Some metal-ion-complexing properties of the ligand 2-(pyrid-2'-yl)-1,10-phenanthroline (MPP) are reported. MPP is of interest in that it is a more preorganized version of 2,2';6,2 ''-terpyridine (tpy). Protonation constants (pK(1) = 4.60; pK(2) = 3.35) for MPP were determined by monitoring the intense pi-pi* transitions of 2 X 10(-5) M solutions of the ligand as a function of the pH at an ionic strength of 0 and 25 degrees C. Formation constants (log K-1) at an ionic strength of 0 and 25 degrees C were obtained by monitoring the pi-pi* transitions of MPP titrated with solutions of the metal ion, or 1:1 solutions of MP P and the metal ion were titrated with acid. Large metal ions such as Ca-II or La-III Law showed increases of log K-1 of about 1.5 log units compared to that of tpy. Small metal ions such as Zn-II and showed little increase in log K-1 for MPP compared to the tpy complexes, which is attributed to the presence of five-membered chelate rings in the MPP complexes, which favor large metal ions. The structure of [Cd(MPP)(H2O)(NO3)(2)] (1) is reported: monoclinic, P2(1)/c, a = 7.4940(13) angstrom, b = 12.165(2) angstrom, c = 20.557(4) angstrom, beta = 96.271(7)degrees, V = 1864.67(9) angstrom(3), Z = 4, and final R = 0.0786. The Cd in 1 is seven-coordinate, comprising the three donor atoms of MPP, a coordinated water, a monodentate, and a bidentate NO3-. Cd-II is a fairly large metal ion, with r(+) = 0.96 angstrom, slightly too small for coordination with MPP. The effect of this size matching in terms of the structure is discussed. Fluorescence spectra of 2 X 10(-7) M MPP in aqueous solution are reported. The nonprotonated MPP ligand fluoresces only weakly, which is attributed to a photoinduced-electron-transfer effect. The chelation-enhanced-fluorescence (CHEF) effect induced by some metal ions is presented, and the trend of the CHEF effect, which is Ca-II > Zn-II > Cd-II similar to La-III > Hg-II, is discussed in terms of factors that control the CHEF effect, such as the heavy-atom effect.