Some metal-ion-complexing properties of the ligand 2,2',6',2 ''-terpyridyl (terpy) in aqueous solution are determined by following the pi-pi* transitions of 2 x 10(-5) M terpy by UV-visible spectroscopy. It is found that terpy forms precipitates when present as the neutral ligand above pH similar to 5 in the presence of electrolytes such as NaClO4 or NaCl added to control the ionic strength, as evidenced by large light-scattering peaks. The protonation constants of terpy are thus determined at. the ionic strength (mu) = 0 to avoid precipitation and found to be 4.32(3) and 3.27(3). The log K-1 values were determined for terpy with alkali-earth metal ions Mg-II, Ca-II, Sr-II, and Ba-II and Ln(III) (Ln = lanthanide) ions La-III, Gd-III, and Lu-III by titration of 2 x 10(-5) M free terpy at pH > 5.0 with solutions of the metal ion. Log K-1 (terpy) was determined for all, Zn-II, Cd-II and Pb-II. by following the competition between the metal ions and protons as a function of the pH. Complex formation for all of these metal ions was accompanied by marked sharpening of the broad pi-pi* transitions of free terpy, which was attributed to complex formation affecting ligand vibrations, which in the free ligand are coupled to the pi-pi* transitions and thus broaden them. It is shown that log K-1(terpy) for a wide variety of metal ions correlates well with log K-1(NH3) values for the metal ions. The latter include both experimental log K-1(NH3) values and log K-1(NH3) values predicted previously by density, functional theory calculation. The structure of [Ni(terpy)(2)][Ni(CN)(4)]center dot CH3CH2OH center dot H2O (1) is reported as follows: triclinic, P (1) over bar, a = 8.644(3) angstrom, b = 9.840(3) angstrom, c = 20.162(6) angstrom, alpha = 97.355(5)degrees, beta = 97.100(5)degrees, gamma = 98.606(5)degrees, V = 1663.8(9) angstrom(3), Z = 4, and final R = 0.0319. The two Ni-N bonds to the central N donors of the terpy ligands in 1 average 1.990(2) angstrom, while the four peripheral Ni-N bonds average 2.107(10) angstrom. This difference in the M-N bond length for terpy complexes is typical of the complexes of smaller metal ions, while for larger metal ions, the difference is reversed. The significance of the metal-ion size dependence of the selectivity of polypyridyl ligands, and the greater rigidity of ligands based on aromatic groups such as pyridyl groups, is discussed.