The document discusses the Valence Shell Electron Pair Repulsion (VSEPR) model for predicting the geometry of main group molecules. It examines hexacoordinated AB6-type molecules, such as SF6 and PF6, which have an octahedral structure due to two d orbitals participating in bonding. Pentacoordinated AB5-type molecules, like halogen pentafluorides XF5, have a trigonal bipyramidal structure with one d orbital involved in bonding. The bonding in these and other complexes is explained using hybrid orbital theory.

2.  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.

3.  According VSEPR model hexa coordinated AB6-type
molecules have
◦ Octahedral structure.
◦ Coordinated by six ligands.
◦ Two d orbitals are required to participate in
bonding.
◦ As a result, two electrons in the s and p subshells
are promoted to higher-level d orbitals forming six
sp3d2 hybrid orbitals in the central atom.

4.  According VSEPR model penta coordinated AB5-type
molecules 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 is
promoted to a higher-level d orbital forming five sp3d
hybrid orbitals in the central atom.

5.  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.

6.  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.

8.  Electronic Configuration of Sulphur is
1s2,2s2,2p6,3s2,3px
2,3py
1,3pz
1
 Electronic Configuration of Phosphorus is
1s2,2s2,2p6,3s2,3px
1,3py
1,3pz
1
 Electronic Configuration of Florine is
1s2,2s2, 2px
2,2py
2,2pz
1

9.  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.

10.  Six S.F bonds in SF6 result from the overlap of
F(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 of
F(p1).P(3px
2).F(p1)
F(p1).P(3py
2).F(p1)
F(p1).P(3pz
2).F(p1)

12.  The halogen pentafluoride XF5 (X = Cl, Br, or
I) belongs to the AB5E type molecule.
 A square pyramidal structure with a lone pair
in the central halogen atom.
 In all the XF5 molecules the basal X.Fb bonds
(all are equivalent) are about 0.1 Å longer
than the axial X.Fa bond.
 This shows that bonding in basal and axial
positions must be different.

14.  Electronic Configuration of Chlorine is
1s2,2s2,2p6,3s2,3px
2,3py
2,3pz
1
 Two sp hybrid orbitals could form on the
central halogen atom pointing toward the
axial positions.
 The shorter axial X.Fa bond is reasonably
formed by the overlap of an sp orbital with
the fluorine p orbital.
 The lone pair resides in another sp orbital in
the opposite axial position.

15.  Two sets of the longer, linear basal Fb.X.Fb
bonds are reasonably formed by the overlap
of the unhybridized, orthogonal px and py
orbitals, respectively, with the terminal
fluorine p orbitals leading to a three-center,
four-electron bond.

16.  Xenon tetrafluoride XeF4 is an example of AB4E2-
type molecules.
 A square planar structure with two lone pairs in
the central xenon atom.
 In XeF4 two sp hybrid orbitals could reasonably
form on Xe pointing toward the axial positions
and holding a lone pair in each of them.
 The two sets of linear F.Xe.F bonds are formed as
three-center, four-electron bonds, the same as
formation of the basal Fb.X.Fb bonds in the
above-discussed halogen pentafluorides XF5.

18.  An alternative model that does not involve d
orbitals in bonding in the pentacoordinated AB5-
type molecules such as phosphorus pentahalides
PX5 (X = F, Cl).
 Instead, three sp2 hybrid orbitals (each contains a
single electron) are formed on the central atom
overlapping with ligand orbitals in the equatorial
positions
 Longer axial bonds are formed via the overlap of
the unhybridized p orbital (containing two
electrons) on the central atom and the two
terminal ligand orbitals, resulting in a three-
center, four-electron bond.