Kinetics and mechanism of ligand exchange in photogenerated (eta(2)-monoalkylarene)Cr(CO)(5) complexes with alkenes: Evidence for involvement of aliphatic chains in the arene and alkene in the exchange process

The kinetics and mechanism of the reaction of photogenerated (eta(2)-C6H5R)Cr(CO)(5) complexes (R = CH3, C2H5, i-C3H7, t-C4H9, c-C6H11, (C6H5)(CH2)(11)CH3), in which C6H5R is a ligand weakly bonded to Cr (L-w), with strongly coordinating ''trapping'' nucleophiles containing an olefinic functionality (L-s = 1-hexene, 1-decene, and 1-tetradecene) have been studied by employing pulsed laser flash photolysis with visible and infrared detection. The substituted arene molecules likely bond to the metal ''edge-on'' via a partially delocalized center of unsaturation in the ring. Kinetics data taken ''neat'' to very high [L-s] and in dilute solution in the ''inert'' solvents (S) agree closely, indicating that influences of S (C6H5R, fluorobenzene, n-heptane) are relatively unimportant. The kinetics data are wholly consistent with a single mechanism involving dissociation of C6H5R from (eta(2)-C6H5R)Cr(CO)(5), followed by competitive reaction of the [Cr(CO)(5)] intermediate thus produced with C6H5R or L-s. Changes in rate constants, k(1), for Cr-L-w bond fission are largely influenced by the electronic properties of R in PhR. Relative rates of attack by L-s at [Cr(CO)(5)] increase in the order 1-hexene < 1-decene < 1-tetradecene. The ''competition ratios'' of the rate constants for interaction of L-s and L-w with the photogenerated intermediate vary with the sum of the number of carbons in the chains for L-w and L-s, strongly suggesting that the interchange takes place, at least in part, through the chains. Carbonyl stretching spectra are quite similar for these (L-w)Cr(CO)(5) complexes and indicate that they are of C-4 upsilon local symmetry (octahedra substituted at an apex) and perhaps suggest similar bonding of PhR to Cr for all complexes, but they provide no information about the influence of R on bonding.