Kinetische und mechanistische untersuchungen von übergangsmetall-komplex-reaktionen: XVII. Assoziativer halogenidaustausch in trans-halogeno(tetracarbonyl)(phenylcarbin)wolfram-komplexen

Abstract

In the presence of free halide ions Y − (Y = Cl, Br, I), the neutral trans-halogeno(tetracarbonyl)(phenylcarbyne)tungsten complexes trans-X(CO) 4WCPh (X = Cl, Br, I) reversibly exchange the halide ligand X − for Y − in homogeneous solution. This substitution reaction follows a second-order rate law, first order in the concentrations of X(CO) 4WCPh and Y − . The activation parameters for the replacement of I − by Br − in 1,1,2-trichloroethane are: Δ H ≠ 56.7 kJ mol −1 and Δ S ≠ −33 J mol −1 K −1. Coordinated iodide is substituted more than three times faster by chloride than by bromide. The influence of the leaving halide is relatively small: k(Br)/ k(I) = 1.2 (Y = Cl). The results are in agreement with a nucleophilic, frontier-orbital controlled attack of the halide ion at the carbyne complex in the rate-determining step.