Question

In polymeric beryllium chloride ${(BeC{l_2})_n}$ there are__________.
(A) three –center two-electron bonds
(B) three-center four-electron bonds
(C) two-center three-electron bonds
(D) two-center two-electron bonds

Answer 1

$BeC{l_2}$ is an electron deficient compound.
It has $sp$hybridization. It combines with another $BeC{l_2}$ molecule to give $B{e_4}C{l_6}$ in solid state.
It can form coordinate bonds with other molecules of the same type and form chain-like structure.

Step by step answer: Let us discuss hybridization ion $Be.$
Atomic number of $Be$ is $- 4$
Its electronic configuration is, $1{s^2}2{s^1}2p_x^12p_y^02p_z^0$
One $s$ and one $p$orbitals hybridized and form two hybrid orbitals.
These hybrid orbitals overlap with $2{p_z}$ orbitals of chlorine atom and form covalent bond

Beryllium has electrons in its orbit in molecule $BeC{l_2}$, therefore $Be$ acts as a deficient center and acts as Lewis acid.
In the solid state $BeC{l_2}$ has polymeric chain structure
$Be$ Atom tetrahedral surrounded by four $Cl$atoms.
Two Cl-atoms are bonded by covalent bond and two by co-ordinate bond.
The polymeric structure of $BeC{l_2}$ is due to its electron deficient nature.
Therefore, bonds of Cl-atoms to Beryllium atoms are one via covalent and other via dative bond. So it is called three-center four electron bonds. [$2Be$ atom and $1Cl$ atom].

Therefore, from the above explanation the correct option is (B) three-center four-electron bonds.

Additional Information: $BeC{l_3}$exist as polymeric structure in solid state and exist as linear structure in gaseous state.
At high temperature solid polymer of $BeC{l_2}$dissociates into linear monomers. The temperature is of the order of $1200k.$

Since $Be$ is $sp$ hybridized therefore bond angle of $BeC{l_2}$ is ${180^0}$ and structure as linear $BeC{l_2}$ is inorganic compound. The formed charge of $BeC{l_2}$ is zero due to the $2$ shared bond. Cl-atom has six electrons and $1$ shared bond making formal charge of both Cl-atoms is zero. This proves that $BeC{l_2}$ has the most stable structure.

Note: $BeC{l_2}$ violate octet Rule. In the molecule $Be$ associated with four electrons and acts as Lewis acid. It does not have complete octet but formal change on $BeC{l_2}$ is zero, so it stable in $BeC{l_2}$ structure