A metaborate is a borate anion consisting of boron and oxygen, with empirical formula BO−2. Metaborate also refers to any salt or ester of such anion (e.g. salts such as sodium metaborate NaBO2 or calcium metaborate Ca(BO2)2, and esters such as methyl metaborate CH3BO2). Metaborate is one of the boron's oxyanions. Metaborates can be monomeric, oligomeric or polymeric.
Quick Facts Names, Identifiers ...
Metaborate
Monomeric metaborate ion |
Trimeric metaborate ion |
| Names |
| IUPAC name
Metaborate |
| Identifiers |
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| ChEBI |
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| ChemSpider |
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1047 |
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InChI=1S/BO2/c2-1-3/q-1 Key: VDLZIANVOYDKFH-UHFFFAOYSA-N
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| Properties |
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BO−2 |
| Molar mass |
42.81 g·mol−1 |
| Related compounds |
Related compounds |
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Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). |
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In aqueous solutions metaborate anion hydrolyzes to tetrahydroxyborate [B(OH)4]−. For this reason, solutions or hydrated salts of the latter are often improperly named "metaborates".
Solid state
In the solid state of their salts, metaborate ions are often oligomeric or polymeric, conceptually resulting from the fusion of two or more BO−2 through shared oxygen atoms. In these anions, the boron atom forms covalent bonds with either three or four oxygen atoms. Some of the structures are:
- A trimer with formula B3O3−6 or (−B(−O−)−O−)3, consisting of a six-membered ring of alternating boron and oxygen atoms with one extra oxygen atom attached to each boron atom. This form is found, for example, in some anhydrous alkali metal salts like sodium metaborate[1] or potassium metaborate,[2][3] in α- and β-barium metaborate Ba3(B3O6)2,[4][5][6] and in the mixed salt potassium cadmium metaborate KCdB3O6.[7] The three B–O distances are nearly equal in the potassium salt (133.1, 139.8, and 139.8 pm) but significantly different in the sodium one (128.0, 143.3, and 143.3 pm).
- A polymer of BO−2 units connected by single shared-oxygen bridges; that is, ···(−B(−O−)−O−)n···. Occurs in calcium metaborate CaB2O4 or Ca(BO2)2.[8][9]
- A tridimensional network of BO4 tetrahedral groups, as in "zinc metaborate", which is actually a mixed salt zinc metaborate oxide, with the formula (Zn2+)4(O2−)(BO−2)6.[10][11]
- A tridimensional regular array of [B(−O−)4]− tetrahedra sharing oxygen vertices, as in the high-pressure and high-temperature γ form of lithium metaborate LiBO2.[12]
Aqueous solution
The cyclic trimer anions dissociate almost completely in aqueous solution giving mainly tetrahydroxyborate anions:[13][14]
- B3O3−6 + 6 H2O
3 [B(OH)4]−
Other molecules and anions, such as B(OH)3, [B3O3(OH)4]−, [B3O3(OH)5]2−, and B4O5(OH)2−4 are less than 5% at 26 °C.[15][16]
In 1937, Nielsen and Ward claimed that the metaborate anion in solution has a linear symmetric structure −O−B+−O− with negative charges on the oxygens and a positive charge on the boron, or O=B−=O with negative charge on the boron.[17] However, this claim has been disproved.
