suggest reasons why the B-F bond lengths in BF3 (130 pm) and BF4 - dif?

Answer 1

In BF3, the B-F "single bond" has some double bond character, not present in BF4-.

If you write a conventional valence bond structure for BF3, with single B-F bonds, there are only six electrons on B and a vacant p-orbital at right angles to the plane of the molecule.

So you can give B an octet by writing a formal negative charge on B, a formal positive charge on one of the F, and a double bond between B and that F.

In molecular orbital terms, you can say that the appropriate p orbitals on F overlap this vacant orbital on B.

Answer 2

BF3 :
Solid state structure
Geometry of boron: 3 coordinate: trigonal
Type of bonding : polar covalent bond . Considering fajan's rules, smaller the size of the cation (and greater the charge) and smaller the size of the anion, greater is the covalent nature of the bond. So in this case, the bonding pair of electrons is pulled closer to the core of the cation, resulting in accumulation of a slight negative charge for the cation? right? But in some other cases, the shared pair(s) of electrons are pulled closer to the anion due to the greater electronegativity of the anion. So.. does this mean that the bonding pair of electrons can be pulled closer to either to the cation or the anion??
hence, relatively larger bond length.
In case of BF4 the bond type is coordinate covalent bond hence relatively less polarised and smaller bond length.

Answer 3

In boron triflouride, each flourine has completely filled unutilised 2p orbitals while boron has a vacant 2p orbital.Since both of these orbitals belong to the same energy level,they can overlap effectively as a result of which the flourine electrons are transferred into the vacant 2p orbital of boron resulting in the formation of an additional pπ-pπ bond.This type of bond formation is known as dative or back bonding.Thus the B-F bond has some double bond character.
Group 13 element halides are not stable and exist as dimers while boron halides exist as monomers.hence BF4- is stable

Answer 4

Dude, BF4 is nonpolar since it has four F's as it completes its octet. On the other hand, BF3 has lone pair of electrons and this pair affects the bond by forming a polar molecule, so their attraction force between shared electrons is changed. Thus, their bond lengths are diff.

Answer 5

It's due to the difference of hybridization. BF3 has sp2 hybrid orbitals at the valence shell and BF4 has sp3