Reactions of the transient species Cr(CO)₅(cyclohexane) with C₄H_nE (n = 4, 8; E = O, NH, S) studied by time-resolved IR absorption spectroscopy

Time-resolved IR absorption spectroscopy is used to investigate substitution of the cyclohexane (CyH) molecule of the photolytically generated alkane-solvated transient intermediate Cr(CO)₅(CyH) by heterocyclic ligands C₄H_nE (n = 4, 8; E = O, NH, S). From the concentration and temperature dependences of the pseudo-first order rate constants, we obtain activation parameters for the reactions, and find that they are consistent with an associative (A) or interchange (I) mechanism. As was the case with ligand substitution reactions at W(CO)₅(CyH), a ligand's reactivity depends both on its electron-donating ability and on its polarizability. We also find that for a reaction with a given ΔH^‡, the activation entropy is higher for reaction of Cr(CO)₅(CyH) than it is for reaction of W(CO)₅(CyH). Comparison of the present results with ligand substitution reactions of W(CO)₅(CyH), CpMn(CO)₂(CyH), and Cr(CO)₅(n-heptane) indicates that for ligand substitution reactions at alkane-solvated transition-metal intermediates, the solvent's effect upon the reaction rate is primarily entropic.