STEREOCHEMICAL NONRIGIDITY AND GEOMETRICAL ISOMERISM IN 6-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-CH3- 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 s hown in formula I. The complexes are proved to be trigonal prismatic i n solution and stereochemically nonrigid at room temperature. An equil ibrium favoring the trans isomer (formula III) is established, with th e concentration of this isomer being three times that of the cis due t o entropy reasons. The kinetics and mechanism of the isomerization is investigated and a scheme is proposed involving the rotation of only o ne ligand around an axis lying on the dithiolenic ring, passing from t he metal to the center of the carbon-carbon bond. This mechanism satis fies energy criteria and is allowed by symmetry selection rules, as th eoretical EHMO calculations indicate.