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Stereochemical Nonrigidity and Geometrical Isomerism in Six-Coordinate Trigonal Prismatic Complexes: The Case of Asymmetric Molybdenum and Tungsten Tris(dithiolenes).

The isomerization of nine asymmetric tris(dithiolenes) of tungsten and molybdenum, of the general formula (R(1)R(2)C(2)S(2))(3)M is studied with NMR methods. In the complexes investigated, R(1) = H, R(2) = p-CH(3)OPh, p-CH(3)Ph, or Ph, and M = W or Mo, or R(1) = H, R(2) = p-ClPh or p-BrPh, and M = W, or R(1) = Ph, R(2) = p-CH(3)OPh, and M = W, as shown in formula I. The complexes are proved to be trigonal prismatic in solution and stereochemically nonrigid at room temperature. An equilibrium favoring the trans isomer (formula III) is established, with the concentration of this isomer being three times that of the cis due to entropy reasons. The kinetics and mechanism of the isomerization is investigated and a scheme is proposed involving the rotation of only one ligand around an axis lying on the dithiolenic ring, passing from the metal to the center of the carbon-carbon bond. This mechanism satisfies energy criteria and is allowed by symmetry selection rules, as theoretical EHMO calculations indicate.