The selectivity of the rigid ligand PDA (1,10-phenanthroline-2,9-dicarboxylic acid) for some M(III) (M=metal) ions is presented. The structure of [Fe(PDA(H)(1/2))(H2O)(3)] (ClO4)(2)center dot 3H(2)O center dot 1/2H(5)O(2) (1) is reported: triclinic, P (1) over bar, a=7.9022(16) angstrom, b=12.389(3) angstrom, c=13.031(3), angstrom, alpha=82.55(3)degrees, beta=88.41(3)degrees, gamma=78.27(3)degrees, V=1238.6(4) angstrom(3), Z=2, R=0.0489. The coordination geometry around the Fe(III) is close to a regular pentagonal bipyramid, with Fe-N lengths averaging 2.20 angstrom, which is normal for a 1,10-phenanthroline type of ligand coordinated to seven-coordinate Fe(III). The Fe-O bonds to the carboxylate oxygens average 2.157 angstrom, which is rather long compared to the average Fe-O length of 2.035 angstrom to carboxylates in seven-coordinate Fe(III) complexes. The structure of 1 supports the idea that the Fe(III) is too small for ideal coordination in the cleft of PDA, and the structure shows that the Fe(III) adapts to this by inducing numerous small distortions in the structure of the PDA ligand. The log K-1 values for PDA at 25 degrees C in 0.1 M NaClO4 were determined by UV spectroscopy with Al(III) (log K-1=6.9), Ga(III) (log K-1=9.7), In(III) (log K-1 = 19.7), Fe(III) (log K-1 = 20.0), and Bi(III) (log K-1=26.2). The low values of log K-1 for PDA with Al(III) and Ga(III) are because these ions are too small for the cleft in PDA, which requires a large metal ion with an ionic radius (r(+)) of 1.0 angstrom. In(III) and Fe(III) (r(+)=0.86 and 0.72 angstrom for a coordination number (CN) of 7) are somewhat too small for the cleft in PDA but may adapt by increasing the coordination number, which increases the metal ion size, and have high log K-1 values. Very large log K-1 values are found, as expected, for Bi(III) (r(+)=1.17 angstrom, CN=8), which fits the cleft quite well. Fluorescence studies show that Y(III) produces the largest CHEF (chelation enhanced fluorescence) effects, followed by La(III) and Lu(III), in the PDA complexes. Metal ions with nonfilled d or f subshells produce very large quenching of the fluorescence, as do heavy-metal ions such as In(III) and Bi(III), which have large spin-orbit coupling effects. The Al(III)/PDA complex produced an intense broad band at longer wavelength than the pi*-pi emissions of the PDA ligand, which is at a maximum at pH 6, and the possibility that this might reflect an exciplex, where one PDA ligand in the Al(III) complex pi-stacks with the excited state of a second PDA ligand, is discussed.