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]2 reve aled the [Sc(C5H5)2Cl]2-type structure, space group P2(1)/c, with a=14 07.6(2) pm, b=1644.7(2) pm, c=1370.6(9) pm, beta=93.46(3)-degrees, V=3 167(2)X10(6) pm3, D(c)=2.322 g cm-3 and Z=6 dimers (R=0.036 for 4627 r eflections with 1>3sigma(I)). The metal centres have the pseudosymmetr y C2v. Magnetic susceptibility data show Curie-Weiss behaviour between 213 and 6 K with theta(p)=-4.5(3) K and a magnetic moment mu=9.8(1) m u(B) close to the Tb3+ free-ion value (9.72 mu(B)). Below 6 K, deviati ons from Curie-Weiss behaviour are observed, and at 5.3 K a maximum in the susceptibility is detected which may be caused by intradimer anti ferromagnetic spin coupling. The magnetic properties are compared with the prediction of various models, starting from cubic crystal fields and isotropic intermolecular exchange interactions, followed by extens ion to lower crystal field symmetry (orthorhombic) and anisotropic con tributions to the spin coupling. However, a reasonable agreement betwe en the measured and calculated data was not obtained. As in [Gd(C5H5)2 Br]2, the low-temperature behaviour is governed by effects which canno t be described by spin coupling models in the generally accepted form.