Orientational and Stereochemical Preferences in [Hydridotris(3,5-dimethyl-1-pyrazolyl)borato](CO)₂W(η³-allyl) Complexes

Irradiation of Tp‘(CO) 3 WH in the presence of alkynes or unconjugated dienes generates η 3 -allyl complexes, Tp‘(CO) 2 W(η 3 -CHRCHCHR‘). A kinetic preference for anti -alkyl substitution due to η 2 -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) 2 W(η 3 -CH 2 CHCHCH 3 ), 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 Hückel molecular orbital) calculations on a model complex [H 3 W(CO) 2 (η 3 -C 3 H 5 )] 2 - show electronic minima at θ = 30° and 115° corresponding to the two orientations.