Four-electron repulsion

Recently, we analyzed the role of electron repulsion relative to bond breaking and antiaromaticity effects on a quantitative basis using Natural Bond Orbital (NBO) analysis.24 Two other destabilizing factors were considered at the initial stage of the cyclization in addition to four-electron repulsion between the filled in-plane acetylenic re-orbitals - distortion/breaking of the acetylenic bonds as a result of their bending, and the fact that, at a distance of ca. 3 A, the in-plane re-orbitals become parallel and reach a geometry that resembles the antiaromatic TS of the symmetry forbidden [2S + 2S] cycloaddition (vide infra). 

Where does the energy increase come from Can we offer an electronic description to the phenomenological term strain A priori, one can consider such factors as four-electron repulsion of filled in-plane orbitals and distortion/breaking of the acetylenic bonds as a result of their bending. 

Due to the lack of increase in stabilizing re-re interactions at the reaction stage where the destabilizing four-electron repulsive interactions increase steadily, the inward bending of alkyne moieites in unstrained enediynes leads to continuously developing reactant destabilization without any compensation from the increased C1-C6 bonding. Only in the 9-membered enediyne the decrease in the C1-C6 distance results in an immediate increase in the extent of C1-C6 cr-bond formation. 

Since four-electron repulsion is the dominant factor in the reactant destabilization, any structural perturbation that either increases electron repulsion in the reactant or decreases the electron repulsion in the TS will decrease the activation energy for the cyclization. One way for placing an accelerating substituent in direct spatial proximity to the in-plane re-orbitals is to use appropriate ortho substituents in benzannelated enediynes. 

The isomerization, itself, originates from the a complex (B in Figure 3). However the total activation energy depends critically on the relative energy of A and B (Figure 3). An alkyne C=C triple bond binds more efficiently to a transition metal complex than a o C-H bond since the % C-C orbital is a better electron-donor and the 71 C-C orbital a better electron acceptor than the a and a C-H orbitals, respectively. However, the difference in energy between the two isomers is relatively low for a d6 metal center because four-electron repulsion between an occupied metal d orbital and the other n C-C orbital destabilizes the alkyne complex. This contributes to facilitate the transformation for the Ru11 system studied by Wakatsuki et al. 

For other systems the transformation is still energetically accessible but die barrier may be higher if the alkyne complex is less destabilized. In d2 and d4 complexes, the four-electron repulsion with the alkyne can be avoided (the alkyne behaves like a 4-electron donor) and this stabilizes the alkyne complex. The intraligand 1,2 shift is then associated to a higher activation barrier. This has been illustrated in a study by Fledos et al. on [Cp2Nb(HC=CH)(L)]+ and [Cp2Nb(C=CH2)(L)]+ (L = CO, PH3) where DFT calculations show that the barrier to isomerization is +29.2 kcal.mof1 (resp. 

Both of these interactions are primarily of type and (see 54 or 59), four-electron repulsive or two-electron attractive interactions. Actually, the energetic and bonding consequences are a little complicated the z2-rrtt interaction would be destabilizing if the antibonding component of this interaction remained filled, below the Fermi level. In fact, many z2-ir0 antibonding states are pushed above the Fermi level, vacated. This converts a destabilizing, four-electron interaction into a stabilizing two-electron one. 

Interaction , peculiar to the solid, is a reorganization of the states around the Fermi level as a consequence of primary interactions , , , and . Consider, for instance, the levels that are pushed up above the Fermi level as a result of interaction , the four-electron repulsion. One way to think about this is the following the electrons do not, in fact, go up past the Fermi level (which remains approximately constant) but are dumped at the Fermi level into levels somewhere in the solid. This is shown schematically in 138. 

Similar starting materials for other early transition metals, such as (silox)3M (M = Sc, Ti, V) react with Py in a conventional manner leading to il (N) derivatives [55]. The absence of four electron repulsion problems in the Sc, Ti and V derivatives has been invoked to account for the selective formation of tl CN) adducts. Reduction of (silox)3NbCl2 with Na/Hg in Py affords a kinetic n (N,C)-Py product which thermally... 

The cleavage of the C-H bond in methane by the /ran5-PtCl2(H20)2 complex has been subjected to quantum-chemical analysis by the extended Hiickel method [36]. It was shown that the approach of the methane molecule to the square planar platinum(II) complex (structure VII-8) leads to strong four-electron repulsion, the main cause of which is the interaction between orbitals ofCR, and the occupied orbitals of the metal complex localized in the region of the free coordination site (HOMO consisting mainly of the orbital and also the... 

Apart from the d ML5(H2) case, other more extreme cases have been analyzed, diO ML3(H2) complexes, in particular [18]. In this case, all metal d orbitals are occupied and donation from the oh2 orbital does not seem favorable. Further, there is a four electron repulsion interaction between two occupied orbitals. For this kind of complex, the presence of a very strong 7t-acceptor like NO" " is required to withdraw electron density from the metal. The tetrahedral arrangement of the ligands around the metal atom is shown to favor the back-donation to the o h2 orbital without significantly altering the three-orbital four-electron interaction where the oh2 orbital is involved. 

The exciplex N... N two-orbital/three-electron bond can be viewed as a mixture of a covalent (N = N - NMe3) and an ionic (N = N - NMe3) electronic configurations. A steep rise of the ground state energy surface towards the conical intersection is due to a destabilizing two-orbital/four-electron repulsive interaction (N = N - NMe3) (see O Fig. 39-19). 

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