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Arrange the following in the correct order of increasing bond angle at the central nitrogen atom: NH3, (CH3)3N, (SiH3)3N.

1. NH3 > (CH3)3N > (SiH3)3N
2. (SiH3)3N > (CH3)3N > NH3
3. NH3 > (SiH3)3N > (CH3)3N
4. (CH3)3N > (SiH3)3N > NH3

Correct answer: (SiH3)3N > (CH3)3N > NH3

Solution

In (SiH3)3N: back-donation of N lone pair into Si empty d-orbitals causes N to become nearly planar (sp2), giving bond angle ~120 deg. In (CH3)3N: methyl groups are electron-donating and bulky; steric repulsion between methyls and inductive effects push the bond angle slightly above that in NH3, giving ~108 deg. In NH3: ~107 deg (sp3 with one lone pair). Order: (SiH3)3N > (CH3)3N > NH3.

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