Condensation of oxoacids

The tendency for an oxoacid to polymerize by condensation is most marked in the less acidic (more highly hydroxylated) acids. There are many stable condensed forms of silicic and boric acid, whereas the condensed forms of phosphoric acid are unstable towards hydrolysis. 

Why do Group VII oxoacids not usually yield condensed oxoacids  

dehydration of perchloric acid, HCIO4, gives dichlorine heptoxide, CI2O7  

Condensation is most marked in structures where the charge on the uncondensed anion is high, because it is able to reduce the charge density on the anion. The dimerization of Si04 can be represented as follows  

Phosphate tetrahedra, P04 , can link up via oxygen-sharing to give polyphosphates in the form of chains and rings. The phosphates, unlike the silicates, contain a central atom with a valency of five. Thus, the maximum number of oxygen atoms that each tetrahedron can share is three, since there must always be one vertex of the tetrahedron that is a terminal oxygen, bound as P=0. 

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