bidentate coordination of unsaturated complex fragments. To our knowledge the only other bridge-forming S4 ligands are found in copper([) polysulfide clusters, in which two copper centers are each bridged by a terminal member of the S, chain.r31
The formation of 1 proceeds via several intermediates. When the dark blue solution of [Cp~Rh,(CO),] in T H F is allowed to react with 118 Ss at room temperature, the color changes to brown within 15 minutes, and the red-violet 2 can be isolated in 46% yield by column chromatography (A1203, toluene)."] The RhzS three-membered ring in 2 can be expanded to a four-membered ring by addition of a further S atom. However, there is only IR spectroscopic proof available (vco frequency at 2015 cm-') for the existence of the resulting complex 3. The Se complex analogous to 3, on the other hand, can be i~olated. '~] When, however, the reaction of [CpfRh,(CO),] with sulfur is carried out at -25 "C instead of at room temperature, about equal amounts of 1 and 4 can be isolated. The symmetrical compound"] 4 may contain two RhS, chelate rings linked together by a Rh-Rh bond and a p-CO group.''] 4 slowly transforms into 1 in solution. The structural changes on going from 3 to 4 are indicated by weak vco frequencies temporarily occurring at 1785 and 1980 cm-', which were observed between -80 and -60°C. Unfortunately, these absorptions disappear in favor of those of 1 and 4 at higher temperatures, so that they cannot be assigned to specific structures. However, it cannot be ruled out that the monosulfide bridges in 2 and 3 are substitution-labile towards larger sulfur units, e.g. S:". An indication in this direction is that the Se analogues of 2 and 3 can be converted into 1 with S8. 171 Although rhodium sulfides are of interest as hydrodesulfurization catalysts,lgl 1 is only the third structurally characterized rhodium polychalcogenide complex, after [CpfRh2Se5]['01 and [Rh(S5)3]3e.[' 'I From the relatively high number of S-S bonds, a considerable reaction potential can be expected for 1. Consistent with this prediction three irreversible oxidation waves were found at El,,= +0.53, + 1.38 and 1.60 V in a cyclovoltammetric experiment (CH2C12, standard calomel electrode).