RELATIONSHIP BETWEEN OLEFINIC PI-COMPLEXES AND 3-MEMBERED RINGS

Compounds with threc-membcred rings can be formulated cither as such or as ir complexes with back-coordination. Dewar first pointed out that olefin transition metal complexes are best represented by the latter structure, a suggestion later confirmed by Chatt and Duncanson in the ease of Zeise&s salt. According to the π-complex theory, compounds of this type can show a continuous range of structures from the π-complex extreme at one end to a “pure” classical ring at the other, so a distinction can be drawn only on the basis of some property that changes along the range. Here we report detailed MNDO calculations for a number of spccies of this type which confirm the postulated continuous transition from w complex to classical ring and which show that the corresponding structural variations can be used to establish a fairly definite order of “π-complex character”. It was originally suggested that the direction of ring opening in unsymmctrically substituted compounds with three-membered rings, brought about by nucleophiles, could be used to distinguish between species best regarded as v complexes and those best regarded as classical microcvcles. A critical survey of the literature suggests that this criterion is generally consistent with the MNDO scale of x-complex character. © 1979, American Chemical Society. All rights reserved.