3 center 4 electrons bond By Malik Xufyan JIAS ACADEMY 03137355727 www.onlychemistrydiscussion.blogspot.com

1. Malik Xufyan JIAS ACADEMY
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3 Center 4 Electrons Bond
(Malik Xufyan)
The 3-center 4-electron bond is a model used to explain bonding in
certain hypervalent molecules such as tetratomic and hexatomic interhalogen
compounds, sulfur tetrafluoride, the xenon fluorides, and the bifluoride ion. It is
also known as the Pimentel–Rundle three-center model after the work published
by George C. Pimentel in 1951,which built on concepts developed earlier by
Robert E. Rundle for electron-deficient bonding. An extended version of this
model is used to describe the whole class of hypervalent molecules such
as phosphorus pentafluoride and sulfur hexafluoride as well as multi-center pi-
bonding such as ozone and sulfur trioxide.
Description
Molecular orbital theory
The model considers bonding of three colinear atoms. For example, in xenon
difluoride (XeF), the linear F−Xe−F unit is described by a set of three molecular
orbitals (MOs) derived from colinear p-orbitals on each atom. The Xe−F bonds
result from the combination of a filled p orbital in the central atom (Xe) with two
half-filled p orbitals on the axial atoms (F), resulting in a filled bonding orbital, a
filled non-bonding orbital, and an empty antibonding orbital. The two lower energy
MO's are doubly occupied. The bond order for each Xe-F bonds is 1/2, since the
only bonding orbital is delocalized over the two Xe-F bonds.
The HOMO is localized on the two terminal atoms. This localization of charge is
accommodated by the fact that the terminal ligands are highly electronegative in
hypervalent molecules. The linear F−A−F axis of the molecules SF and ClF is

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described as a 3-center 4-electron bond. In the xenon fluorides, all bonds are
described with the 3-center 4-electron model. Molecules without an s-orbital lone
pair such as PF and SF are described by an extended version of the 3-center 4-
electron model (See hypervalent molecule).
Valence bond theory
The bonding in XeF can also be shown qualitatively using resonant Lewis
structures:
In this representation, the octet rule is not broken, the bond orders are 1/2, and
there is increased electron density in the fluorine atoms. These results are
consistent with the molecular orbital picture discussed above.
Hypervalent description with d orbitals
Older models for explaining hypervalency invoked d orbitals. As of 2010, these
models still appear in some beginning-level college texts;[6] however, quantum
chemical calculations suggest that d-orbital participation is negligible due to the
large energy difference between the relevant p (filled) and d (empty) orbitals.
Furthermore, a distinction should be made between "d orbitals" in the valence bond
sense and "d functions" that are included in the QM calculation aspolarization
functions. The 3-center-4-electron bonding model has the advantage of dispensing
with the need for d orbitals, which has led to its acceptance.
Other systems
Three-center four-electron interactions can also be considered in the transition state
of S2 reactions and in some (resonant) hydrogen bonding (as in the bifluoride
anion)
Explanation with examples:
 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.
Previous Knowledge About AB6-type Molecules

3. Malik Xufyan JIAS ACADEMY
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 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 sp3
d2
hybrid orbitals in the central
atom.
 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 sp3
d hybrid orbitals in the central atom.
Factors which disfavor the utilization of d orbitals
 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 s2
pn
d0
→ s1
pn-1
d2
[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.
Bonding in the AB6-type Molecules

4. Malik Xufyan JIAS ACADEMY
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 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 S,P and F
 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
 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 of
F(p1
).S(3px
2
).F(p1
)
F(p1
).S(3py
2
).F(p1
)
F(p1
).S(3pz
2
).F(p1
)

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 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
)

6. Malik Xufyan JIAS ACADEMY
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Bonding in the AB5E-type Molecules
 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.
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.

7. Malik Xufyan JIAS ACADEMY
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 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.
 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.
Bonding in the AB4E2-type Molecules
 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.

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Bonding in the AB5-type Molecules
 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.

9. Malik Xufyan JIAS ACADEMY
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Malik Xufyan
M.Sc. Chemistry, B.Ed.
JIAS ACADEMY
JHANG SADAR
CELL # 0313-7355727