Mechanistic studies of substitution reaction of trans-(diaqua)--(N,N'-ethylene-bis-salicylanude)chromium(III) ion

摘要

The kinetics of substitution of trans-[Cr(SaIm)(OH2)2]~+ with some biologically important ligands (Nu~-) viz. glycine (Gly), p-aminobenzoic acid (Paba), glycyl-glycine (Gly-gly), L-histidine (His), L-isoleucine (Ile) has been studied spectrophotometrically over the range : 25 ≤ t ≤ 45 °C, 1.8≤ pH ≤ 5.5, 0.01 ≤ [Nucleophile] ≤0.30, I = 0.5 mol dm~(-3) (KNO3). Although there are two replaceable aqua ligands in the complex, only mono substitution occurred, which is evident from Job's curve. The kinetic studies also showed one aqua ligand substitution. Unlike trans-[Cr(Salen)(OH2)2]~+, the rates of aqua ligand substitution were found slower indicating moderate labilization of axial aqua ligand. The reaction takes place via an outersphere association between the Cr~(III) complex and various nucleophiles followed by transformation of the outer into inner sphere complexes by slow interchange. The anation rate constants (10~4 k_(an)/s~(-1)) at 25 °C were found to be 14.98 (Gly), 8.45 (Ile), 10.52 (Paba), 14.23 (His) and 13.78 (Gly-gly) and the corresponding ΔH ~#(kJ mol~(-1)) values are 57.7 ± 1.6, 38.3 ± 1.3, 43.0 ± 0.8, 34.2 ± 2.7 and 39.7 ± 3.1, the ΔS~# (JK~(-1) mol~(-1)) values are -105 ± 5, -175 ± 5, -157 ± 3, -65.8 ± 9 and -166 ± 11. The higher value of the k_(an) in comparison to k_(ex) and highly negative values of activation entropy and variation of k_(an) by changing nucleophiles, support I_a mechanism for the substitution reactions.