The synthesis and structural characterization of the first three-coordinate dimeric magnesium thiolate [Mg(STriph)(2)](2), 1 (Triph = 2,4,6-Ph(3)C(6)H(2)), and of the four-coordinate etherates Mg{SMes*}(2)(Et(2)O)(2), 3 (Mes* = 2,4,6-t-Bu(3)C(6)H(2)), and Mg{SeMes*}(2)(THF)(2), 4, are described. We also report the synthesis and characterization of the dimeric, presumably three-coordinate Mg thiolate [Mg(SMes*)(2)](2), 2. Compound 1 is prepared in the reaction of Mg(n-Bu/s-Bu)a or [Mg(N(SiMe(3))(2))(2)](2) with 2 equiv of the bulky thiol HSTriph in toluene. The compound exhibits distorted trigonal-planar geometry at the metal center. Compound 2 is accessible by a reaction analogous to that described for 1, using HSMes* instead of HSTriph. Although crystallographic data for 2 are not available, the spectroscopic characterization indicates a dimeric species with a distorted trigonal planar environment at Mg. Compound 3 is synthesized by reacting Mg(n-Bu/s-Bu)(2) or alternatively [Mg{N(SiMe(3))(2)}(2)](2) with 2 equiv of the sterically demanding HSMes* in a hexane/diethyl ether mixture. This compound can also be prepared by the reaction of MgBr2 with 2 equiv of the corresponding lithium thiolate in diethyl ether. Mg(SeMes*)(2)(THF)(2) is prepared by a reaction sequence similar to that described for 3, by the addition of 2 equiv of freshly prepared HSeMes* in a mixture of hexane and tetrahydrofuran. The reaction of MgBr2 with 2 equiv of lithium selenolate in THF affords 4 in good yield. All compounds were characterized by H-1 NMR and IR spectroscopy, as well as elemental analysis; species 1, 3, and 4 were also examined by X-ray crystallography. Crystal data with Mo K alpha (lambda = 0.710 73 Angstrom) at 213 K (for 1 and 3) or CuK alpha (lambda = 1.54178 Angstrom) at 130 K for 4 are as follows : 1, a = 12.055(2) Angstrom, b = 18.341(4) Angstrom, c = 16.635(3) Angstrom, beta = 98.75(3)degrees, V = 3635(2) Angstrom(3), Z = 2, monoclinic, space group P2(1)/n, 1764 (I > 20(I)) data, R = 0.076; 3, a = 11.083(2) Angstrom, b = 13.325(3) Angstrom, c = 16.466(3) Angstrom, alpha = 93.58(3)degrees, beta = 95.00(3)degrees, gamma = 106.41(3)degrees, V = 2314.1(12) Angstrom(3), Z = 2, triclinic, space group P ($) over bar 1, 6028 (I > 3 sigma(I)) data, R = 0.072; 4, a = 16.956(7) Angstrom, b = 16.425(5) Angstrom, c = 17.915(6) Angstrom, beta = 117.30(2)degrees, V = 4434(2) Angstrom(3), Z = 4, monoclinic, space group P2(1)/n, 4118 (I > 2 sigma(I)) data, R = 0.059.