Coordinative bonding approach

Although the simple valence-bond approach to the bonding in coordination compounds has many deficiencies, it is still useful as a first attempt to explain the structure of many complexes. The reasons why certain ligands force electron pairing will be explored in Chapter 17, but it is clear that high- and low-spin complexes have different magnetic character, and the interpretation of the results of this technique will now be explored. 

In the same year (1923) G. N. Lewis first proposed a different approach. In this view, an acid is any species that, because of the presence of an incomplete electronic grouping, can accept an electron pair to give rise to a dative or coordination bond. Conversely, a base is any species that possesses a nonbonding electron pair that... 

The self-association and/or complex formation of organozinc componnds involves considerable rehybridization of the zinc valence orbitals. When only one coordinate bond is formed, the zinc atom becomes sp -hybridized and the resnlting complex is planar or nearly so with bond angles around the zinc of abont 120°. The zinc centre then still has one unoccupied valence orbital and remains coordinatively nnsatnrated. Three-coordinate zinc, however, is relatively rare and only occurs when steric crowding around the zinc prevents the approach of a fourth ligand. 

The synthesis of polycatenanes requires, like the synthesis of catenanes, the preorientation of the macrocycle precursors into a favorable geometry before cycliza-tion (Scheme 4) [5], This pre-orientation is commonly achieved via a template, resulting from rc-donor-acceptor interactions, hydrogen-bonding, and coordination bonds [1-3, 5, 41], The use of a template in catenane synthesis is the subject of Chapters 4 and 6-8 and will not be treated further in this section. The aim of this section is to present the state of the art of the various synthetic approaches leading to the polycatenane polymers and networks. 

In a series of more advanced studies, this problem was partially addressed by systematically adjusting a limited number of internal coordinates (bond lengths, valence angles, and torsion angles) to find the geometry of metal chelates with the lowest energy t5-81. However, due to the computational limitations of the time, the approach was limited in that only a small number of internal coordinates could be adjusted simulta-... 

Molecular mechanics and dynamics studies of metal-nucleotide and metal-DNA interactions to date have been limited almost exclusively to modeling the interactions involving platinum-based anticancer drugs. As with metal-amino-acid complexes, there have been surprisingly few molecular mechanics studies of simple metal-nucleotide complexes that provide a means of deriving reliable force field parameters. A study of bis(purine)diamine-platinum(II) complexes successfully reproduced the structures of such complexes and demonstrated how steric factors influenced the barriers to rotation about the Pt(II)-N(purine) coordinate bonds and interconversion of the head-to-head (HTH) to head-to-tail (HTT) isomers (Fig. 12.4)[2011. In the process, force field parameters for the Pt(II)/nucleotide interactions were developed. A promising new approach involving the use of ab-initio calculations to calculate force constants has been applied to the interaction between Pt(II) and adenine[202]. 

Abstract Porphyrins and their analogues constitute one of the most important families of aromatic macrocycles. The present review discusses aromaticity of porphyrinoids, focusing mainly on non-expanded systems. The effect of structural modifications on the aromaticity-dependent properties of porphyrin-like macrocycles is described. It is shown that delocalization modes observed in porphyrinoids can be classified using a simple valence-bond approach. Aromaticity of porphyrinoids is further discussed as a function of tautomerism, coordination chemistry, and the oxidation state of the macrocycle. 

The use of coordination bonds to form networks, so-called coordination polymers, perhaps represents the most widely studied form of inorganic crystal engineering. This approach has also been examined in a number of reviews [8] and is discussed elsewhere in this volume (see Chapter 5). While coordination chemistry will have an important role to play in a number of the hydrogen-bonded systems presented in this chapter, coordination polymers will only be discussed in the context of their cross-linking or their perturbation using hydrogen bonds. 

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