Substitution and protonation reactions of nitrilotriaceta tochromium(lll) ions

Abstract

The reaction of NH4[Cr(NTA)(H2O)2] (NTA = nitrilotriacetic acid) with H+ has been studied at different temperatures and a rate constant of 1.27(3) x 10(-2) S-1 was obtained at 25.1-degrees-C. It is proposed that the reaction involves the formation of an ion pair, followed by the protonation of one of the carboxylate groups of the NTA ligand bonded to the central metal ion. This results in the formation of the [Cr(eta3-NTA)(H2O)3]+ complex. The substitution reactions between [Cr(NTA)(H2O)2]/[Cr(NTA)(OH)(H2O)]- and different ligands (NCS- and EBT-:EBT-= Eriochrome Black T) were also studied. The second-order rate constant for the reaction of NCS- with the [Cr(NTA)(H2O)2] complex at 35.0-degrees-C was 9.1(1) x 10(-3) M-1 s-1 and that for the reaction of EBT- with the metal complex at 30.0-degrees-C was 9.5(6) x 10(-2) M-1 s-1. The increase in substitution rate for the reaction between [Cr(NTA)(H2O)2] and EBT- is attributed to the chelation of the EBT- ligand during this reaction.