Trisilyl amine, N(SiH3)3 is planar whereas trimethyl amines N(CH3)3 is pyramidal. Explain why?.

In N(SiH₃)₃, N attains sp2 hydridisation and the lone pair of N is involved in d_pi-p_pi  back bonding by getting itself delocalized on to empty 3d – orbitals of silicon.

But in N(CH₃)₃, N is sp3 hybridised in which three of the hybrid orbitals are used in forming s  - bonds with NH₃ groups, while the lone pair is present in the fourth hybrid orbital. Thus the molecule is pyramidal.

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