The kinetics of the reaction between gaseous CO 2 and the cis-[Cr(phen) 2 (OH 2 ) 2 ] 3+ ion leading to the formation of the carbonato complex ion, have been studied over the pH and temperature ranges: 3
Acid-catalyzed hydrolysis of the cis-[Cr(1,10-phenanthroline) 2 (O 2 CO)] + ion studied by the u.v.-vis stopped flow method Abstract The cis-½Cr(phen) 2 ðO 2 COÞ þ ion was prepared through the displacement of two molecules of water from the cis-½Cr(phen) 2 ðOH 2 Þ 2 3þ by the bidentate carbonate anion. It underwent two-phase hydrolysis reactions under acidic conditions (0:1 < ½H þ < 2:7 M) at 5, 10, 15, 20 and 25°C. Via slow carbonato chelate ring opening (first step k slow 1 ) and a second fast decarboxylation (k fast 2 value). The first step was preceded by protonation of the coordinate bidentate carbonate ligand. The second step exhibited no pH dependence, while k slow 1 values increased with acid concentration that suggested the presence of both protonated and deprotonated reactant species. Based on these observations we have proposed a hydrolysis mechanism featuring H 2 O-induced ring-opening of the coordinate CO 2À 3 group in the first step ðk slow 1 Þ followed by loss of CO 2À 3 from two intermediates, ½Cr(phen) 2 ðO 2 COHÞ 2þ ðk slow 1 Þ and ½Cr(phen) 2 ðOH 2 ÞðO 2 COHÞ 2þ ðk fast 2 Þ.
A stopped-flow method was used to investigate the kinetics of the acid hydrolysis of a newly synthesised coordination ion of the cis-[Cr(C 2 O 4 )(AaraNH 2 )(O 2 CO)] ) type, where AaraNH 2 stands for methyl 3-amino-2,3-dideoxy-a -D-arabino-hexapyranoside, over a range of hydrogen ion concentrations 0.01 < [H + ] < 2.7 M and temperatures 5 < T < 25°C. Initiated by perchloric acid (HClO 4 ), the hydrolysis (decarboxylation) of the chromium(III) ion complexed with a bicoordinately linked carbonate ligand turned out to be a two-step process. The kinetic parameters k 1 , k 2 were determined of both steps of the hydrolysis of the cis-[Cr(C 2 O 4 )(AaraNH 2 )(O 2 CO)] ) ion, as was the equilibrium constant K of the protonation of this ion, which precedes the actual two-step hydrolysis. From an analysis of the values of the constants obtained, a mechanism is proposed for the acid hydrolysis of the cis-[Cr(C 2 O 4 )(AaraNH 2 )(O 2 CO)] ) coordination ion.
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