Question

Define back bonding and explain its effects on acid strength.

Answer 1

A bond is a force existing between atoms in a molecule that holds the atoms together. Back bonding makes the compound stable as on its completion the octet rule is satisfied. Also a compound with back bonding has $\pi$-bonding character since it results after formation of sigma bond.

Complete answer:
Back bonding is the bonding of $\pi$-conjugated ligands to a transition metal which involves a synergic process with donation of electrons from the filled $\pi$-orbital or lone pair orbital of the ligand into an empty orbital of the metal (donor-acceptor bond) together with the release (back donation) of electrons from the d orbital of the metal into empty ${{\pi }^{*}}$antibonding orbital of the ligand.
Back bonding decreases down the halogen group.

As for acid strength i.e.$B{{F}_{3}},BB{{r}_{3}}$ etc are lewis acids because the central atom is short of an electron pair. So when the back bonding is effective, the shortage of electrons is compensated and the lewis acid character decreases. Hence acid strength decreases down the halogen group due to the effect of back bonding. **Note:** Back bonding is only effective when the size of valence shell matches. For example in $B{{F}_{3}}$ both boron and fluorine have their valence electrons in 2p but in $BB{{r}_{3}}$, lone pair electrons are in 4p while valence electrons of Boron are in 2p, so the size does not match. So back bonding decreases down the halogen group.