Abstract
Irradiation of Tp‘(CO)3WH in the presence of alkynes or unconjugated dienes generates η3-allyl complexes, Tp‘(CO)2W(η3-CHRCHCHR‘). A kinetic preference for anti-alkyl substitution due to η2-vinyl intermediates exists for the reactions with terminal alkyne substrates. The monoalkyl-substituted allyl complexes are configurationally stable on the NMR time scale but show a thermodynamic preference for syn-substitution upon heating in solution. Separate crystal structures of the syn and anti isomers of the 1-methylallyl complex, Tp‘(CO)2W(η3-CH2CHCHCH3), have revealed isomers which differ by approximately 90° in allyl orientation (θ). The term meso is introduced to emphasize the orthogonal relationship between the anti (θ = 30°, exo) and the syn (θ = 120°, meso) isomers. The change in orientation of the allyl is accompanied by a change in the OC−W−CO angle from acute to obtuse in order to maximize donation from the π-nonbonding orbital of the allyl into the dπ metal orbitals. EHMO (extended Huckel molecular or...
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