Oxyanions four-coordinate

The largest nonmetals show coordination numbers as high as eight in the oc-tafiuoroanions, ]Fjj" and XeFg- (see Chapter 17). The corresponding oxyacids and oxyanions show a maximum coordination number of six [Sb(OH)fi]-, Te(OH)6, OI(OH)s, and [XeO, ]4- Of these, apparently only iodine shows a maximum oxidation state with a coordination number as low as four Periodic acid can exist as either 01(0H)S or HI04. 

The orthophosphate group, P04 , (often shortened to phosphate ) is the most ubiquitous oxyanion of phosphorus. In solid orthophosphates, all four oxygen atoms are usually coordinated to cations resulting in a strongly bonded three-dimensional framework, although layered or chain structures sometimes result. 

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