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J.D. Lee Concise Inorganic Chemistry [3, p. 78] mentions that

The bond length order of the B-N bond in the following compounds is

$\ce{B(NR2)3}$ > $\ce{HB(NR2)2}$ > $\ce{H2B(NR2)}$

and the order of energy required for the rotation of the B-N bond is just reverse, i.e.

$\ce{H2B(NR2)}$ > $\ce{HB(NR2)2}$ > $\ce{B(NR2)3}$

Here the back bonding from N to B is most restricted in case of $\ce{B(NR2)3}$ due to steric crowding and most relaxed in case of $\ce{H2B-NR2}$. Hence, the above orders are well explained.

Here, from the above statement what I analyzed is as follows-

1)Due to steric crowding between bulky $\ce{NR2}$ groups, they rotate in the perpendicular plane and therefore, the lone pair on nitrogen atom is unavailable to back bond with the vacant orbital of boron.

2)And as we replace those $\ce{NR2}$ groups by hydrogen, steric crowding reduces, thereby increasing the planarity of the compound and increasing extent of back bonding. That is maximum in $\ce{H2B(NR2)}$ and due to more efficient back bonding there is more double bond character and lesser bond length.

3)Hence, we will require more energy to be supplied for rotation of B-N bond in the compound which has the most double bond character and less energy for rotation of B-N bond in the compound with more single bond character.

Is my logic correct?

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  • $\begingroup$ This is the second question of yours where the reference counter for some reason starts at 3 and there is no reference to this pointer. Why? $\endgroup$
    – andselisk
    Commented May 19 at 18:16
  • $\begingroup$ Sorry, I'm new here can you please tell me what you mean by "starts at 3" and what the pointer is? $\endgroup$
    – Chetan
    Commented May 20 at 3:54
  • $\begingroup$ The first number in [3, p. 78] is supposed to be the pointer to the literature reference, which is absent. The numbering is usually sequential in the order of references appearing, and starts with 1. $\endgroup$
    – andselisk
    Commented May 20 at 10:59
  • $\begingroup$ okay thanks for information $\endgroup$
    – Chetan
    Commented May 21 at 5:02
  • $\begingroup$ I deleted my answer. Your reasoning is totally correct, given that R is methyl. Other even bulkier R groups may introduce restricted rotation for very different reason. I forgot to pay enough attention to the title. $\endgroup$
    – Paul Kolk
    Commented May 21 at 9:32

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