1 UNR 3-(2-hydroxy-2,2-diphosphonoethyl)-1-methylpyridinium 1.000 2 RIS 1-hydroxy-2-(3-pyridinyl)ethylidene bis-phosphonic acid 0.944 3 M0N (1-hydroxy-2-imidazo[1,2-a]pyridin-3-ylethane-1,1-diyl)bis(phosphonic acid) 0.727