Question

All the ${\rm{Al}} - {\rm{Cl}}$ bonds in ${\rm{A}}{{\rm{l}}_2}{\rm{C}}{{\rm{l}}_6}$ are equivalent
A ) True
B ) False

Answer 1

Electron deficient compounds have coordinate covalent bonds in which number of electrons are lesser than that required for the formation of normal covalent bonds. When the electronegativity difference between two atoms is less than 1.7, coordinate covalent bonds are formed.

Complete answer:
Aluminum has three valence electrons whereas chlorine has seven valence electrons. One aluminum atom shares three electrons with three chlorine atoms to form three ${\rm{Al}} - {\rm{Cl}}$ bonds.
The structure of aluminum chloride varies with change in temperature. It also changes with change in the physical state, solid, liquid or gas. In solid state, aluminum chloride has a cubic close packed layered structure. In a liquid and gaseous state, a dimer is formed. In the dimer, each aluminum atom has tetrahedral geometry. At high temperature, trigonal planar monomer is present.
The given statement is false. All the ${\rm{Al}} - {\rm{Cl}}$ bonds in ${\rm{A}}{{\rm{l}}_2}{\rm{C}}{{\rm{l}}_6}$ are not equivalent.
Some ${\rm{Al}} - {\rm{Cl}}$ bonds are covalent whereas other ${\rm{Al}} - {\rm{Cl}}$ bonds are coordinate.
Terminal ${\rm{Al}} - {\rm{Cl}}$ bonds are normal covalent bonds. They are $2c - 2e$ bonds.
The bonds containing ${\rm{Al}} - {\rm{Cl}} - {\rm{Al}}$ bridges are $3c - 2e$ bonds.
Write down the structure of ${\rm{A}}{{\rm{l}}_2}{\rm{C}}{{\rm{l}}_6}$ dimer as shown below.

Hence, the option B ) is the correct option.

Note: ${{\rm{B}}_2}{{\rm{F}}_6}$ is also an electron deficient compound and it involves halogen bridges. ${\rm{A}}{{\rm{l}}_2}{\rm{C}}{{\rm{l}}_6}$ is an electron deficient compound. The number of electrons in the valence shell is less than that required to complete the octet.