η1/η2-Isomerie bei Pentacarbonyl(thiobenzaldehyd)-Komplexen von Chrom und Wolfram

Abstract

Abstract The benzylidene(pentacarbonyl) complexes of chromium and tungsten (CO)5M = C(H)R [M = Cr, R = C6H4OMe-P (2a); M = W, R = Ph (2b), C6H4CF3-p (2c)] react with SCN- inserting sulfur into the M = C bond to give (CO)5M[S = C(H)R] (3a-c). From the reaction of (CO5W=C(H)R [R = Ph (2h), 2,4,A-C6H2Me3 (2d), C6H4Me-p (2e)] with S8 in CH2Cl2 the thiobenzaldehyde complexes (CO)5W[S=C(H)R] (3b, d, e) are obtained. In 3a and 3d the thiobenzaldehyde ligand is η1-bonded. However, a dynamic equilibrium between the η1- and the η2-isomeric forms is observed for the complexes 3b, 3c and 3e. The equilibrium constant is dependent on the aryl group, the solvent and the temperature. In hexane the η2-isomer is favored by its lower enthalpy. Because of the small but positive entropy of the reaction η2-3→η1-3, increasing temperatures shift the equilibrium towards the η1-isomer.