Luis Fernando Piedra-Garza - Jacobs University

Chapter I. Iso- and Heteropolyoxometalates. An Introduction
1.1 Historical Background
In 1826 Berzelius prepared from the mixture of ammonium molybdate and
phosphoric acid, what is now known as the first POM, a yellow precipitate
which now is known as ammonium 12-molybdophosphonate:
(NH 4 ) 3 [PMoO 40 ]. 2 A few years later, Struve reported the heteropoly
molybdates of Cr 3+ and Fe 3+ (the term heteropoly is referred to POMs with
oxygen, molybdenum in this particular case, and another atom i.e. the
heteroatom: chromium or iron) . 3 Nevertheless, the discovery of the
tungstosilicilic acids and their salts by Marignac in 1862 allowed the precise
analytical composition of such compounds. 4a,b
Rosenheim and Jaenicke 5 published a review in 1917 describing several
dozens of new heteropoly acids and even a greater number of salts derived of
those compounds. By this time, the amount of synthesized polyanions was
fairly numerous and the first attempts to understand their composition were
based on Werner´s coordination theory. Miolati and Pizzighelli 6 developed a
theory which was afterwards adopted by Rosenheim, who was probably the
most influential scientist in the field of polyanion chemistry in the first three
decades of the twentieth century.
In 1929, Linus Pauling suggested a cage structure of MoO 6 octahedra joined
by corners into a shell enveloping the PO 3- 4 ion, opposite to the Miolati-
Rosenheim theory which suggested that heteropoly acids were based on sixcoordinate
heteroatoms with MO 2- 4 or M 2 O 2- 7 anions as ligand or bridging
groups. 7
The first definitive information on a heteropoly compound was provided by
Keggin in 1933 by means of X-ray diffraction proving that the WO 6 octahedral
units in H 3 [PW 12 O 40 ]· 5H 2 O were in fact connected by both shared edges and
corners. 8 The 12-tungstophosphoric acid was studied in the following years by
2