Ligand-Metal Interrelationships. 2. Effect of the Change from Square-Planar to Octahedral Coordination on the pK_a of the Ligand in Unsaturated Tetraaza Macrocyclic Nickel Complexes: A Pulse-Radiolytic Study

We have recently reported that the pK₃ of the macrocylic ligand in the tervalent nickel complex of 11,13-dimethyl-1,4,7,10-tetraazacyclotrideca-10,13-diene, NiL¹³⁺, is higher than the pK_a measured for the divalent complex, NiL₁²⁺ This surprising result, in view of the greater acidity expected for the tervalent nickel complex, was explained by the observation that the pKa predicted for the ligand in this complex coincides with the pH range in which two hydroxyl groups are axially coordinated to the nickel. Thus, the results only indicate that Ni(OH)²⁺ has a smaller effect on the ligand pKa than does Ni²⁺. In addition, we had tentatively assigned the strong absorption, Xmax = 600 nm, observed for the alkaline form of the tervalent complex as a ligand-to-metal chargetransfer transition.¹.