis pairs diminishes as the Lewis base varies from NH3 to MeCN to N2 (Figure ). Specifically, the strong Lewis base NH3 forms only strongly bonded adducts with boron trihalides that go with B-N bond lengths of 1.6-1.7 Å and bonding energies (E) ranging from -18.4 to -26.2 kcal mol -1 . Upon forming strongly bonded adducts, the Lewis acid (BX3) undergoes a higher degree of pyramidalization, deviating from its trigonal planar equilibrium geometry. This structural adjustment involves decreasing the X-B-X bond angle and increasing the rB-X bond length (Table ). We have previously shown that the rigidity of the Lewis acids is the causal factor underlying the stability of the X3B-NH3 adducts. In other words, strongly bonded adducts, which characteristically have higher distortivity, become more stable as X goes from F to Cl to Br to I because the Lewis acid BX3 can more easily deform along this series as the corresponding B-X bond becomes weaker.