Why N-N bond length is greater in N2H4 than in N2F4? And why B-F bond length is greater in BF3 than in BF4^- ?

Dear User,
This can be explained on the basis of Bent's rule. According to the Bent's rule, the orbitals of more electronegative atom has less s character and the orbitals of more electropositive atom has more s character. 
We know that F is more electronegative than H and so hybrid orbitals in N2F4 will have less s character than in N2O4.
This means that in N2F4, N-N bond will have less s character and so the bond length decreases.
Whereas in N2H4, the N-N bond has more s character and therefore its bond length increases.

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