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The tendency of polymerisation is observed as SiO4 4- > PO4 3- > SO4 2- > ClO4 - Explain.

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 XO4 n-  the dX2-y2 and dz2 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,    
            [SiO44- > PO43- > SO42- > ClO41- ]

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
Related Questions:

What are the SESQUI OXIDES ? give the examples.


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