Indian Journal of Chemistry Vol. 26A. March 1987, pp. 205-210 Multidentate Ligand Exchange Kinetics: Kinetics & Mechanism of Formation of Aminocarboxylatonickelate(II) from Reaction of Tetraethylene- pentaminenickel(I1) with Diethylenetriaminepentaacetic (DTPA) & 1, 2-Propylenedinitrilotetraacetic (PDTA) Acids R M NAIK & P C NIGAM· Department of Chemistry, Indian Institute of Technology. Kanpur 208016 Received 12 May 1986; accepted 19 SepJember 1986 The kinetics of formation of N~OTPA)J - and N~POTA)2 - in the reaction of N~tetren)2+ with OTPA 5- and PDTA'~-. respectively have been studied by the cyanide quenching method (tetren or tet refers to tetraethylenepentaamine). The reactions of OTPA 5- and POTA 4- have been studied in the pH ranBCsof 5.5-11.0 and 6.5-11.0 respectively at /=0.1 mol dm -J (NaCI04) and temp = 25VC. The second order rate constants have been calculated by the initial rate method and confirmed by integrated second order plots. The rate of reaction of N~tetren)2 + with OTPA 5- decreases initially with increase in pH. passes through a minimum at pH - 8.5 and again increases. Similar behaviour is also observed in the reaction of N~tetren)2 + and POT A4 -. This type of behaviour is explained on the basis of reactivities of protonated forms of nickel- polyamine complexes as well as protonated forms of OTPA 5- and POT A4 -. A multistep mechanism is proposed which involves formation of a series of mixed ligand intermediates where steric effects prevent full chelation of the six-coordination sites of nickel ion by the two exchanging ligands. The rate-determining step has been identified. The kinetics and mechanism of nickel-polyamine formation and self-dissociation reactions have been studied extensively! -6. The kinetics and mechanism of ligand substitution reactions between nickel(II) polyamines and EDTA 7 have already been reported. We have earlier focussed our attention on the exchange reaction of nickel(II) polyamines with some aminocarboxylates, viz. TMDTA8, HEDTA9 and DTPA !O.In the present study, we report the kinetics and mechanism of reaction (1) in order to further understand the mixed ligand complex intermediate formation in multidentate ligand exchange kinetics. Ni(tetren)2+ + Ln- -+ NiU-n + tetren ... (1) where Ln- represents DTPA 5- (diethylenetriamin- epentaacetic acid) and PDT A 4 - (1, 2- propylenedinitrilotetraacetic acid) while tetren or tet stands for tetraethylenepentaamine. Materials and Methods Reagents Purified and recrystallised samples of DTPA and PDTA (Sigma Chemical Co, USA) were used after standardization by the pH-metric method. A stock solution of Ni(Cl04h was standardised by complexo- metric titration against EDT A using murexide indicator!!. Tetraethylenepentaamine (tetren or tet) (Fluka, AG) was purified by distillation under reduced pressure. The solution of tetren was standardised by the pH-metric method. A solution of nickel-tetren complex was prepared as described for Ni-trien complex earlier9. Sodium perchlorate (E Merck) was used for maintaining ionic strength at 0.1 mol dm -3. Sodium hydroxide or perchloric acid was used to maintain the pH of solutions. Kinetics runs Kinetics were studied spectrophotometrically employing a Toshniwal spectrophotometer model RL- 02 equipped with a circulatory arrangement for thermostating the cell compartment. The progress of reaction was followed at 267 nm (Amax of [Ni(CN)4]2 -) by the cyanide quenching method described in our earlier communications 7 -! o. An Elico digital pH meter model LI-120 was used for pH measurements. Results General data The kinetics of ligand exchange reaction between Nitetren and Ln- was studied over a wide range of pH and temperature. The values of initial rates were obtained from the slopes of concentration versus time plots and reaction orders in [Nitetren] and [L n -] were determined from the plots of initial rate versus concentrations of both reactants. The values of initial rates for both the systems are listed in Table 1. The plots oflog(initial rate) vefsus log (concentration)(Fig. 1) show that both the reactions follow first order kinetics each in [Nitetren2+] as well as [Ln -]. So, a general rate law (2)can be proposed in accordance with 205