STRUCTURAL CHEMISTRY AND MAGNETISM OF DICYCLOPENTADIENIDEHALIDES OF LANTHANIDES .8. TERBIUMDICYCLOPENTADIENIDEBROMIDE [TB(C5H5)2BR]2

A single-crystal X-ray structural investigation of [Tb(C5H5)2Br], reve aled the [Sc(C5H5)2Cl]2-type structure, space group P2(1)/c, with a = 1407.6(2) pm, b = 1644.7(2) pm, c = 1370.6(9) pm, beta = 93.46(3)degre es, V = 3167(2) x 10(6) pm3, D(c) = 2.322 g cm-3 and Z = 6 dimers (R = 0.036 for 4627 reflections with I>3sigma(I)). The metal centres have the pseudosymmetry C2v. Magnetic susceptibility data show Curie-Weiss behaviour between 213 and 6 K with theta(p) = - 4.5(3) K and a magneti c moment mu = 9.8(1) mu(B) close to the Tb3+ free-ion value (9.72 mu(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 cau sed by intradimer antiferromagnetic spin coupling. The magnetic proper ties are compared with the prediction of various models, starting from cubic crystal fields and isotropic intramolecular exchange interactio ns, followed by extension to lower crystal field symmetry (orthorhombi c) and anisotropic contributions to the spin coupling. However, a reas onable agreement between the measured and calculated data was not obta ined. As in [Gd(C5H5)2Br]2, the low-temperature behaviour is governed by effects which cannot be described by spin coupling models in the ge nerally accepted form.