FRANCESCO PAOLO CAVASINO
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The Formation Kinetics of the Nickel Monomalonate Complex Studied by the Temperature-Jump Method
by Francesco Paolo Cavasino Institute of Physical Chemistry, U n h m i t y of Palermo, Palermo, Italy
(Received June 22, 1966)
The kinetics of formation of the nickel monomalonate complex has been examined by the temperature-jump method at 15, 25, and 31’ and at ionic strength 0.1 M. The experimental data are interpretable on the assumption that the following two reactions conMal2- + NiMal and Ni2+ tribute significantly to the complex formation: Ni2+ HMal- ~2 NiMal H+, where Mal2- represents the malonate ion. The results suggest that the rate-determining step is the release of the “first” water molecule from the inner coordination sphere of the nickel ion. The kinetic data for the above reactions are virtually similar to those for the analogous reactions of nickel oxalate formation. All the equilibrium constants at 25’ for each intermediate reaction leading to the nickel malonate and oxalate formation have also been evaluated and the steps, on which the different stability of these complexes depends, fixed.
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Introduction In recent years the study concerning the formation kinetics of metal complexes has been greatly developed thanks to the modern techniques which have allowed the investigation of fast reactions in solution with halftimes down to about lo-$ sec.lr2 In particular, the relaxation methods2-10 have largely contributed to the development of these studies and to a better knowledge of the mechanism of such reactions. The conclusion drawn from these investigations is that the rate-determining step of complex formation is the release of a water molecule from the inner coordination sphere of the metal ion and that the rate for a given metal ion is independent of the nature of the entering ligand. (A dependence of the sate of water subxtitution on the ligands has been found, on the contrary, for metal ions of high charge density, ie., Fe**.) In the case of bidentate or multidentate ligands, two or more water molecules are to be replaced from the inner coordination sphere of the metal ion. I n the few kinetic studies existing at present in this field, the Ni2+ion has usually been used as a coordinating ion. It has been ~uggestedl1-l~that the rate of formation of a complex ion (1: 1) with bidentate (e.g., glycine) or multidentate (e.g., triethylenetetramine) ligands is probably determined by dissociation of the The Journal of Phwical Chemistry
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“first” coordinated water molecule. I n order to obtain further information about the kinetics of formation of such complexes, the reaction of formation of nickel monomalonate has been studied at 15, 25, and 31 O by the temperature-jump relaxation method. (1) L. De Maeyer and K. Kustin, Ann. Rev. Phys. Chem., 14, 5 (1963). ( 2 ) M. Eigen and L. De Mayer in “Technique of Organic Chemistry,’’ Vol. VIII, 2nd Ed., S.L. Friess, E. S. Lewis, and A. Weissberger, Ed., Interscience Publishers, Ino., New York, N. Y., 1963, part 2, p. 896. (3) M. Eigen, Discussions Faraday Soc., 17, 194 (1964); 24, 24 (1957). (4) M. Eigen, 2. Elelctrochenz., 64, 116 (1960). (5) L. De Maeyer, ibid., 64, 65 (1960). (6) M. Eigen in “Advances in the Chemistry of the Coordination 1961, p. 371. Compounds,” The MacmiUm Co., New Yoxk, N. Y,, (7) M. Eigen, Suomen Kemistilehti, 34A, 25 (1961). (8) M. Eigen and R.Tamm, 0.Elebtrochem., 66, 107 (1962). (9) M. Eigen, Pure Appl. C h m . , 6, 97 (1963). (10) M. Eigen and K. Kustin,I.C.9.U. Rm., 5, 97 (1963). (11) M. Eigen and R. G. Wilkins, to be published. (12) G. G. Hammes and J. I. Steinfeld, J . Am. Chem. Hoc., 84, 4639 (1962). (13) J. I. Steinfeld and G . G. Hammes, J. Phgs. Chem., 67, 528 (1963). (14) (a) G. H. Nancollas and N. Sutin, Inorg. Chem., 3, 360 (1964); (b) D . W. Margerum, D . B. Rorabacher, and J. F. G . Clarke, Jr., ibid., 2, 667 (1963).
FORMATION
KINETICS O F THE NICKELhlONOMALONATE COMPLEX
Experimental Section's A stock solution (3.45 X 10-1 M ) containing Ni2+ ion was prepared from Ni(CIOn)z.6Hz0 (Fluka reagent grade) and doubly distilled water, and its concentration was estimated by a complexometric titration.I6 Stock solutions of malonic acid (5.65 X M) and sodium perchlorate (1 M ) were made by dissolving the appropriate weight of the substances (Fluka reagent grade) in doubly distilled water. The temperature-jump apparatus was that described previously." The establishment of the chemical equilibria in solution, after the perturbation produced by the rapid increase in the temperature, was followed by the accompanying shift of hydrogen ion concentration. The shift was observed spectrophotometrically by adding a suitable indicator to the solutions (brom cresol green for pH >4.3 and brom chlorphenol blue for pH
