COVALENT BONDS And DATIVE COVALENT BONDS

 🎗Covalent bonds

Covalent bonds are formed when a pair of electrons is shared between two atoms of the same element or different elements. The sharing pair of electrons contribute one electronfrom each atom to form the electron pair. Consequently, stable electron configurationsare often achieved by both atoms in respect to the total number of electrons in the valence shells.Kossel, Langmuir and Lewis stated that the filling of electrons into a valance shell up to the maximum value of 8 results a stable electron configuration, hence called ‘octet’ rule.According to the current knowledge of electron configurations, the maximum number of valance electrons in 2s and 2p orbitals of elements in the second period (n=2) is 8.Therefore, elements in the second period complete the octet when forming chemical bonds thereby achieving a greater stability. This is more likely for elements such as C, N, O and F which form chemical bonds to complete the octet.

The valance shell of elements in the third period (n=3) and subsequent periods consist of d sub energy level in addition to s and p sub energy levels. Therefore, when forming chemical bonds, there could be instances where the number of electrons in the valance shell may exceed eight. Examples of such molecules are SO2 and SO3. In such molecules the number of electrons in the valance shell of sulphur is greater than eight. The presence of d orbitals in the valence shell of the sulphur atom permits 18 electrons. Since, the dorbitals in the valance shell also participate in bonding, the number of valance electrons in the sulphur atom can exceed the octet. However, for such atoms, it is not always necessary for the d orbitals to participate in bonding. For example, in the H2S molecule, the sulphur atom complete the octet without involving d orbitals.

There are other situations where atoms of some elements do not necessarily complete the octet. Elements like Be, B and Al form some electron deficient compounds such as BeCl2, BH3, BCl3 and AlCl3 are examples of such compounds with an incomplete valance shell. In the case of hydrogen atom where only 1s orbital is present, the stable electron configuration is achieved when the valance shell consists of two electrons. In all instances described above, the number of electrons in the valance shell after forming chemical bonds is an even number. However, this is not always true, as there are compounds such as NO and NO2 each having an odd number of electrons even without completing the octet.

🎗Dative covalent bonds 

In a molecule or ion, dative bonds are formed when atoms with empty orbitals interact with atoms with a lone pair of electrons. In certain cases when the free atom of the element 

has less than four valance electrons (as in Be, B), the number of covalent bonds that the particular atom can form is less than four. This results in incomplete octet with lower 

stability. Therefore, such electron deficient central atoms preferably react with the atom having lone pairs which can donate an electron pair to the central atom to complete the 

octet. The reaction of BH3 with CO to produce borane carbonyl and the reaction with CN- to produce cyanoborohydride are example with such dative bonds. Furthermore, reaction of NH3 with BF3 to form a dative covalent bond between B and N is another example. The dative bond is formed when the empty orbitals in B overlaps with the orbital having the lone pair in the nitrogen atom. In this case a central atom cannot be chosen precisely. 

Since the lone pair on the nitrogen atom is donating the electron pair to B, the bond can be denoted by an arrow. The arrow head is pointed to the electron deficient atom. This 

can be illustrated using formal charges as well as shown below.

Dative covalent bonds are also formed when metal ions or some metal atoms react with molecules or ions having lone pairs (H2O, NH3, CO molecules and CN-ions) in order to 

form complexes. The following illustrates the formation of a complex ion when Cu2+ is reacted with four NH3 molecules to form dative covalent bonds.