The novel five-coordinate aluminum adduct Me2HN.AIL (2) [wherein L = tris(2-oxy-3,5-dimethylbenzyl)amine] containing three six-membered rings has been characterized by spectroscopic and by X-ray means, This adduct of an alumatrane is the first structurally characterized monomeric alumatrane derivative, and unlike its parent alumatrane [Al(OCH2CH2)(3)N](x), 2 is monomeric in the gas, solution, and solid states. The X-ray molecular structure of 2 reveals a tricyclic cage moiety of C-3 symmetry. The aluminum geometry is a slightly distorted trigonal bipyramid in which, quite unexpectedly, the metal atom is located somewhat below the plane formed by three equatorial oxygens and its Al-N-tertiary bond is shorter than that in Me3N.AlH3.NMe3 (Heitsch, C. W.; Nordman, C. E.; Parry, R. W. Inorg. Chem. 1963, 2, 508).