Anisotropic Exchange Effects in Temperature and Pressure Dependences of EPR Zero-Field Splitting in [(C(6)H(5))(3)(n-propyl)P](2)Cu(2)Cl(6).

X-band single-crystal and powder EPR data were collected in the temperature range 4.2-300 K and under hydrostatic pressure up to 500 MPa for [(C(6)H(5))(3)(n-propyl)P](2)Cu(2)Cl(6) (C(42)H(44)P(2)Cu(2)Cl(6)). The crystal and molecular structure have been determined from X-ray diffraction. The compound crystallizes in the monoclinic space group P2(1)/n (Z = 2) and have unit cell dimensions of a = 9.556(5) Å, b= 17.113(3) Å, c = 13.523(7) Å, and beta = 96.10(4) degrees. The structure consists of two controsymmetric Cu(2)Cl(6)(2)(-) dimers well separated by complex anions. EPR spectra are typical for the triplet S = 1 state of Cu(2)Cl(6)(2)(-) dimer 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 contribution from anisotropic exchange but the dipole-dipole Cu-Cu coupling is not much less. The anisotropic exchange integrals were estimated to be as follows: J(xy,x)()()2(-)(y)()()2(an) = -45 cm(-)(1), J(xy,xy)()(an) = +17 cm(-)(1), J(xy,yz)()(an) = +62 cm(-)(1). The D tensor components are strongly temperature dependent and linearly increase on cooling with an anomalous nonlinear behavior below 100 K. The D values increase linearly with pressure, but the effect is much smaller than the temperature effect. This suggests 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.

Angular-overlap analysis of the iron(II) site in [2Fe-2S] clusters.

The angular-overlap model for Fe(II)S4 centres is used to obtain structural information form the experimental data available for the gav approximately equal to 1.96 and gav approximately equal to 1.91 classes of [2Fe-2S] ferredoxin, showing that it is possible to translate the parameters obtained by Bertrand and Gayda in their non-additive ligand field model (Bertrand, P. and Gayda J.P. (1979 and 1980) Biochim. Biophys. Acta 579, 107-121 and 625, 337-342, respectively) into an additive one. The analysis of the e lambda (lambda = sigma or pi) parameters allowed us to conclude that the Fe(II)S4 chromophores of the two types of [2Fe-2S] metallo-protein are similar to each other, being possible to reproduce nicely the different g tensors introducing only small variations in the angular and bonding parameters.