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What is back bonding?

What is back bonding?


1 Answers

40 Points
5 years ago
πbackbonding, also called π backdonation, is a concept from chemistry in which electrons move from an atomic orbital on one atom to a π* antibonding orbital on a π-acceptor ligand.
It is especially common in the organometallic chemistry of transition metals with multi-atomic ligands such as carbon monoxide, ethylene or the nitrosonium cation. Electrons from the metal are used to bond to the ligand, in the process relieving the metal of excess negative charge. Compounds where π backbonding occurs include Ni(CO)4 and Zeise's salt.
The electrons are partially transferred from a d-orbital of the metal to anti-bonding molecular orbitals of CO (and its analogues). This electron-transfer (i) strengthens the metal-C bond and (ii) weakens the C-O bond. The strengthening of the M-CO bond is reflected in increases of the vibrational frequencies for the M-C bond (often outside of the range for the usual IR spectrophotometers). Furthermore, the M-CO bond length is shortened.
 like BF3, the boron atom has an incomplete octet. The fluorine atom on its side has a lone pair which it can donate to boron. But, flurorine is also a very electronegative element. So, it also has a tendency to take back the electrons that it had donated to boron. This way, the lone pair of electrons keep jumping between fluorine and boron. This is called back bonding. This provides the lone pair of electrons more number of exchange positions (which simply means more space). As a result, the molecule becomes more stable.
However, back bonding is effective only when the size of the valence shell matches. In the case of BF3, both boron and fluorine have their valence electrons in 2p. But in BBr3, lone pair electrons are in 4p while valence electrons of Boron are in 2p. So, the size does not match. Also, electronegativity of the halogen decreases down the group. Hence, effectiveness of back bonding with Boron decreases down the halogen group.

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