Inorganica Chimica Acta, 12 (1975) 205- 210 0 Elsevier Sequoia S.A., Lausanne - Printed in Switzerland The Kinetics of the Anation Reaction of Aquopentaamminecobalt(II1) Acetate in Aqueous Acidic Solution P. R. JOUBERT and R. VAN ELDIK Research Unit for Chemical Kinetics, Department of Chemistry, Potchefstroom University for C.H.E., Potchefstroom 2520, South Africa Received July 5, 1974 205 zyxwvutsrqpon bY The anation reaction of aquopentaamminecobalt(IZZ) by acetate has been studied in the temperature range 60-80” C and acidity range 1.0 5pH 5 5.5 for total acetate concentrations S OSM and at ionic strength I.OM. The anation by acetic acid follows second-order kinetics (k,,), whereas the kinetic results for the ana- tion by acetate (Q, k,) provide evidence for the for- mation of an ion-pair with the complex ion. Ty pical experimental results at 70” C are k,, = 5.33 x lO_’ M-’ set-‘, Q, = 5.87M-’ and kI = 1.46 x 1OA see-‘. The activation parameters for the different reaction paths are reported and the results discussed with reference to various other anation reactions of Co(ZZZ) com- plexes. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Introduction The anation kinetics of CO(NH,),OH,~+ has been studied for a variety of entering ligands during the past few years ‘3’ Evidence for the formation of the ion- . pair CO(NH~)~OH, 3+.X’r+ was reported and the Id type of mechanism suggested. Recent results for the anation by oxalate3 indicate a substantial difference between the mechanisms for the anation by H&O4 and that by HC204- and Cz04”. The reaction was studied over an acid range sufficiently wide to follow the reaction path for each of the three species sepa- rately. In succession to this study, we have investigated the anation of CO(NH,),OH,~+ by acetate over a wide pH range to distinguish between the anation kinetics of CH,COOH and CH,COO-. The results are reported in this paper. Experimental The complexes [CO(NH,)~OH,](C~OJ~ and [Co (NH3)s00CCH3](C10& were prepared according to procedures already described in literature4”. Chemical analyses were performed on a Hewlett Packard Model 185B (CHN) Analyzer with the following results: zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA %C %H %N [CO(NH~)~OH,I(C~O~)~: Theoretical 3.72 15.21 Experimental 3.81 15.39 [CO(NH,),OOCCH~](CIO~),: Theoretical 5.98 4.51 17.42 Experimental 6.19 4.31 17.91 From the spectral data in Table I it was decided to follow the anation reaction at 498 nm, using a batch sampling technique and a thermostat controlled within + 0.1’ C accuracy in the temperature range 60 to 80” C. Spectra were recorded on a Unicam SP800 spectro- photometer and absorbance measurements during ana- tion were made with a Zeiss PMQII spectrophotometer. Test solutions comprised 0.01 M CO(NH&OH~~+ in 0.05 5 [total acetate] s OSM (added as acetic acid) at 1.0 5 pH 5 5.5. The acidities were obtained by additions of HNOJ and KOH and measured by means of a Pye Unicam Model 292 pH meter for pH > 1.0. The ionic strength of the test solutions was maintained at l.OM with KNO, since acetate is more soluble in nitrate than in perchlorate medium. No buffer was added due to the selfbuffering property of acetate solu- tions in the pH range concerned. Chemicals of analyti- cal and reagent grade were used throughout the in- vestigation. TABLE I. Spectra1 Data. Complex Ion I max (um) Co(NH&0Hz3+ 491 491 Co(NH,),OOCCH,‘+ 505 505 a Dissolved zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPO in 0.05 M HC104. clllax Reference (M-‘cm-‘) 49.0 3.6 49.0 This work” 74.4 5 75.0 This work