Sodium metaborate

Anhydrous

Solid anhydrous sodium metaborate has the hexagonal crystal system with space group ${\displaystyle R{\bar {3}}c}$. It actually contains a six-membered rings with the formula [B₃O₆]³⁻, consisting of alternating boron and oxygen atoms with one negatively charged extra oxygen atom attached to each boron atom.^[6][1] All nine atoms lie on a plane.^[4] The six oxygen atoms are evenly divided into two distinct structural sites, with different B–O bond lengths: B–O(external) 128.0 pm and B–O(bridge) 143.3 pm.^[6] The density is 2.348 ± 0.005 g/cm³.^[4] The approximate dimensions of the hexagonal cell are a = 1275 pm, c = 733 pm.^[4] However, the true unit cell is rhombohedral and has dimensions: a_r= 776 pm, α = 110.6°, Z = 6 (5.98) molecules KB0^[4]

Dihydrate

The dihydrate NaBO₂·2H₂O crystallizes in the triclinic crystal system, but is nearly monoclinic, with both α and γ very close to 90°. The cell parameters are a = 678 pm , b = 1058A pm, c = 588 pm, α = 91.5°, β = 22.5°, γ = 89°, Z = 4, density 1.905 g/cm³. The refractive indices at 25°C and wavelength 589.3 nm are α = 1.439, β = 1.473, γ = 1.484. The dispersion is strong, greater at red than at violet.^[7]

The transition temperature between the dihydrate and the hemihydrate is 54 °C. However, the crystalline dihydrate will remain metastable until 106 °C to 110 °C, and change slowly above that temperature.^[7]

A compound with an identical empirical formula but a distinct molecular and crystal structure is sodium tetrahydroxyborate (NaB(OH)₄).

Vapor

Infrared spectroscopy of the vapor from anhydrous sodium metaborate, heated to between 900 °C and 1400 °C, shows mostly isolated clusters with formula NaBO₂, and some dimers thereof.^[8] Electron diffraction studies by Akishin and Spiridonov showed a structure O=B−O−Na with linear anion O=B−O⁻ and angle B−O−Na of 90-110°. The atomic distances are O=B: 120 pm, B−O: 136 pm,O−Na: 214 pm^[9]

With water

When sodium metaborate is dissolved in water, the anion combines with two water molecules to form the tetrahydroxyborate anion [B(OH)₄]⁻.^[11]

Electrochemical conversion to borax

Electrolysis of a concentrated aqueous solution of 20%^{[clarification needed]} NaBO₂·4H₂O with an anion exchange membrane and inert anode (such as gold, palladium, or boron-doped diamond) converts the metaborate anion to tetraborate B₄O2−7, and the sodium salt of the later (borax) precipitates as a white powder.^[12]

8 BO−2 → 2 B₄O2−7 + O₂ + 4 e⁻

Reduction to sodium borohydride

Sodium metaborate can be converted to sodium borohydride by several methods, including the reaction with various reducing agents at high temperatures and pressure,^[12] or with magnesium hydride MgH₂ by ball milling at room temperature, followed by extraction of the Na[BH₄] with isopropylamine.^[13][10]

NaBO₂ + 2 MgH₂ → Na[BH₄] + 2 MgO

Another method is the electrolytic reduction of a concentrated sodium metaborate solution,^[10] namely

BO−2 + 6 H₂O + 8 e⁻ → [BH₄]⁻ + 8 OH⁻

However, this method is not efficient since it competes with the reduction of hydroxide:

4 OH⁻ → 2 H₂O + O₂ + 4 e⁻

Nanofiltration membranes can effectively separate the borohydride from the metaborate.^[11]

Reaction with alcohols

Anhydrous sodium metaborate refluxed with methanol yields the corresponding sodium tetramethoxyborate (melting point: 253-258 °C, CAS number: 18024-69-6^[14]):^[15]

Na⁺BO−2 + 4 CH₃OH → Na⁺[B(OCH₃)₄]⁻ + 2 H₂O

The analogous reaction with ethanol yields the sodium tetraethoxyborate.^[15]