The cyclic tetradentate ligand 1,4,7,11-tetra-azacyclotetradecane (isocyclam, = L) has been prepared by the Richman-Atkins procedure. The nickel(II) complexes of the ligand have been studied in detail. The complex [NiL](ClO 4 ) 2 undergoes a square planar octahedral equilibrium in aqueous solution: [NiL] 2 + + 2H 2 O K [NiL(OH 2 ) 2 ] 2 + ; the equilibrium constant K has been determined as a function of temperature and ΔH° = -22.6 ± 1 kJ mol - 1 and ΔS° = - 74.9 ± 2 J K - 1 mol m n 1 . Reaction of ethylenediamine with [NiL](ClO 4 ) 2 leads to folding of the macrocycle and formation of the blue octahedralcis -[NiL(en)](ClO 4 ) 2 . The complex [NiL] 2 + displays a reversible one-electron reduction at - 1.45 V versus SCE in aqueous solution and a reversible oxidation to the nickel(III) complex at +0.85 V versus SCE. [Ni(isocyclam)] 2 + was found to be a good electrocatalyst for the two-electron reduction of CO 2 to CO but is not as efficient as [Ni(cyclam)] 2 + .