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A reaction mechanism for the formation of the paratungstate A ion “HW 6 O 21  5⊖ ” is developed. Because of geometric reasons and in consideration of the formerly  ί 1 stated criterion for stable polytungstates the stepwise aggregation to a hexatungstate ion can proceed only via the tetratungstate ion and - in conformity with the lower rate of formation of the ion - only under condensation of water. Decisive to the way of the aggregation according to this mechanism are therefore the number and the positions of the Η-atoms in the growing polytungstate ion as well as the degree of protonation of the monomer tungstate ion. The rules which are obeyed are deduced. The end of the aggregation with a 7,6-tungstate ion results from a consideration of the free energies of reaction for the formation of the polytungstate ions as they are possible according to these rules. For this it is supposed, that ΔG r r is composed of increments for the characteristic reaction and structure parameters. The correct formula for the paratungstate A ion is [W 6 O 19  (OH) 3 ] 5⊖ . One of the two structures discussed for this ion is in close relation to that of the heptamolybdate ion Μο 7 O 24  6⊖  which is formed under similar conditions.

zeitschrift für naturforschung b

Über den Mechanismus der Bildung von Polyanionen in wäßriger Lösung. Der Kondensationsmechanismus bei der Bildung des Hexawolframations [W 6 O 19  (OH) 3 ] 5⊖  (Parawolframat-A) / On the Mechanism for Polyanion Formation in Aqueous Solution. The Condensation Mechanism for the Formation of the Hexatungstate Ion W 6 O 19  (OH) 3 ] 5⊖  (Paratungstate A)