Some metal ion complexing properties of 2,2'-biimidazole (BIM) are presented. The ligand BIM forms minimum steric strain complexes with hypothetical metal ions with M-N (metal-nitrogen) bond lengths of 4.2 angstrom, in contrast to more usual ligands such as bipy (2,2'-bipyridyl) that prefer metal ions with M-N bond lengths of 2.51 angstrom. This metal ion size-based preference of BIM suggests that ligands with such architecture could be used to produce selectivity (differences in log K-1) for very large metal ions. To test this hypothesis, the crystal structure of [Pb(BIM)(2)(CIO4)(2)](2) (1) was determined as the first example of a complex of BIM with a large metal ion. In addition, formation constants (log KO for BIM with metal ions ranging from the very small Cu(II) to the very large Ba(II) ion were determined to examine the effect of the architecture of BIM on metal ion selectivity. The structure of 1 gave: Triclinic, P (1) over bar, a = 8.314(2) angstrom, b = 8.677(2) angstrom, c = 14.181(3) (angstrom), alpha = 91.143(3)degrees, beta = 104.066(2)degrees, gamma = 106.044(3)degrees, V = 949.5(4) angstrom(3), Z = 1, R = 0.030. Pb(II) in 1 is eight-coordinate, with relatively short Pb-N bonds to the two BIM ligands ranging from 2.366(5) to 2.665(5) angstrom, while the four Pb-O bonds are very long at 2.826(5) to 3.123(5) angstrom. This is typical of the structure of Pb(II) complexes that have a stereochemically active lone pair of electrons, which is postulated to be situated in the vicinity of the long Pb-O bonds. The geometry of the chelate rings formed by BIM with Pb(II) in 1 is analyzed, and it is shown that these are closer in structure to the minimum-strain chelate ring formed by BIM with a very large metal ion than is the case for structures reported in the literature with smaller metal ions. The formation constants (log K-1) determined for BIM at 25 degrees C in 0.1 M NaClO4 by UV-visible spectroscopy are as follows: Cu(II), 6.35; Ni(II), 4.89; Zn(II), 3.42; Cd(II), 3.86; Ca(II), -0.2; Pb(II), 3.2; Ba(II), 0.2. The log K-1 values for BIM complexes show that, as expected from the geometry of the chelate ring formed by BIM, the complexes of BIM with small metal ions such as Cu(II) are considerably weaker than with ligands such as bipy, where the ligand architecture is more favorable for forming chelate rings with small metal ions. In contrast, for very large metal ions such as Pb(II) or Ba(II), the log K-1 values for BIM complexes are larger than for bipy. The use of ligand architecture in BIM-type ligands to engineer selectivity for very large metal ions is discussed. Some fluorescence results for BIM and its complexes are presented. BIM itself fluoresces very strongly, while all of its complexes except for Ca(II) show diminished fluorescence intensity, ranging from small shifts and decreases for Ba(II) to very large decreases for Cd(II), which may be due to the distortion of the ligand geometry in its complexes by metal ions that are too small for low-strain coordination with BIM.