Complexes which natively coordinate K + , the largest ion with an ionic radius of 1.33 Å, exhibit a smaller energetic cost to exchange with the medium-sized Na + ion with an ionic radius of 0.97 Å compared to the smallest Li + ion with an ionic radius of 0.68 Å. For the set of native K +coordinating complexes, it costs on average 1.6 kcal/mol more energy to exchange from K + to Li + than to Na + (i.e., the slope from a linear regression of the binding energies in Figure ). Similarly, complexes which natively coordinate Li + prefer to change to Na + over K + , where it costs 2.5