Theoretical studies of inorganic and organometallic reaction mechanisms. 16. Oxidative promotion of the migratory insertion of carbon monoxide in cyclopentadienylmethyldicarbonyliron (II)

Density functional methodology has been used to study the mechanistic detai ls of the migratory insertion of CO into the transition metal-alkyl sigma b ond of the Fe cyclopentadienyl complexes CpFe(CO)(2)CH3 and [CpFe(CO)(2)CH3 ](+) in the presence of phosphine, PH3. Geometries, energies, and vibration al frequencies of reactants, molecular complexes, transition states, interm ediates, and products are determined. Methyl migration with little PH3 part icipation is the rate-determining step for both the neutral system and the cation system. Addition of PH3 stabilizes the metastable intermediates [CpF e(CO)(COCH3)- -PH3](0/+) and the products [CpFe(CO)(COCH3)(PH3)](0/+). The calculated activation energies of the rate-determining steps are 17.8 and 7 .5 kcal/mol for the neutral and cation reaction, respectively. The signific ant difference in these barriers is not due to the oxidatively promoted rea ction utilizing a significantly different mechanism, but is interpreted in terms of methyl migration in the cation as having more radical character in duced by the unpaired electron on Fe. For the reverse of the neutral migrat ory-insertion reaction, the overall activation energy is 24.3 kcal/mel. Thu s, as is observed, the neutral reaction will be reversible only at higher t emperatures. The overall exothermicities are -4.6 and -29.4 kcal/mol for th e neutral and the cation reaction, respectively. The eta(2)- acyl intermedi ates {CpFe(CO)(COCH3)}(0/+) are more stable by 6.1 and 11.1 kcal/mol than t he eta(2)-methyl (agostic) intermediates, respectively. However, these eta( 2)-acyl intermediates are not involved in the favored pathway of either rea ction due to higher barriers for their formation. Natural bond orbital anal yses for the molecular complexes [CpFe(CO)(2)(CH3)- -PH3](0/+) show a weak donor-acceptor interaction, where the phosphine behaves as acceptor in the neutral complex but as a donor in the cation.