Structural chemistry and magnetism of dicyclopentadienidehalides of lanthanides Part 8: terbiumdicyclopentadienidebromide [Tb(C5H5)2Br]2

A single-crystal X-ray structural investigation of Tb(C₅H₅)₂Br]₂ revealed the Sc(C₅H₅)₂Cl]₂-type structure, space group P2₁/c, with a = 1407.6(2) pm, b = 1644.7(2) pm, c = 1370.6(9) pm, β = 93.46(3)°, V = 3167(2) × 10⁶ pm³, D_c = 2.322 g cm^?3 and Z = 6 dimers (R = 0.036 for 4627 reflections with I > 3σ(I)). The metal centres have the pseudosymmetry C_2v. Magnetic susceptibility data show Curie-Weiss behaviour between 213 and 6 K with θ_p = ?4.5(3) K and a magnetic moment μ = 9.8(1) μ_B close to the Tb³⁺ free-ion value (9.72 μ_B). Below 6 K, deviations from Curie-Weiss behaviour are observed, and at 5.3 K a maximum in the susceptibility is detected which may be caused by intradimer antiferromagnetic spin coupling. The magnetic properties are compared with the prediction of various models, starting from cubic crystal fields and isotropic intramolecular exchange interactions, followed by extension to lower crystal field symmetry (orthorhombic) and anisotropic contributions to the spin coupling. However, a reasonable agreement between the measured and calculated data was not obtained. As in Gd(C₅H₅)₂Br]₂, the low-temperature behaviour is governed by effects which cannot be described by spin coupling models in the generally accepted form.