The reaction of TbCl_3 with K[2,4-(CH_2)_2C_5H_5] at 0℃ in THF followed by crystalliza-tion at -90℃ led to a pale yellow hexagonal prismatic crystalline product [2,4-[CH_3)_2-η~5-C_5H_5]_(2-)Tb·1/2THF, which is highly sensitive to air and water and rapidly efflorescent at ambient temper-ature. The single Crystal X-ray diffraction data of the compound have been collected at low tem-perature (-60℃) and the crystal structure has been solved by heavy atom method. It belongs totriclinic system, space group Pl with lattice parameters a=8.477 ,b=12.583 ,c=12.858 ,a=118.08°,β=91.38°,γ=108.75°,V=1120.36 ~ 3 and Z=2. Least-squares refinement convergedto a final value R=0.043. The compound possesses an idealized G_(2h) symmetry、with three 2,4-dimethylpentadienyl ligands bound to the central terbium atom in a pentahapto mode (η~5). Eachunit cell contains two molecules of[2,4-(CH_3)_2-η~5-C_5H_5]_3Tb and one molecule of solvent THF, of whichthe role in the lattice has been discussed in detail. The reaction of TbCl_3 with K [2,4- (CH_2) _2C_5H_5] at 0 ℃ in THF followed by crystalliza tion at -90 ℃ led to a pale yellow hexagonal prismatic crystalline product [2,4- [CH_3) _2-η ~ 5-C_5H_5] _ (2-) Tb · 1 / 2THF, which is highly sensitive to air and water and rapidly efflorescent at ambient temper-ature. The single Crystal X-ray diffraction data of the compound have been collected at low tem It belongs to totriclinic system, space group Pl with lattice parameters a = 8.477, b = 12.583, c = 12.858, a = 118.08 °, β = 91.38 °, γ = 108.75 °, V = 1120.36 ~ 3 and Z = 2. Least-squares refinement converged to a final value R = 0.043. The compound possesses an idealized G_2h symmetry, with three 2,4-dimethylpentadienyl ligands bound to The central terbium atom in a pentahapto mode (η ~ 5). Eachunit cell contains two molecules of [2,4- (CH_3) _2-η ~ 5-C_5H_5] _3Tb and one molecule of solvent THF, of which the role in the lattice has been discusse d in detail.