Microwave heating is used to synthesize compounds of the type Os 2 (μ-O 2 CR) 2 (CO) 6 , including the first example with aromatic carboxylate ligands. The one-step preparation of these complexes is straightforward and does not require an air-free reaction environment. The compounds for which R = Me (1) or Et (2) are produced in higher yield than previously reported by the microwave irradiation of Os 3 (CO) 12 for less than 15 min in acetic or propionic acid, respectively. This method may also be used to prepare the new compound Os 2 (μ-O 2 CH) 2 (CO) 6 (3) by reaction with formic acid, but a higher yield is obtained when 1,2-dichlorobenzene is used as a co-solvent. Irradiating a mixture of Os 3 (CO) 12 and excess benzoic acid in 1,2-dichlorobenzene gives another new complex Os 2 (μ-O 2 CC 6 H 5 ) 2 (CO) 6 (4). X-ray crystallographic analyses of 3 and 4 reveal molecular structures similar to that of 1 with approximate C 2v symmetry. When the molar ratio of benzoic acid to Os 3 (CO) 12 is 1:1, then the major product is the trinuclear cluster Os 3 (μ-H)(μ-O 2 CC 6 H 5 )(CO) 10 (5) instead of a dinuclear compound. This represents the first instance in which a cluster of this type has been produced directly from Os 3 (CO) 12 instead of through a multi-step procedure.