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Θοδωρής Βαχλιώτης
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Kοίτα τι βρήκα Γιάννη:

1) The following order is experimentally found:

 

BF3<BCl3<BBr3<BI3BFX3<BClX3<BBrX3<BIX3

 

This stands in contrast to what is expected when the electronegativity of the halides is considered. A more electronegative halide should be able to stabilize the negative charge at BB in the Lewis acid-base complex better, and this would suggest that BF3BFX3 would be the strongest Lewis acid.

However, the nature of the B−XB−X bond needs to be taken into account. The change of hybridization from sp2spX2 in BX3BXX3 to sp3spX3 in [BX4]−[BXX4]X− involves the loss of partial ππ bonding which results from the overlap of the pzpXz orbitals of BB and XX. The pzpXz orbitals of BB and FFare of similar size, because both elements are members of the same period. Therefore, they have the best overlap and form the strongest ππ bond. Because the reaction with a Lewis base would require some energy to break this bond, BF3BFX3 is a comparably weak Lewis acid. The larger pzpXz orbitals of the heavier halides have increasingly less overlap with the smaller boron pzpXz orbital. Therefore, the ππ bonding is weaker and formation of the tetrahedral Lewis acid-base adduct via rehybridization becomes energetically more favorable.

Και επίσης:

2) We would expect BF3 to be stronger, because F is more electronegative than Cl.

Chemists explain this unexpected result by an electronic argument and a steric argument.

The electronic argument — backbonding

The boron atom in BF3 is sp2 hybridized, with a vacant 2p orbital.

The F atoms can also be sp2 hybridised, with lone pairs in their 2p orbitals.

upload.wikimedia.orgupload.wikimedia.org

These F orbitals can overlap with the orbital on B, thereby increasing the electron density on the boron atom and making it less acidic.

This effect is called backbonding, because electron density is leaving the more electronegative atom.

In BCl3, the 3p orbitals on Cl are bigger than the 2p orbital on B, so orbital overlap is less efficient, and backbonding is less important.

Hence, the greater backbonding in BF3 makes it a weaker Lewis acid.

The steric argument — ligand close-packing (LCP)

The LCP model is based on the observation that the X atoms (ligands) in AXnsystems are always the same distance from each other.

For example, the distance between the F atoms in BF3 and BF−4 is 226 pm, despite the longer B-F distance in the tetrahedral structure.

Ligand radiiLigand radii
(from alpha.chem.umb.edu)

It is as if the F atoms are closest-packed (like in a crystal), with the F atoms having aligand radius of 113 pm.

When the BF3 forms a Lewis complex, the F atoms remain close-packed, but the B-F bonds must become longer in the new tetrahedral geometry.

Lewis complexLewis complex
(from http://pubs.acs.org/doi/abs/10.1021/ic990713m)

It takes more energy to lengthen the short, strong B-F bonds than the longer, weakerB-Cl bonds.

Hence BF3 is a weaker Lewis acid than BCl3.