Covalent Bond Classification coordinate

The bonds formed between germanium and elements of Group VA can be divided into two categories those in which the element is bonded via a covalent bond and others in which a Lewis acid-Lewis base interaction between the two elements results in coordinate bond formation. The same classification of bond types is also appropriate when discussing organogermanium derivatives of Groups VIA and VII A. 

An overall classification of CT adducts is not such an easy task because the formation of any heteronuclear bond actually involves some form of CT, which will evolve in different ways according to the type of bond (a) in ionic bonds, a complete CT occurs (electron transfer) (b) in heteronuclear covalent bonds, a partial CT occurs until atomic electronegativities are fully equalized (c) in dative or coordination bonds an initial CT is followed by covalent bond formation with donor coordination expansion, as in the series HOCl, HOCIO, HOCIO2, and HOCIO3 (d) in dative or coordination bonds between Lewis acids and bases the so-called Lewis adducts are formed (e.g., in the H3N + BF3 H3N-BF3 reaction) with donor/acceptor coordination expansion (e) in outer [D,A] CT adducts a simple intermolecular attraction occurs without the formation of a new chemical bond. 

Upon interaction of two species, a pair of electrons from one species can be used to form a covalent bond to the other one. The species that donates the electron pair is called a Lewis base, and the speeies that accepts the electron pair is called a Lewis acid. Acids or bases with valence eleetrons that are tightly held and not easily distorted are hard acids or bases. Aeids or bases with valence electrons that are easily polarized or removed are soft. According to the hard and soft acid-base (HSAB) principle, hard acids are preferably coordinated with hard bases and soft acids are preferably coordinated with soft bases [58]. The hardness of an element usually increases with its oxidation state. Selected Lewis acids and bases and their classification into hard, intermediate, and soft acids and bases are listed in Table 4 [59]. 

One advantage of such a classification is that, while it is based upon experimental data, it also has a simple theoretical basis. Acid-base reactions involve coordinate covalent bonds oxidation-reduction reactions involve changes in valence odd-molecule reactions involve simple covalent bonds. The following comparison illustrates the differences between the suggested classification and the older one ... 

A different classification problem is raised by immunogens produced by metal salts. In these cases, autologous carriers are thought to become modified by metal ion interaction, and NAD which contain the metal and others which may not contain it can be envisaged. The question here is rather whether the coordinate bonds which are inferred for the transition metals should not be simply considered as covalent. 

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