Relationship between olefinic .pi. complexes and three-membered rings

Abstract

Compounds with three-membered rings can be formulated either as such or as 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 case of Zeise's salt. According to the -complex theory, compounds of this type can show a continuous range of structures from the -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 species of this type which confirm the postulated continuous transition from complex to classical ring and which show that the corresponding structural variations can be used to establish a fairly definite order of " -complex character". It was originally suggested that the direction of ring opening in unsymmetrically substituted compounds with threemembered rings, brought about by nucleophiles, could be used to distinguish between species best regarded as complexes and those best regarded as classical microcycles. A critical survey of the literature suggests that this criterion is generally consistent with the MNDO scale of -complex character.

Strong Evidence for an Unprecedented Borderline Case of Dissociation and Cycloaddition in Open‐Shell 1,3‐Dipole Chemistry: Transient Nitrilium Phosphane‐Ylide Complex Radical Cations

“…This feature points to an important contribution to the resonance hybrid in 1 ·+ of a π-donor/π-acceptor complex between a nitrile molecule 2 and a radical cationic phosphanylidene complex 7 ·+ (Scheme 5), according to the Dewar-Chatt-Duncanson model. [30] However, we did not find a path for the one-step dissociation of 1 ·+ into 7 ·+ and 2. Despite the stronger character of the (endocyclic) P-C bond in comparison to the P-N bond, we found that the only feasible pathway for dissociation is a stepwise process starting with the initial breaking of the P-C bond to yield the radical cationic nitrilium phosphane-ylide complex 6 ·+ that subsequently dissociates into complex 7 ·+ (see below) and the nitrile 2.…”

Computational Studies on Azaphosphiridines, or How to Effect Ring‐Opening Processes through Selective Bond Activation

“…[48] The aforementioned weakness of the P À N bond in azaphosphiridines 1-5 fully agrees with the highest ring strain associated with the homodesmic P À N bond ROR (Table 2), and also with the relatively low-energy products derived from P À N (and C À N) bond cleavage in 1 (Table 2 and Figure 1). The relatively large negative variation of k o for the C À N bond in 5 upon cyclization, though keeping a moderately positive 1(r c ) increment, cannot be regarded as an index of weakness, but rather as one of flexibility (lower energy change upon distortion of bond length around the equilibrium value r o ) ( Figure 3); this feature is associated with low values of relaxed force constants (i.e., high compliance constants).…”

Violation of Hund's multiplicity rule in the electronically excited states of conjugated hydrocarbons

“…Geometrical structures of the two forms optimized by using the MNDO method (12,13) by assuming the molecule to be planar are shown in Fig. 1, where only the C-C bond lengths are presented.…”