Question

Trisilylamine ${(Si{H_3})_3}N$ :
A.Trigonal pyramidal and acidic
B.Trigonal pyramidal and basic
C.Trigonal pyramidal and neutral
D.Trigonal planar and weakly basic

Answer 1

To answer this question, you should recall the concept of back bonding and Lewis acids and bases. If there is bond formation between two atoms where one atom is having one vacant orbital and another is having lone pair of electrons, then this electron pair over the atom is donated to that respective vacant orbital. Lewis acids are atoms with vacant orbital and Lewis bases are atoms with a lone pair of electrons available for donation.

Complete step by step answer:
The structure of trisilylamine is trigonal planar due to the presence of back bonding. The answer lies in the concept of back bonding. Silicon has vacant d orbitals. The lone pair on nitrogen provides the vacant d orbital of silicon with electrons.
Hence, $N - Si{H_3}$ bond gains partial double bond character, i.e. its hybridisation becomes $s{p^2}$ which is trigonal planar. And due to its ability to donate electron lone pairs, it is basic in nature.

Hence, the correct answer to this question is option D.

Note:
Back bonding occurs between atoms in a molecule in which one atom has a lone pair of electrons and the other has vacant orbital space adjacent to each other. A compound with back bonding has pi-bonding character since it is formed only after the formation of one sigma bond. This allows the molecule to reduce energy, become stable and complete the octet. This phenomenon results in a decrease in bond length and an increase in bond order.