Study of relative stabilities of the intermediates in catalytic carbon monoxide hydrogenation reactions: ab initio calculations on the model compounds [Fe(CO)2(PH3)2H2], [Fe(CO)2(PH3)2(CHO)H], [Fe(CO)2(PH3)2(CH2O)], and [Fe(CO)(PH3)2(CHO)2

Abstract

Ab initio calculations have been performed on the model compounds [Fe(CO)2(PH3)2H2], [Fe(CO)2(PH3)2(CHO)H], [Fe(CO)2(PH3)2(CH2O)], and [Fe(CO)(PH3)2(CHO)2] to investigate the relative stabilities of the intermediates in catalytic carbon monoxide hydrogenation reactions. The results suggest that the formaldehyde compound can be obtained via insertion of CO into the Fe–H bond and subsequent rearrangement of the hydrido–formyl intermediate, in agreement with a proposed mechanism of the Fischer–Tropsch synthesis. Partial geometry optimizations obtained by gradient calculations show that the migration of one hydrogen of the co-ordinated formaldehyde in [Fe(CO)2(PH3)2(CH2O)] to iron to give [Fe(CO)2(PH3)2(CHO)H] is not allowed, while the migration of the same hydrogen to an adjacent CO to yield [Fe(CO)(PH3)2(CHO)2] is energetically favoured.