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

Volumes of activation for the formation and homolysis of the transient complexes (hedta)Fe(III)-CO(2)(2-) and (hedta)Fe(III)-CH(3)(-) (HOCH(2)CH(2)N(CH(2)CO(2-))CH(2)CH(2)N(CH(2)CO(2-))(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 (M(n+)) and ligands (L) points out that (i) the reaction of M(n)L(m) with aliphatic radicals (R(*)) proceeds via an interchange ligand substitution mechanism, i.e. M(n)L(m) + R(*) --> L(m-1)M(n+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 M(n+1) = Cr(III), Co(III), Ni(III) and smaller for Fe(III). The volume of activation for the reaction (hedta)Fe(III)-CO(2)(2-) + CO(2)(*-) + 2H(+) --> Fe(II)(hedta)(H(2)O)(-) + CO + CO(2) was measured, and the results enable a tentative proposal for the nature of the transition state of this interesting reaction.