A phosphoric anhydride contains an acid-anhydride linkage involving phosphoric acid. One type joins two phosphoric groups through oxygen, giving P–O–P connectivity. A mixed anhydride joins a phosphoric group to a different acid-derived group, such as one derived from a carboxylic acid. These are distinct from the P–O–C connectivity of a phosphoric ester.[ref]
Nomenclature of Phosphorus-Containing Compounds of Biochemical Importance by IUPAC and IUBMB. §3, phosphoric anhydrides, types(a) and(b)
A formal neutral-molecule balance illustrates the first type:Two phosphorus centers remain, but one oxygen now bridges them. Both sides contain six hydrogens, two phosphorus atoms and eight oxygens. This is an atom balance, not a claim that dilute aqueous phosphoric acid spontaneously undergoes substantial condensation. Actual speciation and reaction direction depend on conditions.
The P–O–P connections between phosphoric groups in ATP illustrate anhydride connectivity, whereas the connection from its sugar to the first phosphoric group is an ester linkage.[ref]
Nomenclature of Phosphorus-Containing Compounds of Biochemical Importance by IUPAC and IUBMB. §3 and Table2, ATP and other oligophosphoric esters
One molecule can therefore contain more than one linkage class. Identifying the atoms on either side of a bridging oxygen is more informative than merely counting its phosphorus atoms. New to topics? Read the docs here!