Anisotropic exchange in transition-metal dinuclear complexes. 6. Bis(-.mu.-chloro)bis(dichlorocuprates(II))

Single-crystal EPR spectra of [(C6Hs),A]2Cu2C16, A = P, Sb, have been recorded at two different frequencies in a range of temperatures. The zero-field splitting tensors were found to have the largest component perpendicular to the CuCu direction, thus showing the presence of a substantial exchange contribution to the anisotropic spinspin interaction, as was previously observed for the arsonium derivative. In order to have structural parameters available for all the complexes, the X-ray crystal structure of [(C6H5)4Sb]2C~2wCa&s d etermined at room temperature. The crystals are isomorphous with the phosphonium and arsonium derivatives, belonging to the monoclinic space group P2,/n, with a = 13.412 (4) A, b = 19.894 (4) A, c = 9.501 (2) A, fl = 109.6 (3)*, and Z = 2. Least-squares refinement of the structure led to a conventional R factor of 0.025. A reanalysis of the magnetic susceptibility data of the arsonium complex showed that the exchange coupling constant does not vary significantly in the series. The zero-field splitting tensor however does vary, the largest value being observed for the phosphonium and the smallest for the stibonium derivative. This behavior has been related to the increased metal-metal distance observed on passing from the phosphonium to the stibonium through arsonium derivatives in agreement with a general trend observed also for the bis(poxo)- bridged copper(I1) complexes.