Co-ordinate bond is type of bond which is formed between two atoms in which one atom completely donates its two electron or one lone pair to second atom.
What we usually see in covalent bond is that in covalent bond 1 electron is shared by one atom and the another electron is shared by II atom, so in covalent bond there is sharing of electrons.
But in co-ordinate bond, the two electrons come from single atom which is called donor atom(as it donates its lone pair) and the atom to which lone pairs are donated , or we can say the atom which is receiving electrons or lone pair is known as receiver atom.
Now the question arises that oxygen has 6 electrons and if it gets two more electrons, then its octet will be completed and it will become stable.
During the formation of a co-ordinate bond the donor atom donates two electrons which suggests that the donor atom has two free electrons which are not involved with any other atom in bond i.e., they are just free to use.
So why don't these electrons take participate in in double bond formation instead of being complete donatated to the received atom as in the case of N2O, H3O+, H2SO4, etc.
Let's take the case of N2O first:
In N2O
As we can see in this diagram that oxygen can form double bond with nitrogen but there is no formation of double bond(pi bond) between Oxygen and Nitrogen. Instead of this, there is formation of co-ordinate bond. So why oxygen is not forming double bond with nitrogen or we can say nitrogen is not forming double bond with oxygen?
Now let's assume that there is formation of double bond between Oxygen and Nitrogen.
Now have a look at this below diagram and count the number of electrons in oxygen and nitrogen after formation of double bond
In this diagram we can see that initially (before the formation of double bond), oxygen has 6 electrons and after forming double bond with nitrogen Oxygen has 8 electrons this means the octet of oxygen is now completed and it is now stable.
But look at the nitrogen in the middle. This nitrogen had 8 electrons(when it has not formed double bond with oxygen) which means it was already stable but after formation of double bond with oxygen this nitrogen has now 10 electrons due to which its octet has extended or expanded now(now it's valence shell contains more than 8 electons).
As we know, every molecule has a tendency to complete it's octet.
Any atom is said to be most stable only when it's octet is complete i.e., when it contains only 8 electons in its outermost valence shell neither less than 8 nor greater than 8 electons. As in the case of Noble gases or group 18 elements in periodic table, they have only 8 electons in their outermost valence shell and that's why they are the most stable elements in the whole periodic table(except Helium which has 2 electons in outermost valence shell i.e duplet).
When the central atom posses more than 8 electrons or less than 8 electrons then the atom will be no longer stable due to incomplete octet or extended octet.
So for a molecule to be stable, it is mandatory that it should have 8 electrons in its outermost valence shell.
So if nitrogen forms double bond with oxygen it will have 10 electrons in its valence shell which will make it unstable that's why nitrogen do not form double bond with oxygen in N2O.
Now let's have a look when the nitrogen form co-ordinate bond with oxygen.
in this case, we can see that nitrogen is donating its electron pair(not sharing its electron pair) to oxygen.
Now, once again count the number of electrons in the outermost shells of oxygen and nitrogen.
In this case, oxygen got 8 electrons in its valence shell and nitrogen also have 8 electons in its valence shell
And both nitrogen and oxygen have a octet due to which have a stable configuration
You might be thinking that when nitrogen has donated these two electons to oxygen, then why are we counting these donated electrons in the valence shell of nitrogen?
It is important to note that although nitrogen had donated the electrons to oxygen but these electrons will be still counted in the valence shell of nitrogen and oxygen both.
In this way, after formation of co-ordination bond between nitrogen and oxygen, both have 8 electons in outermost valence shell hence both are stable.
That's why a Co ordinate bond is formed between Oxygen and Nitrogen and not Double bond.
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