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

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.