The molecular and electronic structure of M(η3-C3H5)2 (M = Ni, Pd, Pt) has been investigated by means of quasi-relativistic gradient-corrected density functional calculations. Geometries have been fully optimized by considering both trans and cis arrangements of the bis(η3-allyl) moiety. Binding energy differences between isomers are always smaller than 0.2 kcal/mol; in particular, cis-Ni(η3-C3H5)2 is computed to be more stable than trans-Ni(η3-C3H5)2, while a reversed order is obtained for Pd and Pt analogues. Computed geometrical parameters of trans-Ni(η3-C3H5)2 compare very well with available structural data. Moreover, a new assignment of variable energy photoelectron spectroscopy measurements Li, X.; Bancroft, G. M.; Puddephatt, R. J.; Liu, Z. F.; Hu, Y. F.; Tan, K. H. J. Am. Chem. Soc. 1994, 116, 9543−9554 is proposed by assuming that the trans:cis ratio in the gas phase is close to one.