High-pressure pulse-radiolysis study of the formation and decomposition of complexes with iron-carbon σ bonds: Mechanistic comparison for different metal centers

Volumes of activation for the formation and homolysis of the transient complexes (hedta)Fe^III-CO₂^2- and (hedta)- Fe^III-CH₃^- (HOCH₂CH₂N(CH₂CO₂^-) CH₂CH₂N (CH₂CO₂^-)₂ = hedta) were determined using high-pressure pulse-radiolysis techniques. A comparison of the results with those for analogous complexes with other central transition-metal cations (M²⁺) and ligands (L) points out that (i) the reaction of MⁿL_m with aliphatic radicals (R^•) proceeds via an interchange ligand substitution mechanism, i.e. MⁿL_m + R^• → L_m-1Mⁿ⁺¹-R + L, (ii) the homolysis of the metal-carbon bonds naturally follows the same mechanism, and (iii) the volume of activation for the homolysis reaction depends strongly on the nature of the central cation, i.e. larger for Mⁿ⁺¹ = Cr^III, Co^III, Ni^III and smaller for Fe^III. The volume of activation for the reaction (hedta)Fe^III-CO₂^2- + CO₂^•- + 2H⁺ → Fe^II(hedta)(H₂O)^- + CO + CO₂ was measured, and the results enable a tentative proposal for the nature of the transition state of this interesting reaction.