Hexamethyl[9]mercuracarborand-3, [(CH3)(2)C2B10H8Hg](3) (3), was isolated in 60% yield from the reaction of closo-1,2-Li-2-9,12-(CH3)(2)-1,2-C2B10H8 with mercuric acetate. The title compound was characterized by multinuclear NMR spectroscopy and its halide ion complexes by negative ion FAB mass spectrometry, Both 3 and its complexes are air-stable and soluble in poorly or noncoordinating solvents such as CH2Cl2 and toluene. Compound 3 crystallized from acetonitrile in the triclinic space group P (1) over bar with a = 13.362(6) Angstrom, b = 14.071(6) Angstrom, and c = 14.303(6) Angstrom, alpha = 105.15(1)degrees, beta = 95.77(1)degrees, and gamma = 110.14(1)degrees, V = 2383 Angstrom(3), and Z = 2. The final discrepancy indexes were R = 0.059, R(w) = 0.064 for 5987 independent reflections with (I > 3 sigma(I). Halide ion complexes of 3 were formed upon treatment of 3 with salts of the corresponding halide ions. The complexation behavior of 3 in acetone was studied by Hg-199 NMR spectroscopy, and the guest/host ratio of the resulting complexes has been established to be 1, 2, and 2 for chloride, bromide, and iodide, respectively.