The indenyl effect has been introduced to pentadienyl (“open cyclopentadienyl”) chemistry by preparation of the phenylmethallyl (“open indenyl”) ligand oInd^Me. The reaction of its potassium salt K(oInd^Me) with [(η⁵-C₅Me₅)RuCl]₄ afforded the sandwich complex [(η⁵-C₅Me₅)Ru(η⁵-oInd^Me)] (1), which, upon treatment with PMe₃, CO, and 2,6-dimethylphenyl isocyanide (CN-o-Xy), easily underwent η⁵–η³ hapticity interconversion and formed the complexes [(η⁵-C₅Me₅)Ru(η³-oInd^Me)(L)] (2, L = PMe₃; 3, L = CO; 4, L = CN-o-Xy). In these complexes, the η³-bound phenylmethallyl ligand adopts an anti-conformation with regard to the relative positions of the phenyl and methyl substituents. For the PMe₃ complex anti-2, slow conversion to the syn-isomer was observed, and this equilibrium reaction was monitored by NMR spectroscopy at 50 °C to determine a first order rate constant of k_323 K = 6.57 × 10⁻⁶ (± 0.02 × 10⁻⁶) s⁻¹ and an activation barrier of ΔG° = 26.8 kcal mol⁻¹. DFT calculations afforded a stabilization of syn-2 and syn-3 by ΔG₂₉₈ = −1.54 and −1.74 kcal mol⁻¹ over the respective anti-isomer.