Coordinate covalent bonds ammonia molecule

To illustrate further aspects of the interplay between covalent and coordinate bonding, let us consider the successive coordinative additions of ammonia molecules (ammine ligands, NH3) to tungsten hydrides in the series H6 2n W(NH3) , n = 1-3,... 

Therefore it cannot donate electrons to form a bond. But nitrogen (N) in the ammonia molecule has 1 unshared electron pair and it can therefore form a coordinate covalent bond with boron. 

FIGURE 5.16 The electron dot structure of a nitrogen atom (left) and an ammonia molecule (right). The pair of electrons above the nitrogen is the nonbonding pair available for coordinate covalent bonding. 

The solubility of an ionic compound increases dramatically if the solution contains a Lewis base that can form a coordinate covalent bond (Section 7.5) to the metal cation. Silver chloride, for example, is insoluble in water and in acid, but it dissolves in an excess of aqueous ammonia, forming the complex ion Ag(NH3)2 + (Figure 16.13). A complex ion is an ion that contains a metal cation bonded to one or more small molecules or ions, such as NH3, CN-, or OH-. In accord with Le Chatelier s principle, ammonia shifts the solubility equilibrium to the right by tying up the Ag+ ion in the form of the complex ion ... 

The electrons of an electron-pair bond need not be contributed by both of the bonding atoms, as is demonstrated in the formation of the ammonium ion by the addition of a proton to ammonia. The Lewis structures for these two entities are shown in Fig. 9-13 recall that H+ has no electrons. Such a bond is often called a coordinate covalent bond, but is essentially no different from any other covalent bond. The special name indicates that one of the members of the bond brings to the process of bonding any electrons. In this particular case, once the bond has formed, it becomes indistinguishable from the other three N—H bonds in the molecule with the structure being a regular tetrahedron. 

When the ammonia molecule provides both electrons that get shared with the H+ ion, a coordinate covalent bond is formed. 

Figure 8.16 In this reaction between boron trihydride (BH3) and ammonia (NH3), the nitrogen atom donates both electrons that are shared by boron and ammonia, forming a coordinate covalent bond. Interpret Does the coordinate covalent bond in the product molecule satisfy the octet rule ... 

The movement of an electron pair during an acid-base reaction is the basis of the Lewis theory of acidity developed by Gilbert Lewis (see Chapter 4). A Lewis acid is defined as a substance that can accept a pair of electrons from another atom to form a coordinate covalent bond. A Lewis base is defined as a substance that can donate a pair of electrons to another atom to form a dative covalent (coordinate) bond. In the simple examples above, the proton (H" ) is the Lewis acid and the ammonia molecule and water molecule are the Lewis bases. 

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