The pi-contact hypothesis, that quenching of the fluorescence of complexes of photoinduced electron transfer sensors with heavy diamagnetic metal ions may be caused by pi contacts between the metal ion and the fluorophore of the sensor, is examined with a study of the fluorescent properties of the sensor 4-[[bis(2-pyridinylmethyl)amino]methyl]-6,7-dimethoxy-1-benzopyran-2-one (cdpa) and the structures of its complexes with some metal ions. The coumarin-type fluorophore of cdpa is a weaker pi-contact former than the anthracenyl fluorophore of the analogue adpa (Inorg. Chem. 2014, 53, 9014): only Ag-I, the strongest pi contact former, quenches the fluorescence of cdpa, apart from paramagnetic Cu-II and Ni-II, which quench fluorescence by a redox mechanism not requiring pi contacts. The structures of [Ag(cdpa)NO3] (1), [Pb(cdpa)(NO3)(2)] (2), [Zn(cdpa)(NO3)(2)] (3), [Cd(cdpa)Cl-2](2) (4), [Cd(cdpa)(2)H2O] (NO3)(2) (5), and [Hg(cdpa)(2)H2O] (NO3)(2) (6) are reported. Structure 1 shows that Ag-I is the only metal ion studied that forms pi contacts with the fluorophore of cdpa in the solid state: Ag center dot center dot center dot C eta(2) pi contacts of 3.083 and 3.095 angstrom, in line with quenching of the fluorescence of the Ag-I(cdpa) complex. In contrast, Pb-II, Zn-II, and Cd-II show chelation-enhanced fluorescence in their cdpa complexes, and the structures of 2-4 show that the fluorophore of cdpa in each case forms no pi contacts. By contrast, the adpa complexes of Pb-II and C-II show pi contacts with its more strongly pi-contacting fluorophore (Inorg. Chem. 2014, 53, 9014). The structures of 5 and 6 show bis-complexes of cdpa: the coordination geometries of Cd-II and Hg-II are discussed in relation to the number of covalently bound donor atoms present. The preferred hapticity of pi-contacted metal ions is evaluated from the literature structures, suggesting that d(10) metal ions such as Ag-I and Hg-II and tetragonally distorted Cu-II and Pd-II, prefer eta(I) and eta(2) pi contacts, while more ionically bound metal ions such as K-I, Ba-II, and La-III, as well as d(10)s(2) metal ions such as Tl-I, Pb-II, and Bi-III, prefer eta(6) contacts.