Octahedral Metal Carbonyls. XXXVI1Direct Evidence for the Ring-Opening Mechanism: Substitution Reactions of 2, 2, 8. 8-tetramethyl-3, 7-dithianonanetetracarbonyltungsten (0)

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Abstract
It has been proposed in many instances that substitution reactions of (L2)M(CO)4 and related complexes (L2 = 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 parameters3 and the “trapping” of ring-opened species (2a)6 have also been invoked in its support.

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