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What is the nature of bond between B and O in ${{({{C}_{2}}{{H}_{5}})}_{2}}OB{{H}_{3}}$ is:
[A] Covalent
[B] Coordinate covalent
[C] Ionic bond
[D] Banana shaped bond

Answer
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Hint: Borane is a Lewis acid therefore it can accept a pair of electrons and ${{({{C}_{2}}{{H}_{5}})}_{2}}O$ is a Lewis base. There will be sharing of electrons between these two atoms but it is not mutual. Such sharing has a particular name which will be the correct answer.

Complete answer: As we know, a covalent bond is a chemical bond which involves sharing of electrons between two atoms which gives rise to a stable balance between the attractive and repulsive forces hence, forms a stable bond which we know as covalent bonding.
However, in a coordinate covalent bond the sharing is only by one atom i.e. one of the atoms will donate its electron density and the other will accept it. There will be no mutual sharing. We generally find this type of bonding in complex compounds where there is a metal-ligand interaction. We even call this a dipolar bond because the involved atoms are of opposite polarity.
We can define ionic bonding as a bonding where there is a complete transfer of electrons as a result of which we get two oppositely charged ions. Due to this type of bonding the electron acceptor species become negatively charged and the donor species becomes positively charged.
Lastly, a banana bond is also a covalent bond but the geometry resembles the shape of a banana. We generally call this type of bond a bent bond.
As we know that $B{{H}_{3}}$ is a Lewis acid i.e. it can accept a pair of nonbonding electrons. So we can say that it will accept the non-bonding electrons present in the ${{({{C}_{2}}{{H}_{5}})}_{2}}O$ group and simultaneously, ${{({{C}_{2}}{{H}_{5}})}_{2}}O$ is a Lewis base, as it has non-bonding electrons to donate.
Therefore, we can say that oxygen will donate a pair of its lone pair to the boron atom which will give rise to a covalent bond. And as the sharing is only by one atom i.e. oxygen therefore, we can say that it forms a coordinate covalent bond which we can represent as-

     \[{{({{C}_{2}}{{H}_{5}})}_{2}}O\to B{{H}_{3}}\]

The co-ordinate covalent bond is shown by an arrow.

So, the correct answer is “Option B”.

Note: We should not be confused between coordinate covalent and covalent bond even though they are similar as covalent bonding includes sharing between two atoms. In the given question we have a Lewis acid and a Lewis base, sharing is not possible as one species will donate its electron density and the other accepts it. Therefore, a coordinate covalent bonding is formed between the boron and oxygen atom.