# H-N-H bond angle in ammonia boron trifluoride adduct

In the reaction:

$$\ce{NH3 + BF3 -> NH3-BF3}$$ does the HNH bond angle increase or decrease?

Surely, the FBF bond angle decreases because boron changes from $$\ce{sp^2 -> sp^3}$$. But how can we comment upon the NHN bond angle? In my first attempt, I expected it to increase because though N still remains in $$\ce{sp^3}$$ hybridisation lp-bp repulsions are greater than bp-bp repulsions.

But the answer key states that the NHN bond angle will decrease. Please let me know how to determine this.

I couldn't find the bond angle data on net or in JD Lee.

• I think bond angles should increase. Nov 13 '18 at 5:28
• Yes, that's what I think too Nov 13 '18 at 5:50