Question

Dimethyl ether forms adduct to the $B{F_3}$, but silyl ether does not form adduct.

Answer 1

$B{F_3}$ is a Lewis acid which accepts electron pairs from a base. An adduct is a complex formed to stabilize the base by donation of electrons to an electron deficient species.

Complete step by step answer:
Dimethyl ether is indicated as ${\left( {C{H_3}} \right)_2}O$. Here two methyl groups are attached to oxygen atoms via single bonds. Oxygen is an element in the periodic table with atomic number $8$. Its electronic configuration is $\left[ {He} \right]2{s^2}2{p^4}$.
Oxygen has six numbers of electrons in the outermost shell $2$. Out of these six electrons two are bonded with the two methyl groups via sigma bonds and two remain non-bonded. The hybridization of central atom oxygen is $s{p^3}$.
The non-bonded electrons located on oxygen are available for donation to an electron deficient species. Thus it acts as a base for donating electrons.
$B{F_3}$ is a compound composed of boron and fluorine. The central atom in $B{F_3}$ is boron. Its atomic number is $5$ and electron configuration is $\left[ {He} \right]2{s^2}2{p^1}$. The boron is $s{p^2}$ hybridized using the three outermost electrons which are bonded with three fluorine atoms via single bonds.
Boron has an empty $2p$ orbital which accepts electrons when treated with a base like $N{H_3}$ or $M{e_2}O$. Thus the boron atom becomes $s{p^3}$ hybridized by accepting the electron pair from a donor atom. Thus adduct is formed on treating dimethyl ether with $B{F_3}$.
In disilyl ether the molecular formula is ${\left( {Si{H_3}} \right)_2}O$. Here the carbon is interchanged with silicon atom. But the change brings change in the electronic properties of the molecule. Silicon is an element in the periodic table with atomic number $14$ and electronic configuration $\left[ {Ne} \right]3{s^2}3{p^2}$.
The silicon atom in disilyl ether has an extra vacant orbital which is $3d$ orbital. The silica atom is able to accept electrons from a donor atom which is oxygen in this case. Thus in disilyl ether the electrons on oxygen atom are not located on oxygen atom but are involved in $2p\pi - 3d\pi$ back bonding with silicon $3d$ orbital.
Hence the electrons are not available for donation to other electron deficient species like $B{F_3}$. Thus no adduct forms on treating disilyl ether with $B{F_3}$.
Hence the correct option is B.

Note:
The outer vacant $d$ orbitals present in elements down the group like aluminium, silicon or transition metals allow back bonding with atoms, ions or ligands.