ANISOTROPIC EXCHANGE EFFECTS IN TEMPERATURE AND PRESSURE DEPENDENCES OF EPR ZERO-FIELD SPLITTING IN [(C6H5)(3)(N-PROPYL)P]2CU2CL6

X-band single-crystal and powder EPR data were collected in the temper ature range 4.2-300 K and under hydrostatic pressure up to 500 MPa for [(C6H5)(3)(n-propyl)P]2Cu2Cl6(C42H44P2Cu2Cl6) The crystal and molecul ar structure have been determined from X-ray diffraction. The compound crystallizes in the monoclinic space group P2(1)/n (Z = 2) and have u nit cell dimensions of a = 9.556(5) Angstrom, b = 17.113(3) Angstrom, c 13.523(7) Angstrom, and beta 96.10(4)degrees. The structure consists of two controsymmetric Cu2Cl62- dimers well separated by complex anio ns. EPR spectra are typical for the triplet S = 1 state of Cu2Cl62- di mer with parameters g(x) = 2.114(8), g(y) = 2.095(8), g(z) = 2.300(8), and D-x = 0.025(1) cm(-1), D-y = 0.057(1) cm(-1), and D-z = -0.082(1) cm(-1) at room temperature. The D tensor is dominated by a contributi on from anisotropic exchange but the dipole-dipole Cu-Cu coupling is n ot much less. The anisotropic exchange integrals were estimated to be as follows: J(xy,x)'(-y)'(an) -45 cm(-1) J(xy,xy)(an) = +17 cm(-1), J( xy,yz)(an) = +62 cm(-1). The D tenser components are strongly temperat ure dependent and linearly increase on cooling with an anomalous nonli near behavior below 100 K. The D values increase linearly with pressur e, but the effect is much smaller than the temperature effect. This su ggests that the D vs T dependence is dynamical in origin. EPR data, a possible mechanism, and contributions to the observed dependences are discussed and compared to EPR results for similar compounds.