Question

Select the correct statement(s) regarding the structure of $\text{A}{{\text{l}}_{2}}{{\left( \text{C}{{\text{H}}_{3}} \right)}_{6}}$.
(A) All carbon atoms of $-\text{C}{{\text{H}}_{3}}$ groups do not lie in the same plane.
(B) One vacant orbital of each Al – atom is involved in $\text{s}{{\text{p}}^{3}}$ hybridization.
(C) There are only 8 $\text{s}{{\text{p}}^{3}}$ hybridized atoms present.
(D) There are a total of 48 bonding electrons available.

Answer 1

The structure and bonding in $\text{A}{{\text{l}}_{2}}{{\left( \text{C}{{\text{H}}_{3}} \right)}_{6}}$ are similar to that in the structure of diborane $\left( {{\text{B}}_{\text{2}}}{{\text{H}}_{\text{6}}} \right)$. Each boron in diborane is $\text{s}{{\text{p}}^{\text{3}}}$ hybridized and forms four covalent bonds with hydrogen atoms and banana bonds between the two boron atoms. The banana bond consists of 2 electrons shared between 2 borons and a hydrogen atom.

Complete step-by-step answer: Trimethylaluminum or $\text{A}{{\text{l}}_{2}}{{\left( \text{C}{{\text{H}}_{3}} \right)}_{6}}$ is an organometallic compound and exist in a dimer state. The structure of this compound consists of four-terminal methyl groups and two bridges between the two aluminium metals. This structure of trimethylaluminum is analogous to diborane and it is known as a 3-center 2-electron bond or banana bond.
It is called a 3-center 2-electron bond because each aluminium uses two electrons in bonding to terminal methyl groups and one remaining electron in vacant orbital involved in $\text{s}{{\text{p}}^{3}}$ hybridization forms a bond with hydrogens which act as a bridging atom between two aluminium metals. So, each aluminium atom, as well as the methyl groups, are $\text{s}{{\text{p}}^{3}}$ hybridized.
As there are 2 aluminium and 6 methyl groups, so there are a total of 8 $\text{s}{{\text{p}}^{3}}$ hybridized atoms in the compound.

Now, we can see from the structure that there are 22 two-center two-electron bonds and 2 three-center two-electron bonds. So, a total of 24 covalent bonds are present and hence the number of total bonding electrons is 48.
Also, the carbon atoms of methyl groups involved in bridging do not lie in the same plane as that of the other atoms.
Hence, all the statements are correct regarding the structure of $\text{A}{{\text{l}}_{2}}{{\left( \text{C}{{\text{H}}_{3}} \right)}_{6}}$.

Note: The bonds other than the banana bond in the structure of$\text{A}{{\text{l}}_{2}}{{\left( \text{C}{{\text{H}}_{3}} \right)}_{6}}$ are not 3-center 2-electron bonds. The other bonds are the normal 2-center 2-electron covalent bond between aluminium and methyl groups.