Valence, coordinate covalence origin

One final point about covalent bonds involves the origin of the bonding electron pair. Although most covalent bonds form when two atoms each contribute one electron, bonds can also form when one atom donates both electrons (a lone pair) to another atom that has a vacant valence orbital. The ammonium ion (NH4+), for example, forms when the two lone-pair electrons from the nitrogen atom of ammonia, NH3, bond to H +. Such bonds are called coordinate covalent bonds. 

Lewis acids have low-lying empty valence orbitals that readily interact with lone pair(s) on other atoms (Lewis bases) to move towards a closed shell electronic configuration. These definitions do not specify that a lone pair of electrons must be transferred from one atom to another—only that an electron pair, residing originally on one atom (the Lewis base) must be shared with the Lewis acid. Neutralization is defined as coordinate covalent (or dative) bond formation. This results in a bond in which both electrons are furnished by one atom or ion (the Lewis base). 

Borides, in contrast to carbides and nitrides, are characterized by an unusual structural complexity for both metal-rich and B-rich compositions. This complexity has its origin in the tendency of B atoms to form one- two-, or three-dimensional covalent arrangements and to show uncommon coordination numbers because of their large size (rg = 0.88 10 pm) and their electronic structure (deficiency in valence electrons). The structures of the transition-element borides are well established " . 

A model which is often used for covalent or partly covalent crystals is the valence force model. This model has originally been developed for vibrations in molecules [4.19,20] but the formalism can be extended to crystals as well [4.21,22]. The potential energy is expressed in terms of changes in bond distances, bond angles and other so-called internal coordinates, we shall illustrate this model for the linear (SN) chain and discuss the results for diamond and 3-AgI. 

The term covalent was originally applied to the bonds in the ball and stick picture of structure used in organic chemistry. It is therefore interesting to note under what conditions the ball and stick picture can be derived from bond valence theory. The concept of a bond arose in the mid nineteenth century from the study of the chemistry of carbon compounds. The original model was simple it envisioned atoms as having a fixed number of hooks each of which could form a bond by linking to a similar hook on other atoms. This ball and stick picture, in which the balls represent the atoms and the sticks the bonds, can be derived from the bond valence theory if all the atoms have coordination numbers that are equal to their valence. Examples are silicon (V = N = 4), molybdenum (V = N = 6), and the two elements that form hydrocarbons carbon (V = N = 4) and hydrogen (V = N= 1). ... 

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