Octakis(trifluorophosphine)dirhodium, Rh2(PF3)8, reacts with di-tert-butylacetylene, t-Bu2C2, to give a yellow, crystalline complex Rh2(PF3)5(t-Bu2C2). This is believed to contain a bridging acetylene moiety lying over and perpendicular to Rh(PF3)3 and Rh(PF3)2 groups which are joined by a rhodium-rhodium double bond. 19F NMR spectra in the temperature range 24-116° show that the PF3 groups on each rhodium atom undergo intramolecular exchange with identical activation energies but without exchanging between the rhodium atoms, i.e., the ligands rotate about the metal-metal double bond. The free energy of activation ΔG‡ is estimated from line shape analysis to be 15.1 ± 0.5 kcal/mol, being independent of temperature. The complex Rh2(PF3)5(t-Bu2C2) reversibly adds PF3 to give Rh2(PF3)6(t-Bu2C2) which undergoes both intramolecular and intermolecular exchange of PF3. The free energy of activation ΔG‡ for the intramolecular process, estimated as 13.3 kcal/mol, is considerably higher than the values found for other Rh2(PF3)6(ac) complexes, probably owing to steric hindrance by the bulky tert-butyl groups. This feature is probably also responsible for the relative ease of intermolecular PF3 exchange in Rh2(PF3)6(t-Bu2C2) and for the stability of Rh2(PF3)5(t-Bu2C2).