I have read the other answer on chemistrystackexchange regarding the bond angles of $\ce{NH3}$ and $\ce{NCl3}$. The bond angle of $\ce{NCl3}$ is more than $\ce{NH3}$ due to back bonding between p-orbitals and $\ce{N}$ and d-orbitals of $\ce{Cl}$. But recently I came across a question asking for the relation between bond angles of $\ce{NH3}$ and $\ce{NBr3}$. I thought that due to the same reason of back bonding described earlier the bond angle of $\ce{NBr3}$ would be greater than $\ce{NH3}$. But the answer key gave the correct answer as bond angle of $\ce{NH3} > \ce{NBr3}$. Why is it so that the back bonding reason does not work here? Is there any other influencing factor?

And also why is the bond angle of $\ce{PBr3} > \ce{PH3}$ ... which is opposite to the trends shown by corresponding compounds of Nitrogen?



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