Kinetic and mechanistic studies on the formation of bis(dicarboxylato)carbonatochromate(III) complexes

The reaction of the Cr(xx)^sub 2^(H^sub 2^O)^sub 2^ ^sup -^ (xx = oxalate, malonate and methylmalonate) complexes with dissolved CO^sub 2^ was studied by stopped-flow spectrophotometry in the 7 < pH < 9 range and between 20 to 30°C at an ionic strength of 0.5 mol dm^sup -3^ (NaCl). Under the experimental conditions the aqua complex ion consists of a pH-dependent mixture of Cr(xx)^sub 2^(H^sub 2^O)^sub 2^ ^sup -^, Cr(xx)^sub 2^(OH) (H^sub 2^O)^sup 2-^ and Cr(xx)^sub 2^(OH)^sub 2^ ^sup 3-^. The monohydroxo and dihydroxo species undergo CO^sub 2^ uptake and subsequent intramolecular carbonate ligand chelation independently, at rates which are readily distinguishable and are governed by the uptake rate constants k ^sub 1^ and k ^sub 2^ and chelation rate constants k ^sub 3^ and k ^sub 4^, respectively. Only the k ^sub 1^ values for oxalato, malonato and methylmalonato complexes could be calculated; k ^sub 1^ = 1084 and 1333 and 1650 mol^sup -1^ dm^sup 3^ s^sup -1^, respectively. The results obtained were compared with those obtained from other systems that have either cobalt(III), iridium(III) or rhodium(III) as central atoms.[PUBLICATION ABSTRACT]