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]2.