Intramolecular rearrangement in η-allyl complexes of molybdenum(II) containing unidentate amine ligands

Abstract

Allyl and 2-methylallyl complexes [Mo(CO)₂X(η-C₃H₄R)L₂](X = Cl, Br, or I; R = H or Me; L = NC₅H₅, NC₅H₄ Me-4, or NC₅H₃Me₂-3,5) exist in solution at low temperatures in a single isomeric form in which the two carbonyl ligands, the two amine ligands, and the two ends of the allyl ligand are inequivalent. At higher temperatures, rearrangement (shown to be an intramolecular process) has the effect of removing all these inequivalences from the ¹H and ¹³C n.m.r spectra of the complexes. At a given temperature, the rearrangement rate is essentially independent of the nature of L, decreases with increase in the size of the halogen ligand, and is greater for the 2-methylallyl than for the allyl complexes. At the lowest temperatures studied, rotation of one amine ligand about the metal–nitrogen bond is restricted for the 2-methylallyl complexes only. Two structures are suggested for the complexes in solution: in one case (viewed as the more probable of the two) the variation in spectra can be attributed to a trigonal twist of the halogen and amine ligands relative to the remainder of the complex, and in the other to an oscillation of the allyl ligand about the metal–allyl axis.