Bimetallic bridging sulfur complexes having the general structure Cp*M(PR(3))S(2)IrCp* (3, R = Me, M = Ir; 12, R = Me, M = Rh; 15, R = p-tolyl, M = Lr) have been prepared. Heating these compounds leads to the metalla-sulfur cubane complexes (Cp*MS)(2)(Cp*IrS)(2) (10, M = Ir; 13, M = Rh). The observation that diiridium/dirhodium complex 13 is the only product obtained quantitatively from the thermolysis of 12 provides evidence that the dimeric units remain intact on the pathway to cubane formation rather than dissociating to monomeric units followed by reassembly into the tetramer. A mechanistic study of the cubane-assembly reaction was undertaken using complex 15. The reaction is first-order in [15] and zero-order in [PR(3)] when the concentration of phosphine is low. At high (flooding) [PR(3)], the rate is second-order in [15] and inverse second-order in phosphine concentration. These data are consistent with a mechanism in which a doubly unsaturated intermediate, Cp*MS(2)-IrCp*, is generated by reversible loss of phosphine and then in a second bimolecular step dimerizes to form the cubane. The reactions of 3 with methyl bromide, carbon monoxide, and tert-butyl isocyanide are also described. The single crystal X-ray structures of Cp*Ir(PMe(3))S(2)IrCp* (3), Cp*Ir(CN-t-Bu)(mu-S)(2)Ir(CN-t-Bu)Cp* (9), and (Cp*IrS)(4) (10) have been determined. Complex 3 crystallized in the monoclinic system, space group P2(1) with a 8.556(1) Angstrom, b = 14.093(2) Angstrom, c = 10.977(2) Angstrom, beta = 103.28(1)degrees, and Z = 2. Refinement by standard least-squares techniques gave final residuals R = 0.039 and R(W) = 0.040. Complex 9 crystallized in the monoclinic system, space group C2/c with a = 18.798(5) Angstrom, b = 9.554(2) Angstrom, c = 18.219(3) Angstrom, beta = 90.34 degrees, and Z = 8. Refinement by standard least-squares techniques gave final residuals R = 0.034 and R(W) = 0.040. Complex 10 crystallized in the orthorhombic system, space group I ($) over bar 4 with a = 12.026(3) Angstrom, b = 12.026(3) Angstrom, c = 14.844(3) Angstrom, and Z = 2. Refinement by standard least squares techniques gave final residuals R = 0.031 and R(W) = 0.039.