AbstractThe reaction of TbCl3with K[2,4‐(CH3)2C5H5] at 0°C in THF followed by crystallization at −90°C led to a pale yellow hexagonal prismatic crystalline product [2,4‐[CH3)2‐η5‐C5H5]3‐Tb·1/2THF, which is highly sensitive to air and water and rapidly efflorescent at ambient temperature. The single crystal X‐ray diffraction data of the compound have been collected at low temperature (‐60°C) and the crystal structure has been solved by heavy atom method. It belongs to triclinic system, space groupP1 with lattice parameters α=8.477Å,b=12.583Å,c=12.858Å, α=118.08°, β=91.38°, γ=108.75°,V=1120.36Å3andZ=2. Least‐squares refinement converged to a final valueR=0.043. The compound possesses an idealizedC3hsymmetry with three 2,4dimethylpentadienyl ligands bound to the central terbium atom in a pentahapto mode (η5). Each unit cell contains two molecules of [2,4‐(CH3)2‐η5‐C5H5]3Tb and one molecule of solvent THF, of which the role in