Cis Labilization of Ligand Dissociation. 2. Substitution and ¹³CO Exchange Reactions of Cis Monosubstituted Manganese and Rhenium Carbonyl Bromides

The substitution and ¹³CO exchange reactions of cis-M(CO)₄BrL compounds (M = Mn or Re; L = P(C₆H₅)₃, NC₅H₅, or P(OC₆H₅)₃) have been studied in hexane and 1,2-dichloroethane. The reactions are first order in metal carbonyl and zero order in entering ligand. The ligand L labilizes the CO groups toward dissociation, in the order NC₅H₅ > P(C₆H₅)₃ > P(OC₆H₅)₃ ≈ CO. Analysis of the kinetics results reveals that the dissociative loss of the CO groups mutually cis to Br and L is faster than of the other CO groups, and that the five-coordinate intermediate formed is subject to rapid intramolecular rearrangement during its lifetime. The substitution and ¹³CO exchange results are in good accord. Various possible origins of the cis-labilization observed are discussed. The most attractive explanation is that the ligand L stabilizes the five-coordinate intermediate, by taking up a preferred site in the square pyramidal or trigonal bipyramidal structure.