Formation and decomposition of iron-carbon sigma-bonds in the reaction of iron(II)-poly(amino carboxylate) complexes with CO2- free radicals. A pulse radiolysis study

The reactions of ferrous poly(amino carboxylate) complexes with CO2- were studied at neutral pH in aqueous solutions. The results indicate that complexes with metal-carbon σ-bonds are formed as unstable intermediates with maximum absorption bands at 405 nm (epsilon = 620 +/- 50 M-1 cm-1) and at 420 nm (epsilon = 950 +/- 100 M-1 cm-1) for the NTA and HEDTA complexes, respectively. The transient complexes are in equilibrium with the ferrous poly(amino carboxylate) complex and CO2- free radical. The stability constants for the complexes of CO2- with ferrous NTA and HEDTA complexes were determined to be larger than 105M-1. The kinetics of formation and decomposition and possible reaction mechanisms for these intermediates are discussed. The authors determined the specific rate constants for the formation of the iron-carbon σ-bonds (1.5 x 107 and 6.2 x 106 M-1 s-1 for NTA and HEDTA, respectively) as well as for the homolytic cleavage of the metal-carbon bond (140 and 25 s-1 for NTA and HEDTA, respectively). It is shown that the ferrous poly(amino carboxylate) complexes induce the disproportionation of the CO2- free radicals to form CO and CO2 via CO2- + L-Fe/sup III/CO22- with specific rate constants of 1.9 x 107 and 4.5 x 106 M-1 s-1 for L = NTA and HEDTA, respectively