Formation and Decomposition of Iron-Carbon σ-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 (∊ = 620 ± 50 M^-1 cm^-1) and at 420 nm (∊ = 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 10⁵ M^-1. The kinetics of formation and decomposition and possible reaction mechanisms for these intermediates are discussed. We determined the specific rate constants for the formation of the iron-carbon σ-bonds (1.5 X 10⁷ and 6.2 X 10⁶M^-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^IIICO₂^2- with specific rate constants of 1.9 X 10⁷ and 4.5 X 10⁶ M^-1 s^-1 for L = NTA and HEDTA, respectively.