SUBSTITUTION AND PROTONATION REACTIONS OF NITRILOTRIACETATOCHROMIUM(III) IONS

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 tha t 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(eta 3-NTA)(H2O)3]+ complex. The substitution reactions between [Cr(NTA)(H2 O)2]/[Cr(NTA)(OH)(H2O)]- and different ligands (NCS- and EBT-:EBT- = E riochrome Black T) were also studied. The second-order rate constant f or the reaction of NCS- with the [Cr(NTA)(H2O)2] complex at 35.0-degre es-C was 9.1(1) x 10(-3) M-1 s-1 and that for the reaction of EBT- wit h 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 th is reaction.