The acetylene complexes Rh2(PF3)6(ac) (ac = C6H5C2C6H5, C6H5C2CH3, or P-NO2C6H4C2CO2C2H5) react with monodentate tertiary phosphines and arsines (L) to give disubstitution products Rh2(PF3)4L2(ac) and with ophenylenebis(dimethylarsine), o-C6H4[As(CH3)2]2 (diars), to give Rh2(PF3)2(diars)2(ac) (ac = C6H5C2C6H5 or C6H5C2CH3). The reaction with monodentate ligands is reversed by the action of PF3. The crystal and molecular structure of the diphenylacetylene-bis(triphenylphosphine) derivative [L = P(C6H5)3; ac = C6H5C2C6H5] has been determined by three-dimensional X-ray structural analysis using 6834 independent reflections, with I/σ (I) ≥ 3.0, collected by counter methods. The complex crystallizes in the triclinic space group P1 (Ci1, No. 2) with a = 21.186 (9) Å, b = 12.994 (5) Å, c = 12.942 (5) Å, α = 114.10 (2)°, β = 64.36 (2)°, γ = 115.33 (2)°, and Z = 2. The structure was solved by conventional heavy-atom methods and was refined by block-diagonal least-squares methods to final weighted and unweighted R factors of 0.046 and 0.042, respectively. The molecule is structurally similar to cobalt-carbonyl-acetylene complexes such as Co2(CO)6(μ-C6H5C2C6H5) and consists of two [Rh(PF3)2[P(C6H5)3]] moieties bridged by diphenylacetylene, the C=C bond of which is above and approximately normal to the Rh-Rh axis. The triphenylphosphine groups are on the same side of the molecule as the bridging acetylene, and the Rh-Rh distance [2.740 (1) Å] is in the range expected for a rhodium-rhodium single bond. The 19F NMR spectra of the disubstitution products containing unsymmetrical acetylenes, Rh2(PF3)4L2(RC2R′), show signals due to PF3 groups in two different environments at or just below room temperature, but on raising the temperature intramolecular PF3 exchange takes place. This process appears to be initiated by dissociation of the tertiary phosphine or arsine (L). The free energies of activation, ΔG‡, for the intramolecular process have been estimated by approximate line shape analysis of the 19F NMR spectra and are higher for the tertiary phosphine than for the tertiary arsine derivatives. In the one case studied, the electron-withdrawing acetylene ethyl p-nitrophenylpropiolate gives rise to a higher AG* than either diphenylacetylene or 1-phenylpropyne. Possible mechanisms involving the fluxional behavior of a coordinately unsaturated intermediate are discussed. A 1:1 mixture of Rh2(PF3)4[As(CH3)(C6- H52]2(C6H5C2CH3) and Rh2(PF3)6C6H5C2CH3) undergoes intermolecular PF3 exchange in the temperature range 50-82°, perhaps via a pentakis intermediate Rh2(PF3)5[As(CH3)(C6H5)2(C6H5C2CH3). The diars derivatives Rh2(PF3)2(diars)2(ac) show temperature-independent 1H and 19F NMR spectra in the range -90° to room temperature, but their structure could not be determined unambiguously.