Transition Met. Chem., 17, 409-412 (1992) Acid catalysed hydrolysis of [(NH3)sCoCrO3] + 409 Mechanism of the acid catalysed hydrolysis of the chromatopenta- amminecobalt(III) ion Novelette P. Sadler and Tara P. Dasgupta* Department of Chemistry, University of the West Indies, Mona, Kinyston 7, Jamaica Summary The kinetics of the acid hydrolysis of chromatopenta- amminecobalt(III) ion has been studied using a stopped- flow method over the acidity range 0.01-..<[H+]-..< 1.0 tool dm- 3 and 20.0 ~C ~<0 ~< 30.0 ~C at ionic strengths 0.5 and 1.0 mol dm-3 (LiNO3). These studies reveal that the complex is first protonated and subsequently hydro- lysed to the aquapentaammine cobalt(III) ion. The rate constants for the hydrolysis of the mono and diprotonated species at 25 ~ C are 0.83 + 0.01 s- 1 and (1.60 + 0.02) x 104 mol - ~dm- 3 s- 1 respectively. Introduction A series of novel cobalt(III) chromato complexes were synthesised by Brigg ~1~as early as 1918. Since then there has been a resurgence of interest in the kinetic and thermo- dynamic studies of these complexes~2-7( A detailed study of the reaction of aquapentaamminecobalt(III) ion with chromate ion by Haight 14) revealed that the rapid com- plexation leading to the formation of chromatopenta- amminecobalt(III) ion occurs and this was attributed to a mechanism involving substitution at the chromium(VI) center, since substitution at the cobalt(III) center is known to be slow~8( Verification of this mechanism has recently been provided by oxygen exchange studies, carried out by Okimura et al.16~, on the hydrolysis of the chromato- pentaamminecobalt(III) complex. However, no detailed acid hydrolysis study on this compound was described. We report here such an investigation as a part of our investigations into the mechanism of the reactions of complexes with coordinated oxoanions. Experimental Materials All chemicals used were of reagent grade. Deionised H20, obtained by passing distilled H20 through a Milli Q reagent grade water system (Millipore co., Bedford, Mass., U.S.A.), was used for all investigations. Preparation of the complexes [(NH3)sCo(OH2)](NO3)3"H20 was prepared by the method of Basolo and Murmann ~9) and its purity was checked by uv-vis spectroscopy (~49o = 49 mol- 1dm 3 cm- 1 (lit/1~ = 48.6)). [(NH3)sCoOCrO3](NO3).HzO was prepared by a procedure outlined by Briggs m. The purity of the brown- red compound was checked by elemental analysis and by uv-vis spectroscopy. (Found: Co, 17.2; N, 24.6; H, 4.9; Cr, 15.0. CoN6H17CrO8 calcd.: Co, 17.3; N, 24.7; H, 5.0; * Author to whom all correspondence should be directed. Cr, 15.3~ U.v-vis: e54o = 170 mol- 1 dm 3 cm- 1 lit/6) e= 168. I.r. spectrum; Vcr-o 875, 915, 895cm -1 Instrumentation All u,v. vis spectra were recorded on either a Pye-Unicam model SP8-100 spectrophotometer or a Varian-Cary 219 spectrophotometer. For temperature dependent measure- ments, the cell compartment was thermostatted with a constant temperature circulator, HAAKE, model KT33. Temperatures were recorded with an accuracy of + 0.02 ~ C. The i.r. spectra of the complexes were recorded on a Pye-Unicam SP3-300 spectrophotometer using KBr discs. All kinetic measurements were recorded using a Durrum D-110 stopped-flow spectrophotometer connected to a Tetronix D11 single beam storage oscilloscope and a Biomation model 802 transient recorder. The digital- lised information in the transient recorder was redisplayed on a Tetronix D10 oscilloscope before being transferred to a Wang Advanced programming calculator through a DATOS 305 interface unit. The spectrophotometer syr- inges were immersed in a circulating water bath linked to a thermostatted system (GCA Precision Scienfitic, model Lo-Temprol 154) capable of maintaining temperatures within _+ 0.02 ~ C. Semilogarithmic plots for each run were made with a Wang 612 plotter connected to the calcul- ator. Kinetic measurements Weighed amounts of the chromatopentaamminecobalt(IIl) complex were dissolved in H20 and degassed HNO3 solution was prepared in a separate flask. The ionic strength of each solution was adjusted to the required value with appropriate amounts of LiNO 3 and the flasks were thermostatted at the required temperature for 15min before introducing the solutions into storage syringes of the stopped-flow apparatus. The apparatus was equili- brated at the reaction temperature for at least 4-5 h prior to use and the solutions were kept in the thermostatted drive syringes for 5-10 min before each experiment. All runs were made at 540nm, which offers the largest ab- sorbance difference between the reactant and the product (the molar extinction coefficient of the [Co(NH3)50CrO 3] + ion at 540nm, is 170mol-~dm3cm-1 whereas that of the [Co(NH3)5(OH2)] 3+ is 24mol- 1 dm 3 cm- 1). The pseudo-first order rate constants, kob s, were deduced from absorbance versus time data using the computing system described above. Each kob ~ value is an average of at least three successive runs. The error limit in each kob S value is _+ 200. Results and discussion The red brown aqueous solution of the [(NH3)5CoO- CrO3] + ion when treated with dilute HNO 3, immediately turns red due to formation of the [(NH3)sCo(H20)] 3+ ion. This product was confirmed by comparing the final spectrum after hydrolysis with that of an acidic solution 0340-4285 9 1992 Chapman & Hall