Octahedral Metal Carbonyls. XXXVI1 Direct Evidence for the Ring-Opening Mechanism: Substitution Reactions of 2, 2, 8. 8-tetramethyl-3, 7-dithianonanetetracarbonyltungsten (0)

It has been proposed in many instances that substitution reactions of (L 2 )M(CO) 4 and related complexes (L 2 = chelating bidentate ligand; M = Cr, Mo, W), proceed, at least in part, via a ring-opening mechanism (2): 1,3-5 The rate law (3) is obtained under the assumption of a steady-state concentration of (2-a): Under reaction conditions usually employed (large excess of L'), the pseudo first-order rate constant, k obsd , is, and the "reciprocal relationship", is applicable. Based on Eq. 4, for k −1 ≈ k 2 , plots of k obsd vs. [L'] should exhibit complex behavior, limiting second-order behaviour at low [L'] and limiting first-order behaviour at high [L']. The reciprocal plot (Eq. 5) is expected to be linear, with intercept 1/k 1 and slope k −1 /k 1 k 2 . The observation of such rate behaviour has constituted the main line of evidence in support of the ring-opening mechanism, 1,4,5 although activation parameters 3 and the "trapping" of ring-opened species (2a) 6 have also been invoked in its support.