Electrophilic diiodination reactions of icosahedral closo-1,2-C2B10H12 and closo-1,7-C2B10H12 using 2 molar equiv of iodine monochloride in the presence of catalytic amounts of aluminum chloride yielded the corresponding closo-9,12-I-2-1,2-C2B10H10 and closo-9,10-I-2-1,7-C2B10H10 complexes in excellent yields. Palladium-mediated cross-coupling reactions of these diiodocarboranes with a variety of alkyl, aralkyl, and aryl Grignard reagents were reinvestigated, and it was demonstrated that the addition of copper(I) iodide as a cocatalyst is crucial to the success of this reaction. A reaction mechanism involving the initial formation of binary organocopper species (RCu; R = alkyl, aralkyl, aryl) followed by the reaction of (sigma-carboranyl)palladium iodides (L(2)Pd(CbI)I and L(2)Pd(CbR)I; L = PPh(3), Cb = closo-1,2C(2)B(10)H(10), closo-1,7-C2B10H10) via a four-centered transition state is proposed. The molecular structures of closo-9,10-I-2-1,7-C2B10H10, 1, and closo-9,10-(C6H5)(2)-1,7-C2B10H10, 10, have been determined. Crystallographic data are as follows : for 1, monoclinic, space group P2(1)/c, a 13.2719(6) Angstrom, b = 14,9718(6) Angstrom, c 13.4734(6) Angstrom, beta = 114,211(1)degrees, V = 2441.7(3) Angstrom(3), Z = 8, rho(calcd) = 2.15 g cm(-3) R = 0.043, R(w) = 0.064; for 10, tetragonal, space group I4(1)/a, a = 9.446(2) Angstrom, c 39.949(8) Angstrom, V = 3564.1(1) : Angstrom(3), Z = 8, rho(calcd) = 1.10 g cm(-3), R = 0.056, R(w) = 0.063.