Orientational and stereochemical preferences in [hydridotris(3,5-dimethyl-1-pyrazolyl)borato](CO)(2)W(eta(3)-allyl) complexes

Irradiation of Tp'(CO)(3)WH in the presence of alkynes or unconjugated dienes generates eta(3)-allyl complexes, Tp'(CO)(2)W(eta(3)-CHRCHCHR'). A kinetic preference for anti-alkyl substitution due to eta(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(eta(3)-CH2CHCHCH3), have revealed isomers which differ by approximately 90 degrees in allyl orientation (theta). The term meso is introduced to emphasize the orthogonal relationship between the anti (theta = 30 degrees, exo) and the syn (theta = 120 degrees, 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 pi-nonbonding orbital of the allyl into the d pi metal orbitals. EHMO (extended Huckel molecular orbital) calculations on a model complex [H3W(CO)(2)(eta(3)-C3H5)](2-) show electronic minima at theta = 30 degrees and 115 degrees corresponding to the two orientations.