Bonding of halogenated arenes in photogenerated (arene)M(CO)5 complexes (M = Cr, Mo, W)

Detailed kinetics studies of arene displacement by the trapping nucleophile, l-hexene, from (halo-arene) Cr(CO)(5) complexes (halo-arene = X-arene; X = F, Cl, Br) and selected Mo and W analogues generated by pulsed laser flash photolysis have been conducted. All reactions obey a 'reversible dissociation-competition for the [M(CO)(5)] intermediate' mechanism. The systematic variations in rate constants and activation parameters for unimolecular dissociation of haloarenes in the presence of the trap strongly support coordinate covalent bonding of Cl and Br to Cr, where Cr-Br bonds are stronger than are Cr-CI bonds. In contrast, evidence indicates that fluorobenzene coordinates more weakly, bound through an arene ring edge. Carbonyl stretching frequencies for the complexes are diagnostic for the type of bonding, and enthalpies of activation decrease in the order W > Mo > Cr. The failure to observe more than one reaction pathway where more than one complex is generated upon photolysis may suggest very rapid linkage isomerization to afford the thermodynamically most stable (X-arene)Cr(CO)(5) complex. (C) 1998 Elsevier Science S.A. All rights reserved.