Balancing Adduct Formation and Ligand Coupling with the Bulky Allyl Complexes [1,3-(SiMe₃)₂C₃H₃]₂M (M = Fe, Co, Ni)

Passage of CO at atmospheric pressure through solutions of A′₂M (M = Fe, Co, Ni; A′ = [1,3-(SiMe₃)₂C₃H₃]⁻) in hexanes produces the corresponding allyl complexes A′₂Fe­(CO)₂, A′Co­(CO)₃, and A′₂Ni­(CO), respectively. Although the iron and nickel species can be isolated as pure liquids, the cobalt complex is accompanied by the coupling product 1,3,4,6-tetrakis­(trimethylsilyl)-1,5-hexadiene. A′Co­(CO)₃ was independently prepared from the reaction of Co₂(CO)₈, A′Br, and PhCH₂N­(C₂H₅)₃⁺Cl^– in aqueous base. The IR stretching frequencies of A′₂Fe­(CO)₂ and A′Co­(CO)₃ are lower than those in the unsubstituted analogues, indicating that the trimethylsilated allyl ligand is a better electron donor than the parent version. Density functional theory calculations were performed on various conformations of the complexes, which reproduced the frequency-lowering effect of the trimethylsilyl groups. They also indicate that the thermodynamics of the formation of A′₂Ni­(CO) and the unknown (C₃H₅)₂Ni­(CO) are similar, suggesting that the thermal stability of the former is of kinetic origin. Oxidative coupling of the allyl ligands in A′₂Fe and A′₂Co is induced with I₂; this is different from the case with A′₂Ni, which has previously been shown to produce the mixed allyl halide complex [A′Ni­(μ-I)₂]<sub>2