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Coordinate Covalent Bond

Atoms are composed of three fundamental particles, electrons, protons and neutrons. Protons and neutrons are placed at the center of the atom that is called as a nucleus. Electrons are placed around the nucleus in the shell which is further divided in sub-shells and orbitals. The outermost shell of an atom is called as valence shell and electrons in this shell are known as valence shell electrons. The number and position of valence electrons in an atom determines its chemical and physical properties.

All elements tend to get stability by getting the octet configuration that is also known as Noble gas configuration. The valence shell electronic configuration of Nobel gases is ns2, np6. All elements form covalent or ionic bond with other elements to get this octet configuration. The covalent bond is formed by the equal sharing of electrons between two elements. An element can form only that number of covalent bond by which it gets octet configuration. For example, chlorine atom forms one covalent bond with other chlorine as it has 7 electrons in its valence shell and requires only one electron to get the octet configuration.  There is one more type of covalent bond which involves unequal sharing of electrons between bonded atoms. Such type of covalent bonds is known as coordinate covalent bonds. Let’s discuss some features of this type of covalent bond.

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Coordinate Bond

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The chemical bonding exists between the two atoms of same elements or different element. The existence of strong intermolecular forces of attraction between two or many atoms is called as chemical bonding. Chemical bonding is formed so that the resultant molecule is stabilized.

The formation of the octet is generally taken as the criteria for forming a bond. When a chemical bond is formed due to stability it is normally permanent until it is cleaved by giving some external factor like chemical compounds, temperature, heat etc., When the chemical compound is formed from two elements, the property of the combining elements are not seen in the chemical compound, it is the unique property of the chemical compound which is seen.

For example when sodium and chlorine react, the resulting sodium chloride does not show the property of Sodium or Chlorine, but it shows the property of Sodium Chloride.

There are three types of bonding that the atoms may try to form.

  1.     Ionic bond
  2.     Covalent bond
  3.     Coordinate Covalent bond
  Ionic bond is formed by transfer of electron, when an atom can transfer the electron to another atom of the element the ionic bonds are formed.
For example when sodium and chlorine are going to combine to form sodium chloride, Sodium has 1 electron excess than the octet and hence it transfer one electron from its outer shell to Chlorine which needs one electron to complete its third shell. Since sodium gives up the electron it forms positive charged ion, while chlorine gains the electron and forms negatively charged ion.

This type of charged ions attracts each other. The rule is opposite charge attract. So the sodium cation is attracted by the chloride ion by electrostatic forces of attraction or Columbic forces of attraction. This type of bonding normally takes place between a metal and the non metal. Covalent bonding are formed due to sharing of electron between the combining atoms.

The shared electron will make the two atoms stable. For example consider the case of Carbon, carbon has got 4 valence electron, it cannot lose the four valence electron or it cannot gain another four electron. Hence it shares its 4 valence electron with other atoms to attain the stable octet configuration.
  •  In the covalent bonding there is an overlap of the atomic orbital having the electron takes place.
  •  The orbital space is shared between the two orbital and since the electron is between the two atoms the bonding results in equal sharing of the pair of electrons. Generally the valence shells  or orbital is shared.
  • The shared  pair of electron is normally in the middle of the atoms, thereby they can satisfy the octet rule.
  • The covalent compound possesses low melting and boiling point. Ionic compounds which have strong intermolecular forces have high melting and boiling point.
  • The intermolecular forces of attraction between the covalent bond are weak.
  • There are no columbic forces of attraction between the molecules like in the ions.
  • So most of the covalent compounds has low melting and boiling point.
  • Covalent compounds are non conductor of electricity in the solid or in the liquid form where as  Ionic compound conducts electricity in the molten or in the dissolved form.    
  • Covalent bonds are rigid and directional, whereas ionic bonds are non directional.
  • Most of the covalent compounds are non polar and they are not soluble in water, but they are soluble in non polar solvent.
  • While ionic compounds are more soluble in water because they have ion-dipole interaction in the water.
Coordinate covalent bonding is also called as the Dative bonding. It is similar to that of the covalent bond. But the only difference is that the shared pair of electron form one atom or one group of atoms.

When shared pair of the electron comes from only atom and not one each from the two atoms involved in the bonding then it is called as co-ordinate or dative bonding. The atoms which donates the electron to form co-ordinate covalent bonding or dative bonding is called as the donor atom, while the atom which accepts the pair of electron for bonding is called as the acceptor atoms.
Thus co-ordinate bond can also be explained as the bond formed between the donor and acceptor atoms. A co-ordinate bond is represented by an arrow starting from the donor atoms and ending in the acceptor atom. In some cases the donor may be a molecule, an atom in the molecule can donate the pair of electron, in that case the Lewis base can act as a donor atom donating a pair of electron and the acceptor would be the Lewis acid.A co-ordinate covalent bond has all the characteristic of the covalent bond. They have low boiling and melting point. Since the shared pair is between two atoms there are no columbic forces of attraction. They do not conduct electricity in the liquid or in the dissolved state. The compounds are that much soluble in water. 

Coordinate Covalent Bond Examples

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Lewis acid base reaction is an excellent example of the co-ordinate covalent bond. For example the bond

H3N: BF3

is a coordinate bond. Here nitrogen acts a donor atom.

The lone pair of electron in the nitrogen is donated to the vacant p orbital of the boron. Here ammonia is Lewis base and BF3 is Lewis acid.

The formation of the hydronium ion when the water reacts with proton is also an example of the co-ordinate bonding

H2O + H+ H3O+

There two lone pair of electron in the oxygen atom which is used to form coordinate bond with the proton. Here water acts the donor atom, more specifically it is the oxygen atom in the water that acts as the donor atom and the proton accepts the electron.

All the coordination complex has the co-ordinate bond. In the complex the donor atom is called as ligand. For example consider the complex [Ag(NH3)2]Cl. Here the ammonia act as the donor atom or ligand and the metal acts the Lewis acid or acceptor.

Ammonium Ion Coordinate Bond

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  It is formed by the combination of the ammonia molecule and a hydrogen ion. In ammonia, the nitrogen atom has a lone pair of electrons after completing its octet. It donates this lone pair to the hydrogen ion. 

Thus the nitrogen atom becomes the donor. The hydrogen atom becomes the acceptor. The linkage between N and H atoms is called coordinate bond. It is represented by an arrow →.

Coordinate Bond in Ammonia

Hydronium Ion Coordinate Bond

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It is formed by the combination of water molecule and hydrogen ion.

Coordinate Bond in Water
  • The oxygen atom in a water molecule has two lone pairs of electrons. 
  • It donates one pair to the hydrogen ion. 
  • Oxygen is thus the donor and hydrogen ion, the acceptor. 
  • The hydrogen ion carries over its charge to the hydronium ion.
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