KINETICS AND MECHANISM OF CO SUBSTITUTION-REACTIONS OF M3(CO)12 (M = FE, RU, OS) IN THE PRESENCE OF (P-CH3OC6H4)2TEO

The rates and activation parameters for CO substitution of M3(CO)12 (M =Fe, Ru, Os) in the presence of (p-CH3OC6H4)2TeO are reported. The rat es are first-order in concentrations of M3(CO)12 and telluroxide, and zero-order in entering ligand. The DELTAH(double dagger) values for th ese reactions vary from 11 to 14 kcal/mol, with DELTAS(double dagger) values changing between -12 and -22 cal/mol K. The reactivities Of M3( CO)12 towards R2TeO decrease in the order Fe>Ru>Os. A mechanism was pr oposed in which tbe rate-determining step involves nucleophilic attack of the O atom of R2TeO at a C atom of a carbonyl group. This is then followed by O atom transfer to form the good leaving group CO2 and a r eactive intermediate which readily reacts with the entering ligand to afford the formation of monosubstituted products. Compared with simila r reactions of (CH3)3NO, the R2TeO reagent has greater O atom transfer selectivity towards M3(CO)12 complexes. This is discussed in terms of CO bridging effects in the reaction transition states.