Vinylidene carbonylation at a manganese-iron complex: A density functional study of mechanism

Treating the phenylvinyldene manganese complex Cp(CO)₂MnC¹C²HPh, 1, with Fe(CO)₄] yields the binuclear μ-vinylidene complex Cp(CO)₂MnFe(μ-C¹C²HPh)(CO)₄, 2, that further isomerizes to the carbonylated product η⁴-Cp(CO)₂MnC¹(CO)C²HPh]Fe(CO)₃, 3. In a computational study of the mechanism using a hybrid density functional method, we considered two stereoisomers for species 2 and 3 where the phenyl group at center C² is oriented in cis (E form) or trans (Z form) fashion to the Cp(CO)₂Mn unit. Isomers 2E and 2Z were calculated to be degenerate whereas the experimentally detected species 3E is 8 kcal/mol more stable than its isomer 3Z. The two-step pathway 1 → 2Z → 3E was calculated to be the lowest-energy route with the highest activation barrier at 12 kcal/mol. The activation energy of the alternative single-step pathway 1 → 3E is 19 kcal/mol. We rationalized the stabilization of the ground state of 3E and the transition states leading to or starting from isomer 2Z as conjugation effect between the Mn-CC metallaallene fragment and the co-planar phenyl ring.