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Question: Answered & Verified by Expert
In $\mathrm{BF}_{3^{\prime}}$ the $\mathrm{B}-\mathrm{F}$ bond length is $1.30 Å$, when $\mathrm{BF}_3$ is allowed to be treated with $\mathrm{Me}_3 \mathrm{~N}$, it forms an adduct, $\mathrm{Me}_3 \mathrm{~N} \rightarrow \mathrm{BF}_{3^{\prime}}$ the bond length of $\mathrm{B}-\mathrm{F}$ in the adduct is
Chemistryp Block Elements (Group 13 & 14)AIIMSAIIMS 2013
Options:
  • A greater than $1.30 Å$
  • B smaller than $1.30 Å$
  • C equal to $1.30 Å$
  • D none of these.
Solution:
1817 Upvotes Verified Answer
The correct answer is: greater than $1.30 Å$
In $\mathrm{BF}_3$, there is back bonding in between fluorine and boron due to presence of $p$-orbital in boron.
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back bonding imparts double bond characteristics.
As $\mathrm{BF}_3$ forms adduct the back bonding is no longer present and thus double bond characteristic disappears. Hence, bond becomes a bit longer than earlier $(1.30 Å)$.

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