Tris(tetramethylcyclopentadienyl)aluminum, Cp 3 Al (Cp = C 5 Me 4 H), reacts with t-butyl isocyanide to form (η 1 -C 5 Me 4 H) 2 Al{C( NtBu)--C( NtBu)(C 5 Me 4 H)} which results from the double insertion of the isonitrile at a single Al-cyclopentadienyl ring position. Double isonitrile insertion occurs in preference to a single insertion such that only unreacted Cp 3 Al and the title compound are observed when fewer than two equivalents of the isonitrile are consumed. The molecular structure of (η 1 -C 5 Me 4 H) 2 Al{C( NtBu)--C( NtBu)(C 5 Me 4 H)} has been determined by single-crystal X-ray diffraction. The aluminum center is tetracoordinate with two η 1 -cyclopentadienyl rings and the coupled isonitrile fragment bonding through a carbon of one iminoacyl group and a nitrogen of the other to form a four-membered azametallacycle.