Steric and electronic effects in linkage isomerization reactions of M(CO)₅(L) (M = Cr, Mo, W; L = 2-methyl-2,3-dihydrofuran, 2,3-dihydropyran)

Time-resolved infrared absorption spectroscopy is used to investigate the kinetics of the linkage isomerization reaction M(CO)₅(η ¹-L) → M(CO)₅(η²-L), where M = Cr, Mo, or W, and L = 2-methyl-2,3-dihydrofuran or 2,3-dihydropyran. Photolysis of M(CO)₆ in cyclohexane in the presence of excess L produces the η¹ (O-bound) isomer as the sole kinetic product. The kinetic product subsequently reacts intramolecularly to form an equilibrium mixture of the η¹ and η² (C=C bound) isomers. Activation and equilibrium parameters are derived for the reactions and compared with previous results for linkage isomerization of the M(CO)₅(dihydrofuran) complexes. The results of the experimental study, supported by DFT calculations, indicate that for M = W the kinetics of the η¹ → η² isomerization depend primarily on how close the double bond is to the metal atom in the η¹ isomer, while the thermodynamics of the η¹ ⇄ η² equilibrium are determined primarily by electronic factors. In contrast, for M = Cr and Mo, both steric and electronic factors play a role in the isomerization kinetics.