We know that the pπ-dπ bonding is much more
significant than the pπ-pπ bonding from the standpoint of overlap integral.
In pπ-pπ bonding, the
overlapping lobes are parallel and as a result, there occurs a pure sideways
overlap. On the other hand, in pπ-dπ bonding, the lobes
of the overlapping d-orbitals are at an angle <<180° to the lobes of
the p-orbital. Thus in the pπ-dπ interaction, the
ovelap is better.

Now let us consider the O(2p)
- X(3d)
π-bonding (where, X= Si, P, S, Cl).In the tetrahedral structure
of XO

_{4}^{n-}the d_{X}^{2}-_{y}^{2}and dz^{2}orbitals of X can participate in the said pπ-dπ bonding interaction. On moving from left to right along a period, the effective nuclear charge generally increases and consequently the energy of the 3d-orbitals decreases from Si > P > S > Cl. The overlap becomes better when the overlapping orbitals have comparable energies. Thus the π-bonding efficiency increases from Si < P < S < CI. Now let us consider the oxyanions in the order of increasing tendency of polymerization,
[SiO

_{4}^{4-}> PO_{4}^{3-}> SO_{4}^{2-}> ClO_{4}^{1-}]
Here, besides the different periodic positions of
Sl, P, S and Cl, it is worth mentioning that the 1 positive oxidation state of
the central element increases as, Si(+4), P(+5), S(+6): Cl(+7). With the increase of positive oxidation state, the
energy of the 3d-orbitals gradually decreases and it favours the π- bonding
interaction. Thus the 2pπ(O) - 3dπ(X) bonding
in X - O increases
from Si to Cl. This is why, the systems where the π-bonding is not effective,
the stabilization is attained through the single bonded structure, i.e. through
the polymerization. Thus the concept of pπ-dπ can
explain the observed sequence of tendency towards polymerization

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