The synthesis and characterization of several sterically crowded aluminum and gallium thiolates are described. Ther major reason for these studies was the investigation of the possible occurrence of pi-bonding in Al-S and Ga-S bonds and the determination of the steric requirements for the isolation of monomeric heavier main group 3 thiolates in the solid state. The compounds examined were RA1(SMes*)(2) (R = n-Bu, 1; t-Bu, 2), Mes*(2)GaSR (Mes* = 2,4,6-t-Bu(3)C(6)H(2), R = Me, 3; Ph, 4), n-BuGa(SMes*)(2), (5) (t-Bu(2)AlSTrip)(2) (6) (Trip = 2,4,6-i-Pr3C6H2), (THF)Al(STrip)(3) (7). They were characterized by X-ray crystallography (1-3, 5-7) and by NMR and IR spectroscopy. The data indicate that the M-S, p-p, pi-bonding is weak and has an upper limit of 8-9 kcal mol(-1). Restricted rotation around an M-S bond was detected only in the cases of Mes*(2)GaSMe (3) and Mes*(2)GaSPh (4). Crystal data at 130 K with Mo K alpha (lambda = 0.71069 Angstrom A) (2, 5-7) or Cu K alpha (lambda = 1.54178 Angstrom A) (1, 3) radiation; 1, n-BuAl(SMes*)(2), C40H67AlS2, a = 18.563(5) Angstrom A, b = 27.171(11) Angstrom A, c = 32.089(4) Angstrom A, orthorhombic, Z = 16 (two independent molecules), space group Pbca, R = 0.087 for 6746 (I > 2 sigma(I)) data; 2, t-BuAl(SMes*)(2), C40H67AlS2, a = 17.375(9) Angstrom A, b = 27.982(10) Angstrom A, c = 17.868(8) Angstrom A, beta = 112.29(2)degrees, Z = 8 (two independent molecules), monoclinic, space group P2(1)/c, R = 0.082 for 8425 (I > 2 sigma(I)) reflections; 3, Mes*(2)GaSMe, C37H61GaS, a = 33.654(8) Angstrom A, b = 10.433(4) Angstrom A, c = 20.258(8) Angstrom A, Z = 8, orthorhombic, space group Pbcn, R = 0.066 for 3164 (I > 2 sigma(I) reflections; 5, n-BuGa(SMes*)(2), C40H67GaS2, a = 18.521(8) Angstrom A, b = 27.342(10) Angstrom A, c = 32.046(12) Angstrom A, orthorhombic, Z = 16 (two independent molecules), space group Pbca, R = 0.141 for 3297 (I > 2 sigma(I) reflections; 6, (t-Bu(2)AlSTrip)(2), C46H82Al2S2, a = 20.820(8) Angstrom A, b = 14.598(6) Angstrom A, c = 16.118(4) Angstrom A, Z = 4, orthorhombic, space group Pna2(1), R = 0.062 for 2469 (I > 2.5 sigma(I)) data; 7, (THF)Al(STrip)(3) . 0.5C(6)H(14), C52H84AlOS3, a = 15.589(6) Angstrom A, b = 13.622(5) Angstrom A, c = 26.308(12) Angstrom A, beta = 99.88(2)degrees, Z = 4, monoclinic, space group P2(1)/c, R = 0.075 for 5697 (I > 3 sigma(I)) data.