Kinetics and mechanism of CO substitution reactions of M 3(CO) 12 (MFe, Ru, Os) in the presence of ( p-CH 3OC 6H 4) 2TeO

Abstract

The rates and activation parameters for CO substitution of M 3(CO) 12 (MFe, Ru, Os) in the presence of ( p-CH 3OC 6H 4) 2TeO are reported. The rates are first-order in concentrations of M 3(CO) 12 and telluroxide, and zero-order in entering ligand. The Δ H‡ values for these reactions vary from 11 to 14 kcal/mol, with Δ S‡ values changing between −12 and −22 cal/mol K. The reactivities of M 3(CO) 12 towards R 2TeO decrease in the order Fe > Ru > Os. A mechanism was proposed in which the rate-determining step involves nucleophilic attack of the O atom of R 2TeO at a C atom of a carbonyl group. This is then followed by O atom transfer to form the good leaving group CO 2 and a reactive intermediate which readily reacts with the entering ligand to afford the formation of monosubstituted products. Compared with similar reactions of (CH 3) 3NO, the R 2TeO reagent has greater O atom transfer selectivity towards M 3(CO) 12 complexes. This is discussed in terms of CO bridging effects in the reaction transition states.