What
are three center four electron(3c-4e) bonds?
Explain
with the help of XeF2 molecule.
The
three center four electron (3c-4e) bond:
Piemental and Rundle independently introduced the 3-center
4-electron bond in 1951. Ordinary two center two electron (2c-2e) bond is
formed by the overlap of a p-orbital of the central atom with a s-orbital of an
outer atom. While three center four electron bond is formed by the overlap of
p-orbital of central atom and the s-orbital of two outer atoms.
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| (3c-4e bond formation in SeF4) |
Three center four electron (3c-4e) bond model approach is
based on the two main ideas:
(i)
Use of outer d-orbitals of the central atom of
non-transition metal ion is so slight that these may be neglected altogether.
(ii)
Presence of bond angles close to 900
and 1800 in AB4 molecules suggests that orbitals
perpendicular are being used.
For molecules with an octet, or less, of electrons in the
valence shell of the central atom, the hybridization employing sp, sp2
and sp3 orbitals remain valid. The 3c-4e model has been proposed for
molecules in which five or more electron pairs on central atoms must be accounted
for chemical bonding.
XeF2:
VSEPR description of XeF2 molecule:
The simplest example of a compound containing 3c-4e bond is
XeF2.The geometry according to VSEPR for the AE3X2 linear.
If both of the bonds are 2-center-2-electron bonds, the Xe atom has 10
electrons(four in Xe-F bonds and six in lone pairs). However main group
elements have only four valence orbitals and can accommodate only eight electrons.So
how can bonding in this and related compounds can be understood?
According
to hybridization theory:
Bent’s rule suggests that lone pairs are best housed in
s-orbitals and due to primogenic and relativistic effects this is especially
true for the heavier p-block elements. Considering there are three are three
lone pairs on Xe to support, the bonding in the molecule is very likely done by
a p-orbital on xenon. Since the fluorines have three lone pairs each, we will
consider the bonding by fluorine to be done by pure p-orbital as well. We realize
that there is one orbital on each atom that can participate because all others
are holding electron pairs. So hybridization for central xenon atom here must
be higher than even sp3.
Molecular Orbital theory:
The valence shell electronic configuration of Xe and F are
Xe:5s2 5p6 , F:2s2 2p5. If the
bonds lie in x-direction, this 2px orbital of each fluorine atom
bearing unpaired electron and the filled 5px orbital of Xe will
participate in forming three center sigma MOs. Similarly the 2py orbital of fluorine along with 5py orbital
of xenon will form three center pi MOs. Thus by using pz orbital of
xenon and fluorines we can get another set of pi-MOs. The corresponding ∏y
and ∏z MOs are degenerate as usual. Here the pi system is of no use
from the stand point of stability of molecule, as the bonding, non-bonding and
anti bonding pi-MOs are completely filled in and yield the pi- bond order zero.
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| MO energy diagram of XeF2 molecule |
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