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Three Isomers in Equilibrium: Phosphine Exchange of Coordinated and Pendant Phosphines in a Unique Complex, (OC)5WL (L = 1,2-Bis(diphenylphosphino)-1-di‑p‑tolylphosphinoethane), and Implications for Understanding the k2 Term in the Rate Law for Phosphine Substitution in Group 6 Metal Carbonyl Complexes

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posted on 2018-10-15, 19:48 authored by Bret R. Van Ausdall, Michael F. Cuddy, Joel S. Southern, Kraig A. Wheeler, Ellen A. Keiter, Edward M. Treadwell, Richard L. Keiter
The linkage isomers [(OC)5W­{κ1-PPh2CH2CH­(PPh2)­[P­(p-tol)2]}] 3a, [(OC)5W­{κ1-P­(p-tol)2CH­(PPh2)­CH2PPh2}] 3b, and [(OC)5W­{κ1-PPh2CH­[P­(p-tol)2]­CH2PPh2}] 3c isomerize at ambient temperatures to give a mixture of the three isomers in equilibrium. Rates of isomerization and equilibrium constants have been measured at temperatures from 10 to 55 °C, and from these, enthalpies and entropies have been determined. The isomerization reactions are initiated by a nucleophilic attack of a pendant phosphine on a bound carbonyl group followed by dissociation of a coordinated phosphine. These studies lead us to suggest that the k2 term in the rate law for CO replacement by phosphines is based on a similar mechanism and offers a plausible clarification of the currently accepted understanding. The crystal structure of 3b, the most stable of the three isomers, has been determined.

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