p-Block Elements: Back Bonding and Dimerisation

These flashcards cover important concepts of p-Block elements, specifically focusing on the mechanisms and examples of back bonding and dimerisation.

Back bonding

A $\pi$-co-ordinate bond formed between 2 adjacent bonded atoms in a covalent molecule/ion by colateral overlapping.

Conditions for back bonding

(i) One bonded atom must possess a vacant orbital and the other must possess a lone pair. (ii) Atoms must belong to the 2nd period, or one to the 2nd and the other to the 3rd period.

Effects of back bonding

As a result of this overlapping between bonded atoms, the bond length decreases and the bond energy increases.

Order of strength for p\text{\pi}-p\text{\pi} back bonding

$$2p - 2p > 2p - 3p > 2p - 4p$$

Inorganic Benzene

Also known as borazene or borazole ($B_3N_3H_6$), it features $sp^2$ hybridisation for both $B$ and $N$ and is more reactive than benzene due to polar bonds.

Trimethyl amine ($(CH_3)_3N$)

A molecule where $N$ is $sp^3$ hybridised with a trigonal pyramidal structure; it acts as a stronger Lewis base because back bonding is not possible.

Trisilyl amine ($(SiH_3)_3N$)

A molecule where $N$ is $sp^2$ hybridised with a trigonal planar structure due to p\text{\pi}-d\text{\pi} back bonding; it is a weaker Lewis base.

Silyl alcohol ($SiH_3-O-H$)

A molecule that is more acidic than methyl alcohol because back bonding is present in its conjugate base.

Banana Bond

A $3$-centre-$2e^-$ bond involved in the dimerisation or polymerisation of molecules like $B_2H_6$ and $(BeH_2)_n$.

Diborane ($B_2H_6$)

An electron-deficient Lewis acid where Boron is $sp^3$ hybridised; it contains 4 terminal $2$-centre-$2e^-$ bonds and 2 bridging $3$-centre-$2e^-$ bonds.

$$(BeH_2)_n$$

A polymer of $BeH_2$ in the solid state which is non-planar and features $sp^3$ hybridisation of Beryllium.

$$Al_2Cl_6$$

The dimer of $AlCl_3$ in liquid or solid state featuring $sp^3$ hybridised Aluminium and $3$-centre-$4e^-$ bonds; its octet is complete, making it not electron deficient.

$$(BeCl_2)_2$$

The dimer of $BeCl_2$ in the vapour state which is planar, electron deficient, and features $sp^2$ hybridisation of Beryllium.

$$(BeCl_2)_n$$

The polymer of $BeCl_2$ in the solid state which is non-planar, not electron deficient, and features $sp^3$ hybridisation of Beryllium.

$$I_2Cl_6$$

The dimer of $ICl_3$ where Iodine is $sp^3d^2$ hybridised; it is a planar, electron-rich molecule with $12 e^-$.

Steric Repulsion in Boron Halides

The reason why $BCl_3$, $BBr_3$, and $BI_3$ do not form dimers, due to the small size of boron and the large size of the halogens.

Ionic nature of $AlF_3$

The specific property that prevents $AlF_3$ from forming a dimer, unlike other aluminium halides.