The kinetics and mechanism of ligand exchange in photogenerated ( η1-bromobenzene)Cr(CO) 5/alkene systems

• Published in: Inorganica Chimica Acta 1998, Vol.267 (1), p.49-54
• Main Authors: Ladogana, Santino, Nayak, Suresh K., Smit, Jaap P., Dobson, Gerard R.
• Format: Article
• Language: English
• Subjects: Alkene complexes; Carbonyl complexes; Chromium complexes; Kinetics and mechanism
• ISSN: 0020-1693; 1873-3255
• DOI: 10.1016/S0020-1693(97)05550-3

The kinetics and mechanism of the reaction of photogenerated ( η 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-trimethyl-pentene), has been studied by employing pulsed laser flash photolysis with visible and infrared detection. The kinetics data are ( η 1-PhBr)Cr(CO) 5 → ( η 2-L s)Cr(CO) 5 consistent with a single mechanism, involving dissociation of PhBr from ( η 1-PhBr)Cr(CO) 5, followed by competitive reaction of the [Cr(CO) 5] intermediate thus produced with PhBr or L 5. 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) 6] increase in the order 1-hexene < 1-decene < 2,2,4-trimethylpentene. Carbonyl stretching spectra for (C 6H 5X)Cr(CO) 5 complexes (X = H,F, Cl, Br) are quite similar, and indicate that these L w all are weak σ and π 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. However, activation parameters for complexes exhibiting the two bonding modes are diagnostic for each linkage.