N —> cr* interactions

In the case of A = F, there is apparently no local minimum corresponding to the HO- HF isomer, and instead the proton transfers to form HOH- F- as the only stable equilibrium species. However, in the case of the weaker HA Lewis acids, stable HO- HA structures are found. Figure 5.7 displays optimized structures of these complexes and the dominant n-cr interaction in each case, while Table 5.9 summarizes energetic and structural properties of these complexes for... 

In this chapter, we demonstrated that the restriction of building a compound with only one type of an element is not a restriction at all and a multitude of neutral, cationic as well as anionic polychalcogen structures is currently known. As expected for the more electronegative nonmetal (S) and meta metals (Se, Te), the bonding within these moieties is covalent and a small number of interactions, namely, p2-rap2 lone pair repulsion, n- and n -n bonding as well as p2- cr interactions, are sufficient to rationalize the structures and account for the bond lengths alternations or weak transannular interactions that are often found. 

The first example of lithium-NHC complexes, in which the lithium is coordinated only to carbon centres, was reported by Arduengo and coworkers.10 Stable NHCs were reacted with lithium 1,2,4-n-A(trimethylsilyl) cyclopentadienide to give 2 (Fig. 2). A single crystal X-ray structure reveals a complex in which the lithium centre is coordinated in a r 5-fashion to the cyclopentadienyl ring, with a single cr-interaction between the lithium and carbene centre. The lithium centre lies 2.155(4) A from the carbene centre hence has a closer contact than in the previous example, possibly as a result of the carbene interacting with only one lithium centre. 

Salts containing the cation [PhgP—N= PPh3]+ (hereafter described as C+) are gaining wide acceptance in the preparation of anionic transition-metal species, and details of several crystal structures have appeared. There is now evidence for the formation of ion pairs in the complex salt C+[Fe(CO)NO], with a N—-O- C+ interaction that can determine the reactivity of the anion, for example in reactions with iodine. Associative effects have also been observed in the species C+A A = BPh4, HFe(CO)4, Co(CO)4, or p-H[Cr(CO)5]2 when in solution... 

Reactions occur readily when there is congruence between the orbital symmetry characteristics of the reactants and the products, and only with difficulty when that congruence does not obtain. In other words, the orbital symmetry is conserved in concerted reactions. How exactly is the orbital symmetry conserved and what are its further ramifications are important issues which we will examine in detail by considering a few examples. For a better grasp of the subject, let us first understand orbitals and their interactions in relation to n and cr bond formation. 

It may appear a priori that whereas n + combination enjoys from two cr-type of interactions, Jt - Jt2 combination benefits from two tr-type of interactions, and since a cr bond resulting from cr-interaction is lower in energy than a n bond originating from Tt-interaction, n + n 2 must be lower on the energy scale in comparison to the Jt - n2 level. The fact, however, remains that this cr-interaction is in the very early stage and, hence, its contribution to lowering of the energy is considerably small. 

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