The synthesis and structural characterization of the compounds Mes*AlCl2(THF) (1), Mes*AlCl2 (2), Mes*Al(H)Cl(THF) (3a), Mes*Al(H)Cl (4a), and (Mes*AlH2)(2) (5) (Mes* = 2,4,6-t-Bu(3)C(6)H(2)(-)) are described as well as those for two compounds 3b and 4b that are analogs of 3a and 4a but have H:Cl ratios that are less than 1:1. All compounds were characterized by H-1, C-13 NMR, and IR spectroscopy, and 1, 2, 3a, and 4b were additionally characterized by X-ray crystallography. Compound 1 is best synthesized by the reaction of [(THF)(2)LiH(3)AlMes*](2) (6) with 6 equiv of Me(3)SiCl. A more conventional route involving the addition of (THF)(2)LiMes* to 2 equiv of AlCl3 in toluene usually affords a mixture of 1 and AlCl3 . THF. Recrystallization of 1 from n-hexane results in a species that has less than 1 equiv of THF per Mes*AlCl2. The THF free complex 2 may be obtained in quantitative yield by heating 1 for 20 min at 90 degrees C under reduced pressure. Compound 3a may be obtained by treating a 1:1 mixture of Mes*Li(THF)(2) and LiAlH4 with 2 equiv of Me(3)SiCl or by the addition of slightly less than 4 equiv of Me(3)SiCl to 6. The THF can be removed from 3a by sublimation to give 4a. The related compounds 3b and 4b, which display an aluminum-bound H:CI ratio that is deficient in H, can be obtained by reactions with slightly mon than 2 equiv of Me(3)SiCl. Crystal data at 130 K with Cu K alpha (lambda = 1.541 78 Angstrom) radiation : 1, C22H37AlCl2O, a = 11.889(3) Angstrom, b = 9.992(3) Angstrom, c = 19.704(5) Angstrom, orthorhombic, space group Pca2(1), Z 4, R = 0.068 for 1556 (I > 2 sigma(I)) data; 2, C18H29AlCl2, a = 12.147(5) Angstrom, b = 18.042(6) Angstrom, c = 17.771(7) Angstrom, beta = 95.77(3)degrees, monoclinic, space group P2(1)/n, Z = 8, R = 0.032 for 4610 (I > 2 sigma(I)) data; 3a, C22H38AlClO, a = 16.887(7) Angstrom, b = 16.333(6) Angstrom, c = 8.739(3) Angstrom, beta = 101.41(3)degrees, monoclinic, space group P2(1)/c, Z = 4, R = 0.073 for 2752 (I < 2 sigma(I)) data; 4b, C18H29.64AlCl1.36, a = 12.077(3) Angstrom, b = 17.920(3) Angstrom, c = 17.634(5) Angstrom; beta = 95.21(2) Angstrom,monoclinic, space group P2(1)/n, Z = 8, R = 0.070 for 4261 (I > 2 sigma(I)) data.