Vanadates

Structure of orthovanadic acid H ₃ VO ₄

Vanadates are the salts of the vanadium acids H _x V _y O _z , which do not exist in free form . The vanadates themselves are stable in aqueous solution, the forms that occur depending on the pH of the solution. Some vanadates also occur naturally as vanadium minerals , such as vanadinite , descloicite and carnotite .

Structure of the orthovanadate anion [VO ₄ ] ³⁻

presentation

Many vanadates can be prepared from vanadium (V) oxide by heating with the appropriate amount of a metal oxide:

${\displaystyle \mathrm {3\ CaO\ +\ V_{2}O_{5}\ {\xrightarrow {\Delta T}}\ Ca_{3}(VO_{4})_{2}} }$

Calcium oxide and vanadium (V) oxide react to form calcium orthovanadate

${\displaystyle \mathrm {2\ Na_{2}O\ +\ V_{2}O_{5}\ {\xrightarrow {\Delta T}}\ Na_{4}V_{2}O_{7}} }$

Disodium oxide and vanadium (V) oxide react to form tetrasodium divanadate

Poorly soluble variants - such as ammonium metavanadate - can be precipitated from vanadate solutions by reacting them with another metal salt:

${\displaystyle \mathrm {NaVO_{3}\ +\ NH_{4}Cl\ {\xrightarrow[{wsrg.Lsg.}]{\ }}NH_{4}VO_{3}\downarrow \ +\ NaCl} }$

Ammonium chloride precipitates from an aqueous sodium metavanadate solution, slightly soluble ammonium metavanadate

Types of vanadates

Soluble vanadates

Numerous different vanadate ions can be present in aqueous solutions, the distribution being strongly dependent on the pH value :

• at pH> 13, the solution mainly contains the tetrahedral orthovanadate anion [VO ₄ ] ³⁻
• between pH 7 and 13, divanadate or pyrovanadate [V ₂ O ₇ ] ⁴⁻ and metavanadate [VO ₃ ] ^- occur in addition to orthovanadate ; the lower the pH, the higher the proportion of protonated anions such as [HVO ₄ ] ²⁻ and [HV ₂ O ₇ ] ³⁻
• From pH 7, mainly orange-colored decavanadates [V ₁₀ O ₂₈ ] ⁶⁻ , [HV ₁₀ O ₂₈ ] ⁵⁻ and [H ₂ V ₁₀ O ₂₈ ] ⁴⁻ are present.

Slightly soluble vanadates

In the industrial production of vanadium, the ammonium metavanadate NH ₄ VO _{3 is} important, which precipitates as a poorly soluble, yellow, rhombic platelet-forming compound when ammonium salts are added from vanadate solutions. When adding soda (Na ₂ CO ₃ ) or soda solution, a yellow, flaky sodium orthovanadate Na ₃ VO ₄ also precipitates . Both salts, usually white compounds in the pure state, are yellowish in color due to a low content of polyvanadates.

Calcium vanadates can be prepared by reacting calcium oxide (CaO) with vanadium (V) oxide (V ₂ O ₅ ); the selected stoichiometric ratio determines whether ortho-, pyro- or metavanadates are formed.

Similarity to phosphates

The various vanadates are structurally and in their chemistry very similar to the phosphates ; all anions are also made up of (partly condensed) tetrahedra, with the condensed vanadates being linked by an oxygen bridge. Here, too, there are primary ( dihydrogen ), secondary ( hydrogen ) and tertiary (non-protonated) forms of the orthovanadates . The divanadates and higher vanadates also have quaternary and pentary forms depending on the number of protons that can be absorbed.

Primary vanadates (di- / trihydrogen vanadates )	Secondary vanadates (di- / hydrogen vanadates )	Tertiary Vanadates (Hydrogen Vanadates )	Quaternary Vanadates
Sodium dihydrogen orthovanadate, NaH 2 VO 4	Disodium hydrogen orthovanadate, Na 2 HVO 4	Sodium orthovanadate, Na 3 VO 4	nv
Sodium trihydrogen divanadate, NaH 3 V 2 O 7	Disodium dihydrogen divanadate, Na 2 H 2 V 2 O 7	Trisodium hydrogen divanadate, Na 3 HV 2 O 7	Tetrasodium divanadate, Na 4 V 2 O 7

Polyoxovanadates

Polyoxovanadates (POVs) belong to the group of polyoxometalates (POMs); they generally consist of several [MO _x ] units (with M = V, Nb, Mo, Ta, W and x = 4 - 7). Mostly they have an octahedral or square-pyramidal coordination geometry. The metal centers are often in a d ₀ or d ₁ electron configuration. The POVs show a wide variety of species. There is also the possibility of incorporating heteroatoms into polyoxovanadate anions. Often some vanadium atoms in the structures are replaced by arsenic, antimony, germanium or silicon, for example, so that what are known as heteropoly anions are formed. It is also possible to bind organic molecules or transition metal complexes to the POVs. This results in new physical and chemical properties.

In contrast to their counterparts with molybdenum or tungsten, polyoxovanadates are often produced solvothermally . Many synthesis parameters are decisive for the formation of the polyoxovanadates, including the pH value, as individual species can transform into one another by changing the pH value. If the pH value is very high, orthovanadates [VO ₄ ] ^{3− are} predominantly present, which when acidified into polyvanadates, such as B. decavanadate [V ₁₀ O ₂₈ ] ⁶⁻ , skip over.

Vanadium minerals

Because of Diadochie, the group of vanadate minerals shows a large variety of similar minerals in which elements of similar ionic radii and the same charge number can be exchanged. A naturally occurring vanadate is the red-colored vanadinite occurring in the hexagonal crystal system with the chemical composition Pb ₅ (VO ₄ ) ₃ Cl, which forms barrel-shaped crystals. Carnotite or uranium mica is a vanadium-containing, yellow to greenish, monoclinic mica with the composition K ₂ (UO ₂ ) ₂ (VO ₄ ) ₂ 3H ₂ O. Descloizit can be differently colored and crystallizes in orthorhombic crystals with the composition PbZn (VO ₄ )(OH).

Biological significance and toxicology

Although vanadium is essential for the metabolism , orthovanadates ([VO ₄ ] ³⁻ ) inhibit membrane-bound enzymes ( P-ATPases , also P-type ATPases ) in higher concentrations and are therefore toxic to almost all eukaryotes and prokaryotes . This is based on the similarity of the vanadate and the phosphate anions . Some aquatic creatures, such as sea ​​squirts , are able to accumulate vanadates from seawater. These living things contain large amounts of metalloproteins called vanabins , which contain vanadium.

Individual evidence

1. ↑ ^{a b} Vanadium minerals. In: Lexicon of Geosciences. Science online dictionaries
2. ↑ Vanadates. In: Lexicon of Chemistry. Science online dictionaries
3. ↑ D.-L. Long, R. Tsunashima, L. Cronin: Angew. Chem. 2010, 122, 1780.
4. ^ Y. Hayashi: Coord. Chem. Rev. 2011.
5. ^ Mineral Atlas: Vanadinite.
6. ↑ Mineral Atlas: Carnotite.
7. ^ Mineralienatlas: Descloizit.
8. ↑ T Ueki, T Adachi, S Kawano, M Aoshima, N Yamaguchi, K Kanamori, H. Michibata: Vanadium-binding proteins (vanabins) from a vanadium-rich ascidian Ascidia sydneiensis samea . In: Biochim Biophys Acta . tape 1626 , no. 1–3 , 2003, pp. 43-50 , doi : 10.1016 / S0167-4781 (03) 00036-8 , PMID 12697328 .