Spectrochemical studies of coordination complexes: I. Stereospecific kinetics of the ligand-ligand exchange reactions of D*-(-)- and L*-(+)-1,2-propylenediaminetetraacetatolead (II) with D-(-)-trans-1,2-cyclohexanediaminetetraacetic acid

Abstract

The ligand-ligand exchange reactions of D-(-)-trans-1,2-cyclohexanediaminetetraacetic acid (D(-)CDTA) with D*-(-)-[Pb(L(+)PDTA)] and L*-(+)-[Pb(D(-)PDTA)] (PDTA denotes 1,2-propylenediaminetetraacetic acid) over the pH range 11.1 to 13.4 have been shown to be pH dependent, and the rates of exchange increase with increasing pH. The ligand-ligand exchange reactions were monitored by means of polarimetry at 290 nm. The D*-(-)-[Pb(L(+)PDTA)] exchange reaction is slower than the L*-(+)-[Pb(D(-)PDTA)] exchange reaction at the lower experimental pH values, but their rates of exchange with D(-)CDTA approach each other with increasing pH until they become experimentally indistinguishable at pH 13.2. The experimental pH range (pH 11.1 to 13.4) is divided into three regions for each lead-PDTA exchange reaction, with each region characterized by a particular exchange reaction which were described as a aquo-assisted tetra-negative D(-) CDTA attack, a monohydroxy-assisted reaction, and a dihydroxy assisted reaction. A rate expression was derived for the exchange reactions considering the three proposed reaction pathways. Computer-assisted least squares regression analysis showed agreement between the proposed rate expression and the experimental rates of reactions. Using molecular models and the steric influences of the entering and leaving ligands to yield the most favorable intermediates and to account for the correct stereochemical products, reaction mechanisms were proposed for each lead-PDTA isomer and each proposed reaction pathway. ...