Intramolecular and Intermolecular Ligand Exchange in Trifluorophosphinerhodium Complexes of Di-tert-butylacetylene. Rotation of Trifluorophosphine Groups about a Rhodium-Rhodium Double Bond

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).