The coordination behaviour of the ditopic receptor of 1‐[p‐(2,2′:6′,2′′‐terpyrid‐4′‐yl)tolyl]‐1,4,8,11‐tetraazacyclotetradecane (L1) towards Co2+ in acetonitrile/water (70:30, v/v) has been investigated. At acidic pH values, the L1−H+−Co2+ system shows an oxidation wave at 150 mV vs. SCE, characteristic of Co2+ in a bis(terpyridine) environment. Upon addition of OH−, a reduction in the intensity of the wave at 150 mV is observed and a new oxidation wave appears at 1.30 V. This new wave is close to that found for [Co(cyclam)]2+ under similar working conditions. The electrochemical data thus suggest that there is a pH‐controlled translocation of the Co2+ ion from the bis(terpyridine) to the cyclam environment. A similar inference can be made from the results of UV/Vis studies. A thermodynamic characterization of the L1−H+−Co2+ system has also been carried out. Stability constants have been determined in acetonitrile/water (70:30, v/v, containing 0.1 mol dm−3 nBu4NClO4 at 25 °C) using potentiometric techniques. The following species were found: [Co(L1)2H4]6+, [Co(L1)2H3]5+, [Co(L1)H]3+, [Co(L1)]2+, and [Co(L1)(OH)]+. The 2:1 ligand‐to‐metal species [Co(L1)2H4]6+ and [Co(L1)2H3]5+ are assigned to complexes where Co2+ is coordinated by two terpyridine units. The [Co(L1)H]3+, [Co(L1)]2+, and [Co(L1)(OH)]+ species are assigned to complexes where the Co2+ cation is in the cyclam binding domain. The pH‐induced Co2+ jumping between coordination sites is discussed in terms of the different basicities of cyclam and terpyridine. Co2+ jumping between coordination sites has also been studied by the sequential acidification of basic mixtures of L1 and Co2+. Under these conditions, the translocation was observed at a different pH value than when an acid‐to‐basic path was followed. The effect of anions such as phosphate and chloride on the translocation process has also been studied. In the presence of phosphate, translocation occurs at pH = 11 rather than at pH ≈︁ 8. The phosphate−L1 and phosphate−L1−Co2+ systems have been characterized by potentiometry and a remarkably good agreement has been found between the electrochemical and potentiometric data.

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