Pentadienyl-Metal-Phosphine Chemistry. 8.¹Synthesis and Solution Dynamics of (n³-2,4-Dimethylpentadienyl)Rh(PR₃)2 Complexes

(2,4-Dimethylpentadienyl)Rh(PR₃)₂(R = Me, Et, i-Pr, i-Bu) complexes have been synthesized by reacting potassium 2,4-dimethylpentadienide-tetrahydrofuran with [(PR₃)₂RhCl]₂. These complexes are all ground-state 16e species in which the dimethylpentadienyl ligand (2,4-Me₂pd) is bonded to the rhodium center in an n3-fashion. The dominant 2,4-Me₂pd geometry in each case is anti (U-shaped), but small quantities of the syn (W-shaped) isomers have been detected. In solution, the anti isomers undergo a fluxional process, which exchanges the ends of the 2,4-Me₂pd ligand via 18e (n⁵-2,4-Me₂pd)Rh(PR₃)₂intermediates. The free energy of activation for the exchange process, AG*, increases as the cone angle of the phosphine ligand increases. At higher temperatures, equilibration of anti and syn isomers is observed by NMR, and this process is believed to involve intermediates with sickle-shaped 2,4-Me₂pd ligands.