Ligands incorporating two sulfur atoms directly connected to the carbon atoms of the unit 7,8-dicarba-nido-undecaborate(1-) react with [IrCl(CO)CPPh(3))(2)] in the presence of AgNO3 in ethanol to form stable metal complexes. Each Ir(I) atom is four-coordinated to two phosphorus atoms of the PPh(3) groups and the two sulfur atoms of the 7,8-dicarba-nido-undecaborate(1-)-containing ligand. The influence of the S,S’ outer chain was studied by reacting these Ir(I)-dithioether complexes with O-2. The complex with a short chain ligand (six element mesocycle) decomposed, complexes with seven or eight chain atoms in the ligand did not react, and those with a noncyclic ligand reacted by oxidative addition forming a stable complex. The crystal structure of [IrO2(L(s2n)) (PPh(3))(2)] Me(2)CO verified unambiguously the coordination of this peroxo complex. The compound C43H51B9IrP2O3S2, M(r) = 1031.45, crystallizes in the monoclinic space group P2(1)/n (No. 14), with a = 15.369 (1) Angstrom, b = 19.481(2) Angstrom, c = 16.027(4) Angstrom, beta = 101.92(1)degrees, V = 4695(1) Angstrom(3), Z = 4, R = 0.039, and R(w) = 0.044.