Crystal structures of tris(tert-butyl)boron, -aluminum, -gallium, and -indium:: Nonplanarity of the AlC3 skeleton and evidence of inter- and intramolecular "Agostic" or hyperconjugative interactions

X-ray crystal structures determined at low temperatures (150-220 K) for the tris(tertbutyl) derivatives of boron, aluminum, gallium, and indium (M) reveal essentially monomeric molecular units throughout with consistently longer M-C bonds than in the corresponding monomeric trimethyl derivatives. Comparison of the three structures shows a significant strengthening of intermolecular M(...)beta-CH3 binding in the order M = B similar to Ga < In < Al, resulting in a distinctly nonplanar MC3 skeleton with the M atom displaced 0.25 angstrom above the plane of the quaternary C atoms in the case where M = Al. Tilting of the tert-butyl groups about the M-C bonds, which is concerted when M = B or Al, appears to reflect the influence of intramolecular hyperconjugation or "agostic" bonding.