DICYCLOPENTADIENIDEHALIDES OF LANTHANIDES .9. EXCHANGE COUPLING IN [DY(C5H5)(2)(MU-BR)](2) AND [DY(C5D5)(2)(MU-BR)](2)(2)

Single crystal X-ray structural investigation of [Dy(C5D5)(2)(mu-Br)]( 2) revealed the [Sc(C5H5)(2)(mu-Cl)](2)-type structure, space group P2 (1)/c, with a=1397.7(3), b=1635.7(2), c=1366.7(6) pm, beta=93.44(3)deg rees, V=3119(3)x10(6) pm(3), D-c=2.380 g cm(-3) and Z = 6 dimers (R = 0.033 for 2745 reflections with I>3 sigma(I)). Elastic neutron-scatter ing investigation of a polycrystalline sample at 8K and 1.5K disclosed the absence of three-dimensional antiferromagnetic ordering. From the oscillating form of the difference intensity pattern Delta I=1(1.5 K) -I(8 K) antiferromagnetic short-range order between the metal centres within the dimers is deduced. Magnetic susceptibility measurements (SQ UID magnetometer, 1.7 K less than or equal to T less than or equal to 300 K, 0.01T less than or equal to B less than or equal to 1T) and sim ulation calculations based on a cubic crystal held and isotropic intra dimer exchange interactions ((H) over cap(ex)= -(2K/(h) over bar(2))(g -1)(2) (J) over cap(1) .(J) over cap(2)) confirmed the antiferromagnet ic spin-spin coupling (K approximate to-0.15 cm(-1)).