3 center 4 electrons bond

The Valence Shell Electron Pair Repulsion (VSEPR)

model has long been used successfully to predict the

geometry of various main group molecules.

Hexa coordinated AB6-type main group molecules

belong to an important class of compounds. This type

of molecule possesses six electron pairs in the central

atom.

Similarly the penta coordinated AB5-type molecules

possessing five electron pairs are predicted to have

trigonal bipyramidal structure.

According VSEPR model hexa coordinated AB6-type molecules have

◦ Octahedral structure.

◦ Coordinated by six ligands.

◦ Two d orbitals are required to participate inbonding.

◦ As a result, two electrons in the s and p subshellsare promoted to higher-level d orbitals forming sixsp3d2 hybrid orbitals in the central atom.

According VSEPR model penta coordinated AB5-typemolecules possess

◦ Five electron pairs

◦ Trigonal bipyramidal structure

◦ Central atom coordinated by five ligands

◦ One d orbital is required to participate in bonding.

◦ As a result, one electron in the s or p subshell ispromoted to a higher-level d orbital forming five sp3dhybrid orbitals in the central atom.

Two factors which disfavor the utilization of d orbitals

and have made d-orbital participation in bonding in

main group compounds a controversial issue.

1. Energetically unfavorable s2pnd0→ s1pn-1d2 [n = 3(P),

4(S), or 5(Cl, Br. I)] excitation, which requires a large

promotion energy.

2. d orbitals in nonmetals are heavily shielded by the

more penetrating s and p electrons and extremely

diffuse. As a result, they have poor overlap with the

orbitals of neighboring atoms.

SF6 and PF6- anion are often chosen as representatives

of AB6-type main group molecules.

Six fluorine ligands approach to the central sulfur or

phosphorus atom along the x, y, and z axes forming

an octahedral structure.

Electronic Configuration of Sulphur is

1s2,2s2,2p6,3s2,3px2,3py

1,3pz1

Electronic Configuration of Phosphorus is

1s2,2s2,2p6,3s2,3px1,3py

1,3pz1

Electronic Configuration of Florine is

1s2,2s2, 2px2,2py

2,2pz1

Ligand’s p orbitals have effective overlap with all the

orthogonal sulfur or phosphorus 3p (3px, 3py, 3pz)

orbitals in six orientations (±x, ±y, ±z).

A filled 3p subshell in the central atom (achieved

readily by s-p transition) is required, because the

entire S.F or P.F bond distances in SF6 or PF6. are

equal.

Six S.F bonds in SF6 result from the overlap ofF(p1).S(3px

2).F(p1)F(p1).S(3py

2).F(p1)F(p1).S(3pz

2).F(p1)

Six P.F bonds in PF6- are formed by the

overlap ofF(p1).P(3px

2).F(p1)F(p1).P(3py

2).F(p1)F(p1).P(3pz

2).F(p1)

The halogen pentafluoride XF5 (X = Cl, Br, orI) belongs to the AB5E type molecule.

A square pyramidal structure with a lone pairin the central halogen atom.

In all the XF5 molecules the basal X.Fb bonds(all are equivalent) are about 0.1 Å longerthan the axial X.Fa bond.

This shows that bonding in basal and axialpositions must be different.

Electronic Configuration of Chlorine is

1s2,2s2,2p6,3s2,3px2,3py

2,3pz1

Two sp hybrid orbitals could form on thecentral halogen atom pointing toward theaxial positions.

The shorter axial X.Fa bond is reasonablyformed by the overlap of an sp orbital withthe fluorine p orbital.

The lone pair resides in another sp orbital inthe opposite axial position.

Two sets of the longer, linear basal Fb.X.Fbbonds are reasonably formed by the overlapof the unhybridized, orthogonal px and pyorbitals, respectively, with the terminalfluorine p orbitals leading to a three-center,four-electron bond.

Xenon tetrafluoride XeF4 is an example of AB4E2-type molecules.

A square planar structure with two lone pairs inthe central xenon atom.

In XeF4 two sp hybrid orbitals could reasonablyform on Xe pointing toward the axial positionsand holding a lone pair in each of them.

The two sets of linear F.Xe.F bonds are formed asthree-center, four-electron bonds, the same asformation of the basal Fb.X.Fb bonds in theabove-discussed halogen pentafluorides XF5.

An alternative model that does not involve dorbitals in bonding in the pentacoordinated AB5-type molecules such as phosphorus pentahalidesPX5 (X = F, Cl).

Instead, three sp2 hybrid orbitals (each contains asingle electron) are formed on the central atomoverlapping with ligand orbitals in the equatorialpositions

Longer axial bonds are formed via the overlap ofthe unhybridized p orbital (containing twoelectrons) on the central atom and the twoterminal ligand orbitals, resulting in a three-center, four-electron bond.