Reinvestigation of Decarboxylation Kinetics from the Hydrogencarbonatocobalt(III) Intermediate

Acid-catalysed hydrolysis has been studied by a stopped-flow method at 25 °C, ionic strength of 2.0 (NaClO4) and oxonium ion concentration of 0.02—1.0 M for monodentate carbonato complexes ( [Co(CO3)(NH3)5]+, [Rh(CO3)(NH3)5]+, and [Ir(CO3)(NH3)5]+) and for didentate carbonato complexes ( [Co(CO3)(NH3)4 ]+, [Co(CO3)(en)2]+, α, β-[Co(CO3)(trien)]+, [Co(CO3)(tren) ]+ and [Co(CO3)(nta)]2- (M = mol dm-3, en = ethylenediamine, trien = 1,8-diamino-3,6-diazaoctane, tren = tris(2-aminoethyl)amine, and H3nta = nitrilotriacetic acid) ). The experimental first-order rate constant of decarboxylation from the monodentate hydrogencarbonato intermediate depends on the acidity of the solution, which we ascribe to its amphiprotic character. The deprotonation constant depends critically on an aqua ligand cis to the hydrogencarbonato ligand: 0.004—0.6 M with it, whereas about 10-6 M without it. We interpret that the former strong acidity originates from the stability of the aquacarbonato complex, owing to a strong (21—33 kJ mol-1) intramolecular hydrogen bond. The basicity has been confirmed by the acidity dependent zero-time spectra, the protonation constant being 0.1—0.7 M-1. The hydrolysis rate constant of the monodentate carbonic acid complex has been estimated to be smaller than 0.3 s-1 at 25 °C.