Gas phase
The vapor of cesium metaborate has neutral monomers CsBO2 and dimers Cs2[B2O4] as well as ionized versions thereof.[18] The same situation holds for thallium metaborate TlBO2.[19]
In 1964 Hisatsune and Surez investigated the infrared spectrum of metaborate anions in dilute solid solutions of potassium salt in alkali halides such as potassium chloride KCl.[20]
M. Marezio, H. A. Plettinger and W. H. Zachariasen (1963): "The bond lengths in the sodium metaborate structure", Acta Crystallographica, volume 16, pages 594-595. doi:10.1107/S0365110X63001596 W. H. Zachariasen (1937): "The Crystal Structure of Potassium Metaborate, K3(B3O6)". Journal of Chemical Physics, volume 5, issue 11, page 919. doi:10.1063/1.1749962 W. Schneider and G. B. Carpenter (1970) "Bond lengths and thermal parameters of potassium metaborate, K3B3O6". Acta Crystallographica - Section B volume B26, pages 1189-1191 doi:10.1107/S0567740870003849 C. S. Willand and A. C. Albrecht (1986): "The second-order nonlinear optical properties of β-barium metaborate: A semiempirical theoretical investigation". Optics Communications, volume 57, issue 2, 15 February , Pages 146-152. doi:10.1016/0030-4018(86)90146-X Y. Roussigné, R. Farhi, C. Dugautier, and J. Godard (1992): "A Raman study of both phases of barium metaborate (BBO)". Solid State Communications. volume 82, issue 4, pages 287-293. doi:10.1016/0038-1098(92)90643-N P. Ney, M. D. Fontana, A. Maillard, and K. Polgár (1998): "Assignment of the Raman lines in single crystal barium metaborate (β-BaB2O4)". Journal of Physics: Condensed Matter, volume 10, issue 3, pages 673-. doi:10.1088/0953-8984/10/3/018 Shifeng Jin, G. Chai, J. Liu, W. Wang and X. Chen (2009): "The centrosymmetric metal meta-borate KCdB3O6". Acta Crystallographica Section C, volume C65, issue 7, pages i42-i44. doi:10.1107/S0108270109021477 W. H. Zachariasen (1931): "The Crystal Lattice of Calcium Metaborate, CaB2O4". Proceedings of the National Academy of Sciences (PNAS), volume 17, issue 11, pages 617-619. doi:10.1073/pnas.17.11.617 M. Marezio, H. A. Plettinger and W. H. Zachariasen (1963): "Refinement of the calcium metaborate structure". Acta Crystallographica, volume 16, pages 390-392. doi:10.1107/S0365110X63001031 (1964): "The crystal structure of anhydrous zinc metaborate Zn4O(BO2)6". Zeitschrift für Kristallographie, volume 119, issue 5-6 doi:10.1524/zkri.1964.119.5-6.375 M. Marezio and J. P. Remeika (1966): "Polymorphism of LiMO2 Compounds and High‐Pressure Single‐Crystal Synthesis of LiBO2". Journal of Chemical Physics, volume 44, issue 9, pages 3348-. doi:10.1063/1.1727236 Robert K. Momii and Norman H. Nachtrieb (1967): "Nuclear Magnetic Resonance Study of Borate-Polyborate Equilibria in Aqueous Solution". Inorganic Chemistry, volume 6, issue 6, pages 1189-1192. doi:10.1021/ic50052a026 Fayan Zhu, Yongquan Zhou, Chunhui Fang, Yan Fang, Haiwen Ge, and Hongyan Liu (2017): "Ion association in lithium metaborate solution: a Raman and ab initio insight". Physics and Chemistry of Liquids, volume 55, issue 2, pages 186-195. doi:10.1080/00319104.2016.1183003 Yongquan Zhou, Chunhui Fang, Yan Fang, Fayan Zhu, Song Tao, Sha Xu (2012): "Structure of aqueous sodium metaborate solutions: X-ray diffraction study". Russian Journal of Physical Chemistry, volume 86, issue 8, pages 1236–1244. doi:10.1134/S0036024412060349 Yongquan Zhou, Souta Higa, Chunhui Fang, Yan Fang, Wenqian Zhang, and Toshio Yamaguchi (2017): "B(OH)4− hydration and association in sodium metaborate solutions by X-ray diffraction and empirical potential structure refinement". Physical Chemistry Chemical Physics, volume 19, pages 27878-27887 doi:10.1039/C7CP05107G J. Rud Nielsen and N. E. Ward (1937): "Raman Spectrum and Structure of the Metaborate Ion". The Journal of Chemical Physics, volume 5, Issue 3, page 201 doi:10.1063/1.1750008 Mitsuru Asano, Yoshihiko Yasue, and Kenji Kubo (1984): "Mass Spectrometric Study of Ions Formed from Cesium Metaborate Vapor under Electron Impact". Journal of Nuclear Science and Technology, volume 21, issue 8,pages 614-624. doi:10.1080/18811248.1984.9731090 David H. Feather and Alfred Buechler (1973): "Gaseous thallium(I) metaborate and thallium(I) aluminum fluoride". Journal of Physical Chemistry, volume 77, issue 12, pages 1599–1600. doi:10.1021/j100631a027 I. C. Hisatsune and Noelia Haddock Suarez (1964): "Infrared Spectra of Metaborate Monomer and Trimer Ions". Inorganic Chemistry, volume 3, issue 2, pages 168-174. doi:10.1021/ic50012a003
