Dimethyl acetylenedicarboxylate, CH3O2CC2CO2CH3, and methyl propiolate, HC2CO2CH3 (ac), react with dirhodium octakis(trifluorophosphine), Rh2(PF3)8 below room temperature to give red complexes of empirical formula Rh2(PF3)5(ac)2 (1, ac = CH3O2CC2CO2CH3; 2, ac = HC2CO2CH3). Above room temperature explosive polymerization of the acetylenes ensues. The complexes are assigned a metallocyclopentadiene structure (F3P)3Rh(μ-C4X2Y2)Rh(PF3)2 (X = Y = CO2CH3; X = CO2CH3, Y = H) on the basis of 1H and 19F NMR spectra and a preliminary single-crystal X-ray study of the bis(triphenylphosphine) complex of 2, the acetylene units in 2 being arranged in a “head-to-tail” manner. The Rh(PF3)3 unit in 1 and 2 shows fluxional behavior in the 19F NMR spectra while the Rh(PF3)3 unit remains essentially unchanged, and an intramolecular tritopal rearrangement is suggested. Line shape analysis gives the free energy of activation (ΔG‡) of the process in 1 and 2 as ca. 10.5 kcal/mol.