Isomeric p- and o-phenylenediamine- containing macrocyclic hosts ( Wurster's thiacrown ethers L-1 and L-2, respectively) were prepared and studied as Hg-II ionophores. The distinct electrochemical properties of the two hosts allowed for the formation of a coordination compound with the ortho- Wurster's thiacrown ether but not the para isomer. In the latter case, the Hg-II ion served as an oxidant in an electron-transfer reaction with the host. Solutions of the HgII complex of L-2 were studied by C-13 NMR spectroscopy and cyclic voltammetry and revealed a strong interaction between the redox-active phenylenediamine subunit and the bound metal cation. An X-ray analysis confirmed the participation of the three macrocyclic S atoms and both phenylenediamine N atoms in the stabilization of the complex.