Fig. 1. 'H-NMR spectra of the diastereotopic protons of complexes (1)-(3) at 27 "C
determining step being the dissociation of the Mo-N bonds at a rate which is rapid on the NMR time scale. The values of AG+ for (Ib) and (3b) could be determined from the coalescence temperatures by simultaneous decoupling of the methyl protons (Table 1). The values obtained support the suggested mechanism, they are clearly higher than those of the noncomplex bonded ligand~"~.
Finally, it is interesting that substitution of both oxo-oxygen atoms in (lb) by two sulfur atoms in (3b) lowers the dissociation barrier of the Mo-N bond. An increase in the electron density at the molybdenum(v1)-centre weakens the Mo-N bond.
Procedure
(3 g, 5.4 mmol) and (Ib)l8] (3 g, 10 mmol) respectively, in 150 cm3 benzene over 10 g dry Na2S04. The mixture is stirred for 12 h in a closed flask, the deep red solution filtered off and the solvent removed. One half of the residue is dissolved in 20 cm3 benzene and chromatographed at 20 "C on 150 g aluminum oxide 90 (activity grade 11-111); the other half is dissolved in 20 cm' CHC13 and chromatographed on 100 g silica gel 60. The red-violet cis-dithio compounds (.?a) .C6H6 (yield 15%) and (3bj.0.33 C6Hb (13%) are obtained from the first half. The faster eluting violet fraction from the chromatographed chloroform solution is rejected: solvent is removed from the succeeding yellow fraction until crystallization just begins. Yellow crystals of ( 2 4 (yield 10%) and (2b) (8%) respectively are filtered off and air-dried.
H2S is passed for 30 mins into a stirred solution of