Linear free energy relationships between reaction rate constants and equilibrium constants of complex compounds: III. Kinetics and mechanisms of ternary complex formation between (5‐X‐1, 10‐phenanthroline)copper(II) and threonine

The kinetics of ternary complex formation involving Cu(5‐X‐1, 10‐phen) and threonine (CuAL, A=5‐X‐1, 10‐phen; L=threonine or represented by O‐N; X=NO 2 , Cl, H, CH 3 ) has been studied by temperature‐jump and stopped‐flow methods. The formation rate constants, k f (M −1· s −1 ), for the complexation reaction, CuA + LCuAL, are as follows; X=NO 2 , 8.68×10 8 ; X = Cl, 7.13×10 8 ; X=H, 6.12×10 8 ; X=CH 3 , 5.42×10 8 . The rate constants for zwitterion attack are nil within experimental error. It has been found that a linear free energy relationship exists between the stability (log K CuA CuAL ) of the complexes CuAL and log k f as follows: log K CuA CuAL = 0.13+0.83 log k f , r = 0.99. It suggested that the formation rate governed the stability of the ternary complexes. The rates of formation of the ternary complexes increased with decreasing electron‐donating property of the substituents. A linear relationship was found to exist as expressed by the following equation: log( k R f / K O f = 0.097σ, r = 0.96. A mechanism involves a rapid equilibrium between CuA and L followed by a slow ring closure of L.