The idea that M-C it contacts between diamagnetic heavy metal ions such as Pb(II), Ag(I), Pd(II), or Hg(II) and the anthracenyl fluorophore of adpa ((N-(9-anthracenylmethyl)-N-(2-pyridinylmethyl)-2-pyridinemethan- amine) are responsible for quenching the fluorescence of the complexes of these metal ions with adpa is explored crystallographically. The structures of [Pb(adpa)(NO3)(2)] (1), [Ag(adpa)NO3] (2), [Pd(adpa)NO3]NO3 (3), [Zn(adpa)(NO3)(2)] (4), and [Cd(adpa)Br-2] (5) are reported. The pi contacts with the fluorophore are for 1 are a Pb center dot center dot center dot C pi contact of 3.178 angstrom; for 2, an Ag center dot center dot center dot C pi contact of 3.016 angstrom; and for 3, a Pd center dot center dot center dot C pi contact of 2.954 angstrom on the axial site of the Pd(II) ion. The Zn(II) ion in 4 has no Zn center dot center dot center dot C pi contact, with the anthracenyl fluorophore rotated completely away from the Zn(II) ion. These structures confirm that in the Pb(II), Ag(I), and Pd(II) complexes of adpa, which experience strong quenching of fluorescence, there are strong M center dot center dot center dot C pi contacts, as expected if it is the pi contacts that quench fluorescence. In contrast, for the Zn(II) adpa complex, which forms no pi contact, there is a strong increase in fluorescence intensity. The structure of 5 shows a long Cd center dot center dot center dot C pi contact at 3.369 angstrom, in contrast to a previously reported structure with two coordinated nitrates where the Cd center dot center dot center dot C pi contact is 3.097 A. The long Cd center dot center dot center dot C pi contact in [Cd(adpa)Br-2] suggests how coordination of Br-, as well as other more covalently bound ligands such as Cl-, SCN-, and S2O32-, cause an increase in fluorescence intensity, reported for the Cd(II)adpa complex in 50% CH3OH/H2O. Coordination of covalently bound ligands to the Cd(II) weakens the Cd center dot center dot center dot C pi contact and so enhances fluorescence, whereas more ionically bound ligands such as SO42-, NO3-, or H2O produce a strong Cd-C pi contact and weakened fluorescence. Complexes of the Cd(II)/adpa type may form the basis for a new type of anion/small molecule sensor. The tendency of metal ions to form pi contacts with aromatic groups is analyzed in terms of the frequency of occurrence of pi contacted structures in the literature, as well as by DFT calculations on the adpa complexes.