Allyl complexes of heavy group 13 elements: Structure and bonding in [1,3-(SiMe3)2C3H3]3Ga

With the exception of triallylboron compounds, allyl derivatives of the group 13 elements are uncommon and none have been structurally authenticated. A substituted allyl complex of gallium, tris(1,3-bis(trimethylsilyl)allyl)gallium, [1,3-(SiMe3)2C3H3]3Ga, has been synthesized from the reaction of K[1,3-(SiMe3)2C3H3] and GaCl3 in THF and characterized with X-ray crystallography and NMR spectroscopy. In the solid state, the complex displays σ-bound ligands in a trigonal planar arrangement around the metal, with an average Ga–C distance of 1.980(5) Å. In solution at room temperature, the molecule is fluxional and the ligands appear as if symmetrically bound in a π-type fashion. The fluxionality is observed even at −75 °C. Density functional theory calculations indicate that the structure found in the solid state, with one allyl ligand antiparallel to the other two, is the lowest in energy, but an alternate arrangement with all three ligands parallel (C3 symmetry) is less than 3 kcal mol−1 higher in energy.

The first structurally characterized example of a gallium allyl complex, [1,3-(SiMe3)2C3H3]3Ga, has been synthesized from GaCl3 and K[1,3-(SiMe3)2C3H3]. The complex is fluxional on the NMR timescale, and X-ray crystal data reveals a σ-bonded structure, confirmed as the most stable geometry by density functional theory calculations.Figure optionsDownload as PowerPoint slide