The kinetics and mechanism of ligand exchange in photogenerated (eta(1)-bromobenzene)Cr(CO)(5)/alkene systems

The kinetics and mechanism of the reaction of photogenerated (eta(1)-bromobenzene)Cr(CO)(5)(bromobenzene=PhBr), which coordinates weakly (=L-w) to Cr through a 'dative' Br --> Cr bond, with more strongly bonding alkene 'traps' (L-s: 1-hexene; 1-decene; 2,4,4-trimethylpentene), has been studied by employing pulsed laser flash photolysis with visible and infrared detection. The kinetics data (eta(1)-PhBr)Cr(CO)(5)<(L-s)under right arrow>(eta(2)-L-s)Cr(CO)(5) are consistent with a single mechanism, involving dissociation of PhBr from (eta(1)-PhBr)Cr(CO)(5), followed by competitive reaction of the [Cr(CO)(5)] intermediate thus produced with PhBr or L-s. Data taken 'neat' to very high [L-s], and in dilute solution in the 'inert' solvents (S: fluorobenzene, n-heptane) agree closely, indicating that solvent effects are relatively unimportant. Relative rates of attack at [Cr(CO)(5)] increase in the order 1-hexene < 1-decene < 2,2,4-trimethylpentene. Carbonyl stretching spectra for (C6H5X)Cr(CO)(5) complexes (X = H, F, Cl, Br) are quite similar, and indicate that these L-w all are weak sigma and pi bonders, but do not enable one to distinguish between 'edge-on' binding of the ring to the metal (H, F) from dative X --> Cr bonding (Cl, Br). However, activation parameters for complexes exhibiting the two bonding modes are diagnostic for each linkage. (C) 1998 Elsevier Science S.A.