Bonding topological considerations

The RS system is based on purely topological considerations and is consequently independent of reference substances. The individual ligands of a chiral center are ordered according to a sequence in this system whereby the assignment of R- or S- can be derived from the order. The ligands are given different seniorities in the system such that the position in the periodic system of the atom bonded directly to the chirality center determines the seniorities ... 

Bonding to a metal surface can also be described in terms of frontier orbital interactions similar as in the cluster case. Symmetry considerations now apply to the coefficients of the metalsurface molecular orbitals at the Fermi elevels that interact with the HOMOs and LUMOs of the adsorbing molecule or atom. Such a theory helps to predict bonding topology of adsorbates, as will be illustrated for bonding of CO to the transition metals. 

Amidase enzymes catalyze the hydrolysis of amide bonds with considerable divergence noted within the family with respect to substrate spedficity. All amidase enzymes, however, maintain the core a,p,a structure, where the topologies of the C and N terminal halves are similar. 

A further example of an alkyl lithium compound with ambiguous chemical bonding that was resolved on the basis of electron density topology considerations is given in the on-line supplementary section for chapter 10. 

Recently considerable attention has been directed at anion binding ligands. Macrobicyclic 27 29) and macrotricyclic amines 30,31) were topologically designed to host anions such as spherical Cl-, linear Nf 32). These anion substrates are incorporated into macrocyclic cavities lined with appropriate anion-binding sites capable of forming hydrogen bonds like those of protonated amines (see /, below). 

In the case of TGT structures which are acyclic or which contain isolated rings, the disconnection of non-ring bonds must be examined to identify those disconnections which may be most effective on topological grounds. However, for such acyclic disconnections the topological factors may be overshadowed by other structural considerations. For instance, if a powerful stereosimplifying disconnective transform, such as stereospecific organometallic addition to carbonyl... 

The high electrophilicity of the positively charged element can be modified by intramolecular donation from remote donor substituents. This interaction leads to solvent-free cations with coordination numbers for the positively charged element > 3 and to a considerable electron transfer from the donor group to the element. Frequently used donor substituents utilize heteroatoms with lone pairs (e.g. amino, hydrazino, methoxy, carboxy, phosphino, etc.), in many cases in combination with pincer-type topology of the ligand, for the stabilization of the cationic center. These strongly stabilized cations are beyond the scope of this review and instead we will concentrate on few examples where we have weak donors such as CC multiple bonds, which stabilize the electron-deficient element atom. 

Topological analysis of the total density has a considerable advantage over the use of the deformation densities in that it is reference-density independent. There is no need to define hybridized atoms to analyze the nature of covalent bonding, and the ambiguity when using the standard deformation density, noted above in the discussion on propellanes, does not occur. 

Many of the currently available studies of metal-metal bonding were completed before the multipole model and the topological analysis of the total density were fully developed. For this reason, the discussions reported below focus on the deformation density distributions, and their comparison with theoretical results, though a more quantitative analysis is now possible and would be of considerable interest. 

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