Mechanism of Halogenation of Dimanganese, Manganese-Rhenium, and Dirhenium Decacarbonyls

Article abstract of DOI:10.1021/ja00317a023

The reactions between halogens (X2 = Cl2, Br2, I2, and ICl) and M2(CO)10 (M2 = Mn2, MnRe, and Re2) in CH3CN solvent yield equal amounts of M(CO)5X and ⁺.For X2 = I2 the rate law was shown to obey second-order kinetics, k.For M2 = Re2 the activation parameters were ΔH^<*> = 5.8+/- 0.2 kcal/mol and ΔS^<*> = -34.9 +/-0.9 cal/(mol K), for M2 = MnRe ΔH^<*> = 6.3 +/- 1.5 kcal/mol and ΔS^<*> = -41 +/- 6 cal/(mol K), and for M2 = Mn2 ΔH^<*> = 8.5 +/- 1.4 kcal/mol and ΔS^<*> = -39 +/- 5 cal/(mol K).The relative rates of halogenation follow the order Re2(CO)10 > MnRe(CO)10 > Mn2(CO)10, which opposes the order expected based upon homolysis of the metal-metal bond.The rate of iodination varies by more than a millionfold depending on the solvent.In nonpolar solvents such as decalin or toluene the oxidation proceeds slowly to yield M(CO)5I, whereas in CH3CN or CH3NO2 the reaction proceeds rapidly to produce both M(CO)5I and ⁺.The evidence suggests that halogen attacks the metal-metal bond via one end of the dihalogen molecule and that heterolytic cleavage of the halogen-halogen bond takes place to yield free halide ion and a halogenium-bridged metal dimer.In many respects this resembles the mechanism for the bromination of alkenes.