[en] Eleven chelating tetrazole-containing ligands have been synthesized, and their complexes with palladium (II) and ruthenium (II) prepared. Proton n.m.r. spectroscopy, electronic absorption spectroscopy and cyclic voltametry have been used to study the nature of the metal-ligand interactions in these complexes. The negatively charged tetrazolate group is shown to be a strong electron donor with very different properties to those of the protonated or alkylated tetrazole group. This leads to pH control of the properties of transition metal complexes containing such ligands. 13 refs., 2 tabs., 3 figs.,ill