tion to completion. The increase in activation energy at the highest pressures does seem to imply the existence of a secondary pressure effect, such as could arise from intramolecular distance changes, which may prevent the ultimate realization of the class II -> III transition.In summary, the results of this study indicate that the solid-state properties of the M(A)X3 complexes are extraordinarily sensitive to pressure, presumably due to the effect of pressure in reducing the geometric inequivalency in the environment about the metal atoms while at the same time increasing the intrachain metal-halogen orbital overlaps. In the context of the energy level diagram in Figure 8, at high pressures the system is effectively being driven toward a onedimensionally metallic M(III) state whose ultimate attainment, however, is apparently prevented by the action of secondary effects which may arise from extrachain internu-clear distance changes. The application of high pressures also serves to raise the level of conductivity of the M(A)X3 complexes into a more easily measurable range, revealing apparent trends in conductivity with the M, (A), and X groups. These trends are further investigated in the following paper which deals with the electrical properties of these complexes at ambient pressures.
The structures of two neutral bis (ethylene-1,2-dithiolene) complexes, MS4C4H4, where M = Pd or Pt, have been determined by single-crystal X-ray diffraction using complete three-dimensional data obtained on a GE 490 automated diffractometer. The compounds are isostructural and crystallize in the monoclinic space group P2\/n with Z = 4; for PdS4C4H4, a = 9.916 (3) Á, b = 11.788 (4) Á, c = 6.472 (3) A, ß = 92.21 (2)°; for PtS4C4H4, a = 9.973 (5) A, b = 11.868 (8) A, c = 6.477 (6) A, ß = 92.12 (5)°. The structure was solved by Patterson and Fourier methods and refined by least squares to a final R factor of 0.059 (1349 reflections) for PdS4C4H4 and 0.081 (1116 reflections) for PtS4C4H4. The molecular units in each case have an approximately square-planar arrangement of S about the metal, with two MS4C4H4 units in eclipsed relationship joined by Pd-Pd and Pt-Pt bonds of 2.790 (2) and 2.748 (2) A, respectively, to form a dimeric structure.These dimers have a distorted cubic arrangement of S atoms with metal atoms pulled in toward each other from the centers of two opposite faces. They represent the first examples of dimeric metal-metal bonded 1,2-dithiolene complexes. Bond distances and angles within the chelate rings are similar to those observed for other neutral 1,2-dithiolene complexes.
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