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Molybdenum and tungsten have the widest distribution of oxidation states, from -2 to +6
.
There are many compounds with low oxidation states.
1.
Compounds in +6 oxidation state
The molybdate solution was acidified to give a white "molybdic acid" (H 2 MoO .
4 · H 2 O) precipitate tungstate solution was acidified yellow "acid" (H 2 WO .
4 · xH 2 O) precipitation
.
Aluminic acid and tungstic acid are dehydrated to obtain oxides.
MoO 3 (white powder) and WO 3 (yellow powder) have high thermal stability.
MoO 3 does not decompose at the boiling temperature (1155°C), and WO 3 melts but does not decompose when heated to 1473°C
.
Both MoO 3 and WO 3 are insoluble in water, but easily soluble in strong alkali and ammonia
Reduced with H 2 at a certain temperature , MoO 3 is reduced to brown MoO 2 , and WO 3 is reduced to blue WO 2
.
The halide of hexavalent molybdenum is only MoF 6 , a colorless liquid, with a low melting point (17.
4°C) and boiling point (34°C), and it is easily hydrolyzed
2.
Isopolyacid and heteropolyacid
Polyacids include homopolyacids and heteropolyacids
.
The acid formed by the dehydration and condensation of simple oxygen-containing acid molecules of the same element is called homopolyacid.
Oxyacids such as V, Cr, Mo, W, B, Si, P, As are easy to form homopolyacids, such as H 2 Cr 2 O 7 , (HPO 3 ) 4 , H 5 P 3 O 10 , H 6 Si 3 O 9 , H 4 Si 4 O 10 , H 6 V 10 O 28 , H 5 Mo 7 O 24 and so on
.
The polymerization of oxyacid radicals is related to the acidity of the solution
.
E.
The polyacid formed by the dehydration and condensation of oxoacid molecules of two different elements is called heteropolyacid, such as H 3 [PMo 12 O 40 ], H 6 [TeMo 6 O 24 ] and so on
.
Heteropolyacid is a special kind of complex, in which P or Si is the central atom of the complex, and polymolybdate or polytungstate is the ligand
Polyacids often exist in the form of salts, such as Na 5 P 3 O 10 , Na 6 Si 3 O 9 , K 6 V 10 O 28 , (NH 4 ) 3 [PMo 12 O 40 ], Na 6 [TeMo 6 O 24 ], K .
5 CoW 12 is O 40 and the like
Heteropoly compounds have diverse structures.
Two common composition types that have been studied more are anions with Keggin structure [XM 12 O 40 ] and anions with Dawson structure [X 2 M 12 O 62 ]
.
There are five isomers of anions with Keggin structure.
Figure 16-3 a-Keggin structure (a) and M 3 O 13 unit (b)
Heteropoly compounds have many excellent properties, such as high-efficiency homogeneous catalysts in the petrochemical industry, dye precipitation agents, and novel resin exchangers
.
In addition, heteropoly compounds of molybdenum can also be used as flame retardants
Ammonium molybdate and phosphate ions can produce yellow (NH 4 ) 3 PO 4 ·12MoO 3 precipitation, and this reaction can be used to identify PO 4 2-
.
3.
Compounds in other oxidation states
The halides of trivalent molybdenum are relatively stable, such as MoF 3 (brown yellow), MoCl 3 (dark red), MoBr 3 (green), Mol 3 (black); there are only two halides of trivalent tungsten, WCl 3 ( Red), WBr 3 (black)
.
Except for iodide, other halides of tetravalent molybdenum and tungsten are relatively stable
.
For example, MoF 4 (light green), MoCl 4 (black), MoBr 4 (black), WF 4 (red-brown), WCI 4 (black), WBr 4 (black)
.
The halides of pentavalent molybdenum are only MoF 5 (yellow) and MoCl 5 (black); the halides of pentavalent tungsten are only WCI 5 (black)
.
The color of molybdenum and tungsten halides darkens as the number of halogen and metal cycles increases, and the color of halides with high oxidation numbers for the same metal is generally darker
.
Related links: divalent chromium, pentavalent chromium and tetravalent chromium compounds
