Cluster condensations. The dimerization of [Os3(CO)9(μ3-CO)(μ)3-S)]

UV irradiation of [Os3(CO)9(μ3-CO)(μ3-S)] (1) under a CO atmosphere has yielded the new hexaosmium cluster complex [Os 6(CO)20(μ3-S)(μ4-S)] (2) in 13% yield (30% based on the amount of 1 consumed). Compound 2 is a dimer of 1 based on its formula, but its structure is considerably different. Compound 2 was characterized by a single-crystal X-ray diffraction analysis. Crystal data: Os6S2O20C20·1/3CHCl3·H2O, M = 1759.0, rhombohedral, R3 (No. 148), a = 43.333(9), c = 10.203(1) A, V = 16592(9) A3, Dc = 3.17 g cm−3, Z = 18, λ(Mo-Kα) = 0.71069 A (graphite monochromator) μ = 202.4 cm−1. Methods: MITHRIL, difference Fourier, full-matrix least-squares. Refinement of 2754 reflections [F2 ⩾ 3σ(F 2)], (1 ⩽ 2θ ⩽ 43°), gave R and Rw values of 0.041 and 0.042, respectively. Data/parameters ratio = 11.1, highest peak in f inal difference Fourier = 1.60 e A−3. The molecule consists of one cluster of four metal atoms arranged in the form of a planar spiked- triangle, that is linked to a second group of two metal atoms by one quadruply-bridging sulphido ligand and one triply bridging sulphido ligand. The dimerization involved the transfer of one metal atom from one cluster to the second cluster. When 2 is photolysed under an atmosphere of nitrogen, it loses 3 mol of CO and condenses to the known cluster complex [Os6(CO)17(μ4-S)2] (3).