The emission spectrum and luminescent lifetime of (bpy)(2)Ru-II(binicotinic acid) is affected by the presence of heavy metal ions in solution. As little as 1 muM Pb2+ causes a red shift in emission, an increase in the emission quantum yield, and an increase in the room-temperature lifetime. A smaller red shift is observed in the 4,4'-dicarboxy analogue in the presence of large quantities of lead; however, the emission lifetime and intensity are dimished. An X-ray determination of the ground-state geometry shows that the bipyridine rings of the binicotinic acid are twisted along the 2,2' bond by 19.3 degrees. The interaction between lead and the binicotinic acid complex was modeled by molecular mechanics and extended Huckel calculations. The calculations show that interaction with lead flattens the bridged ring system of the binicotinic acid ligand, which affects the pi* energy levels of the ligand, the d-orbital energies of the Ru(II), and the vibrational modes available to the substituted bipyridine ligand. The inverse energy gap law behavior observed in the binicotinic acid complex is explained in terms of an allosteric interaction between lead and the binicotinic acid complex.