Irradiation of toluene solutions of the complexes [M(2)Cp(2)(CO)(6)] (M = Mo, W; Cp = eta(5)-C5H5) with UV-visible light at -35 degrees C gives the new trinuclear compounds [M(3)(mu-eta(1),eta(5)-C5H4)Cp(2)(CO)(6)] in moderate (M = Mo) or medium (M = W) yields, along with the known complexes [MH(CO)(3)Cp], [W-2(mu-H)(2)(CO)(4)Cp(2)], and [M(2)Cp(2)(CO)(4)]. Similar treatment of equimolar mixtures of [W(2)Cp(2)(CO)(6)] and [Mo(2)Cp(2)(CO)(4)] gives a complex mixture containing (in order of decreasing yield) the mixed-metal compounds [Mo2W(mu-eta(1),eta(5)-C5H4)Cp(2)(CO)(6)], [MoW2(mu-eta(1),eta(5)-C5H4)Cp(2)(CO)(6)] (two isomers), and [W-3(mu-eta(1),eta(5)-C5H4)Cp(2)(CO)(6)], along with [WH(CO)(3)Cp] as dominant hydride species. The structure of the new trinuclear compounds has been determined by single crystal X-ray diffraction studies on those species with W-3, Mo2W, and W2Mo metal cores. Crystals of the W2Mo compound were found to be an equimolar mixture of both isomers detected in solution, so that two of the three metal positions are best described as 0.5 Mo + 0.5 W. Otherwise, the three crystal structures are virtually identical. These 46 electron molecules exhibit a V-shaped metal core (angle ca. 105 degrees) with a short (ca. 2.52 Angstrom) and a long (ca. 3.12 Angstrom) intermetallic separation, the central position being always occupied by a tungsten atom. Each of the metal atoms involved in the short bond carries out a cyclopentadienyl group and a carbonyl ligand which bridges that bond in a linear semibridging fashion. The outer metal atom has also a terminal carbonyl group. Finally, the third metal atom bears a mu-eta(1),eta(5)-cyclopentadienylidene ligand, sigma-bonded to the central tungsten atom, and three terminal carbonyl groups. Variable-temperature H-1 NMR spectroscopy reveals that all trinuclear compounds exhibit fluxional behavior in solution, which is thought to result from two independent rearrangements. In addition, the trimolybdenum complex undergoes a slower dynamic process which implies a proton exchange between cyclopentadienylidene and cyclopentadienyl ligands in the molecule, as suggested by 2D EXSY experiments.