P. Abu Backer and L. V. Natarajan New College Madras 600014, India
I
A Simple Spatial Diagram for 12-Heteropolyanion
When a few d r o ~ of s nitric acid solution of a phosphate is added to excess df ammonium molybdate a bright yellow compound is obtained. This is due to the formation of ammonium 12-molybdophosphate, a salt of 12-molybdophos~ h o r i acid c in which the ratio of the hetero atom phosphorus i o molybdenum is 1:12. More than thirty elements such as P(V), As(V), Si(N), Ge(IV), Ti(N), andZr(N) are known ( 1 ) to function as hetero atoms. Phosphorus with tungsten forms 12-tungstophosphoric acid, the structure of which is discussed below. Various theories have been proposed to explain the structure of 12-heteropoly acids. The first attempt was made by Miolati and Rosenheim (2) who postulated that the 12-heteropoly acid with the composition H7[P(W207)6] could he obtained by the replacement of oxygen atoms by ( W ~ 0 7 ) ~ ion from the hypothetical acid H7[P06]. This theory was rejected since there was no direct evidence for the existence of ( W 2 0 ~ ) ions ~ - in the acid. Pfeiffer (3)suggested that WO3 molecules might he coordinated around the central PO6 anion in a second shell in order to give H ~ [ P O S ( W O ~Based ) ~ ~ ] .on X-ray studies, Pauling (4) proposed that in 12-heteropoly acids 12 W 0 s octahedra are linked by each sharing three corners with neighboring octahedra. The remaining three oxygen atoms are saturated by hydrogen ions giving a neutral [W1201a(OH)36]group. The tetrahedral ( P o d ion is accommodated in the center of the structure. This model reauires that each molecule of the 12-heteropoly acid must have a t least eighteen molecules of water. Ocmrrence of anhvdrous acids such as 12-tungstosilicic acid (5) makes this structure untenable. The most satisfactory structure is the one proposed by Keggin (6) based on the X-ray diffraction study of the crystalline pentahydrate of 12-tungstophosphoric acid, of composition H3[P04W12036]5H20.According to Keggin the tetrahedral PO4ion is at the center and it is surrounded by twelve WOs octahedra in four groups. Thus each corner of the tet-
642 / Journal of Chemical Education
rahedron is shared with three WOF,ortahedra. Out of the six corners of the WOI;ocrahedronone is linked tn the c~mtral1'0.j anion, two are linked to the octahedra of its own group, two are linked to the neighboring octahedra of the next group, and the sixth corner is free. The composition of the anion [P04W12036I3-can he arrived at as follows. For each WO6 octahedron the effective number of oxygen atoms is (1 X %) (4 X %) + 1= 3 l13. For one group of three octahedra the total number of oxygen atoms is 3 X 3 '/n = 10. Hence the formula of one group of three octahedra is [W3Ol0I2-. There are four such groups. Hence the molecular formula is [ W 1 2 0 ~ ~ ] Including ~-. the central hetero atom ohosohorus. the formula becomes IPW120401~- or
+
~PO~W~~O~~]~-. This arrangement leaves vacant space inside the structure
The srmchrre of [FQ4W,20m]S-. Ea. phosphws; 0 .tungsten; 0,oxygen. The W-0-W cross linking is shown by dashed line faclarity.
which is filled with many molecules of water. The composition of the hydrate may he written as H:3[P04W12036].29 H20. This hydrate readily loses water and gives the penta hydrate, H~[PO~W~~O H20. & ] This . ~ structure of Keggin has been fully substantiated (7) hy the X-ray study of numerous 12heteropoly acids and their salts. ~~~~i~~~structure has been represented by cubo.octahedral. &lvhedral. and soatial diaerams. The cubo-octahedral and kepolyhed" (a) '?low the external appearance of the molecule, and hence the internal linkages between the atoms are less evident. Thespatialdiagram (8)lacks simplicity because of its unsvmmetric nature. In this article we are mesenring another simplt!spatial dingrnni of symmetric nature, n hich elearl" shows the variuus linknge3 herween the atoms. \\'e have founcl from experience rhnt this simple diagram
diagrams
enables the undergraduate students to have a clear insight into the structure and composition of 12-heteropoly acids as compared to the previous models.
US
We are grateful t o Dr. N. S. Gnanapragasam for providing with the three dimensional model of 12-heteropoly acid.
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8 i+m.a+n,.a cnmd
C.W.,and Figg1a.L S.,J Amer Chem. S o r . 92.3794,(1970). 1%) ~ n h e i m , ~ . , ~ ~ e t e m p o i y s a u r e n ~ i n ~ ~ ~ ~ ~ . ~ . , ' ~ a n d b u c h d e r ~ ~ r g ~ i w h e n ~ h o m Vol. 4,Pt. ii,S. Hind.Leipzig,1921,pp.917-1065.
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