Three Compounds containing equilateral-triangular trimolybdenum clusters capped on both sides by ethylidyne (CCH3) groups are described: (I) , [MO~(CCH~)~(~~CCH~)~(H~~)~] (CF3S03)2 (2), and [MO~(CCH~)~(~~CCH~)~(H~~)~](P-CH~C~H~S~~)~~~OH~~ (3). All three are obtained by suitable workup of the reaction mixture after refluxing M o ( C O ) ~ with a 1O:l v/v mixture of acetic acid and acetic anhydride. For the last two permanganate oxidation is used. The crystal structures of the three compounds have been solved and refined. In 1, where there are five electrons for the M o ~ cluster, the Mo-Mo bond orders are 5 / 6 and the mean Mo-Mo distance is 2.814 ( 5 ) A; this is about 0.06 A longer than that in a closely related cluster with Mo-Mo bond orders of 1.0. In 2 and 3, which each contain a bicapped M o ~ cluster with only four electrons, and hence a bond order of 2/3, the mean Mo-Mo distances are 2.883 (1) 8, and 2.892 (1) A, Le., about 0.13 A longer than for the Mo-Mo bonds of order 1.0. The cation in 1 has one unpaired electron showing an essentially constant g factor slightly above 2.0 in the range 4-300 K according to bulk susceptibility measurements. It shows no EPR spectrum but has a sharp 360-MHz IH NMR spectrum in which the resonance of the 02CCH3 protons is shifted to 14 ppm downfield while that of the CCH3 protons is broadened and shifted 85 ppm downfield. The cations in 2 and 3 appear to be paramagnetic, but the temperature dependence is unusual and still under study. The cation gives an 'H NMR spectrum with lines at 15.6 and 27.1 ppm with relative intensities of 3.3 to 1.0.