Anomalous Properties found on the basic chemistry and electrocatalytic activity towards CO₂ reduction of a series of square planar Ni(II), Cu(II) complexes ([ML_n]²⁺ and [ML_n]⁺ where M= Cu and Ni) of different shift bases (L_n) are reported. All the complexes show a significant affinity for H⁺ association regardless of the +II charge of the complexes. It seems that such an association is not hindered by the positive charge of the complexes and the ligand (L_n) is susceptible for the protonation. Ni(II) and Cu(II) complexes of L_n show reversible colour changes in the presence of acids and bases containing a coordinating anion. This behavior is nicely reflected in their UV-Visible spectral changes. The broad spectral band that appeared in the visible region for all these complexes due to their d-d electronic transitions decreases its intensity, shows a red shift and grows as a new band at the low energy end of the Visible region on adding HCl into the solutions of each of the complexes. Such behavior is not seen on adding acetic acid in which acetate anion is not a good coordinating anion. The intensity of all the original intense bands at wavelength < 400 nm decreases and a new band appears at the low energy end of UV region on stepwise addition of HCl. Original spectrum reappears on adding NaOH to the acidified solution proving the reversibility of the process. In addition, all the complexes show a significant blue shift in its d-d band on adding anions such as SCN^- and Br^- successively. This behavior and inferred electrophilicity of the complexes towards the anions is seen on adding these anions to the solutions of the complexes only after acidifying the solutions with HCl. In contrast, [ML₅]⁺ shows anion association even in the absence of acidic conditions. Anion association to the [CuL_n]²⁺ and [NiL_n]²⁺ complexes, especially for [ML₅]⁺, is probably largely ionic as in the ion pair at the beginning because the dz² orbital, which has the proper symmetry to be bonded with incoming anion, in both complexes is doubly occupied for the square planar geometries of d⁸ and d⁹ configurations. If any change in either the geometry or electronic spin state of the square planar [CuLn]²⁺ and [NiL_n]²⁺ complexes occurs at low pH allowing SOMO, dx²-y² to have the proper symmetry to be bonded with an approaching anion, a covalent interaction with [ML_n]²⁺ complexes giving an axial coordination of anions can be expected. H1NMR spectroscopic data provides evidence for such a geometry change at low pH. Interaction of all these complexes with CO₂ is always larger than that of their starting metal salts under the same condition. However, spectroscopic and electrochemical data suggest that the incorporation of a different metal ion in the cavity of these macrocyclic ligands alters their catalytic properties very significantly though it does not alter their acid–base properties and anion binding capacities qualitatively. All the Ni²⁺ complexes of L_n show some similarity in their electrochemistry and catalyze the CO₂ reduction at ~-2.2 0.2 V irrespective of L_n whereas all Cu²⁺ complexes of L_n except [CuL1]²⁺show some anomalous electrochemistry. [CuL1]²⁺ is the only complex that catalyzes the CO2 reduction at a significantly lower potential, -1.2 V than that of other [ML_n]²⁺ complexes studied.

Keywords: CO₂ reduction, Complexes of shift bases, Transition metal catalysts.