The order of B-N bond length in B(NR2)3, HB(NR2)2 and H2B-NR2 is as follows:

1. H2B-NR2
2. HB(NR2)2
3. B(NR2)3

Explanation:

H2B-NR2:
The B-N bond length in H2B-NR2 is the shortest among the three molecules. This is due to the presence of a double bond between boron and nitrogen. The double bond results in a stronger bond and shorter bond length.

HB(NR2)2:
The B-N bond length in HB(NR2)2 is longer than in H2B-NR2 and shorter than in B(NR2)3. This is because HB(NR2)2 has a single bond between boron and nitrogen, which is weaker than the double bond in H2B-NR2.

B(NR2)3:
The B-N bond length in B(NR2)3 is the longest among the three molecules. This is because B(NR2)3 has three nitrogen atoms bonded to one boron atom, resulting in a weaker bond due to increased electron density around boron.

In summary, the order of B-N bond length in these molecules is determined by the strength of the bond, which is influenced by the number of nitrogen atoms bonded to boron and the type of bond (single or double).

Due to back bonding.. answer is 1>2>3 answer .ok