Trimethylsilylated Allyl Complexes of Nickel. The Stabilized Bis(π-allyl)nickel Complex [η³-1,3-(SiMe₃)₂C₃H₃]₂Ni and Its Mono(π-allyl)NiX (X = Br, I) Derivatives

Reaction of 2 equiv of K[1,3-(SiMe₃)₂C₃H₃] with NiBr₂(dme) in THF at −78 °C produces the orange π-allyl complex [1,3-(SiMe₃)₂C₃H₃]₂Ni (1). Unlike the pyrophoric (C₃H₅)₂Ni, the trimethylsilylated derivative only slowly decomposes in air (from hours to days). Both eclipsed (1a) and staggered (1b) conformations are found in solution; the eclipsed form irreversibly converts to the thermodynamically more stable staggered conformation when heated above 85 °C. Single-crystal X-ray structures obtained for both 1a and 1b confirm that the allyl ligands are bound in a trihapto manner to the metals and that trimethylsilyl substituents are in syn, anti arrangements. Density functional theory calculations performed on the bis(allyl)nickel complexes indicate that the substituents exert little effect on the basic metal−ligand geometries. Trimethylphosphine is converted to tetramethyltetraphosphane, (MeP)₄, on reaction with 1. In toluene, 3-bromo-1,3-bis(trimethylsilyl)propene reacts with (COD)₂Ni to produce the dimeric purple complex {[1,3-(SiMe₃)₂C₃H₃]NiBr}₂ (2a). Both NMR and X-ray crystallographic data establish that the allyl ligands are staggered and that the trimethylsilyl substituents are in a syn, syn conformation. NMR data indicate that the reaction of one equivalent of 1 with Br₂ in benzene produces an analogous complex (2b) with the allyl ligand substituents in a syn, anti configuration. When 1 equiv of 1 is treated with I₂ in hexanes, the dark red dimeric complex {[1,3-(SiMe₃)₂C₃H₃]NiI}₂ (3) is formed. Its X-ray crystal structure demonstrates that both eclipsed (3a) and staggered (3b) allyl conformation are present. The trimethylsilyl groups on the allyl ligands are in syn, anti arrangements in the two forms.