Calcium hexaboride

Adsorption / desorption

Administrative data

Endpoint:

adsorption / desorption: screening

Data waiving:

study scientifically not necessary / other information available

Justification for data waiving:

the study does not need to be conducted because the physicochemical properties of the substance indicate that it can be expected to have a low potential for adsorption

Cross-reference

Cross-reference 1

Reason / purpose for cross-reference:

data waiving: supporting information

Reference

Endpoint:

melting point/freezing point

Type of information:

not specified

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

data from handbook or collection of data

Principles of method if other than guideline:

no data

GLP compliance:

no

Type of method:

other: no data

Key result

Melting / freezing pt.:

2 235 °C

Remarks on result:

other: no data on pressure

Conclusions:

The melting point of CaB6 is reported as 2235 °C.

Executive summary:

The melting point of CaB6 is reported as 2235 °C. Because this data is available in a handbook, it is considered reliable with restrictions.

Cross-reference 2

Reason / purpose for cross-reference:

data waiving: supporting information

Reference

Endpoint:

water solubility

Type of information:

other: review of literature

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

data from handbook or collection of data

Qualifier:

no guideline followed

Principles of method if other than guideline:

Molecular Orbital Theory of metal-metal bonds

GLP compliance:

no

Remarks:

(Theoretical consideration of chemical structure)

Type of method:

other: literature review

Key result

Remarks on result:

not determinable

Remarks:

Based on molecular orbital theory, the bond energy of the B-B bonds is too high to be cleaved by solvation. Only hydrolysis in strong acid would slowly cleave the B-B bonds. Therefore, CaB6 is insoluble in water.

Water solubility of calcium hexaboride is not a relevant property. Rather than being an ionic solid consisting of calcium and hexaboride ions, calcium hexaboride consists of calcium ions [Ca(II)] packed into an anionic crystalline 3-D polymeric lattice formed by hexaboride. As such, calcium may be exchanged with other cations, but the hexaboride lattice will remain intact. The crystal structure of calcium hexaboride consists of calcium cations in a close packed 3-D structure of covalently-linked octahedral hexaboride units (1). “The presence of a strong bond between boron atoms in hexaborides creates a strong frame work of boron atoms, in which each boron atom is bonded to four neighboring boron atoms in the same octahedron and to one boron atom in the next octahedron. (3 1/3 bonding electrons per boron)” (2). This results in seven intra-unit bonding orbitals forming each octahedral hexaboride and six outwardly directed orbitals forming one bond with each of six neighboring octahedral hexaboride units, for each boron in an individual octahedron (3). The result is a bond dissociation energy of ca -6.5 eV for each B-B bond within a hexaboride octahedron, a -13 eV orbital consisting of two electrons shared six ways and six hexaboride-hexaboride bonds connecting the octahedral with a dissociation energy of ca. -8 eV (3). These energies can be compared to the 4.40 eV bond energy for the covalent C-H bond and a 3.91 eV bond energy for the covalent C-C bond (4). “Dissolution” would therefore involve the breaking of strong covalent bonds within the hexaboride 3-D lattice, i.e. hydrolysis, rather than the disruption of weaker intramolecular ionic involved in dissolving ionic salts or organic molecules. However, sufficient energy to do this is not available in aqueous solution. In order to dissolve one the hexaboride, more energy is required than for water to break all C-C and C-H bonds in ethane. As ethane is not atomized in water, it is safe to assume that a hexaboride octahedron would not be cleaved from the crystal lattice in water.

Calcium hexaboride will dissolve slowly in non-oxidizing acids (1, 2). The proposed mechanism is intrusion of protons into the lattice which possibly forces the adoption of a Hexaboride Lanthanum type electronic structure. This is followed by more proton intrusion and subsequent loss of Ca(II). Lastly, water intrudes and dissolution occurs. It is unclear exactly when the boride octahedral is broken down or when oxidation of released boron occurs.

1. G. Raynor, K. Trask. Chemistry and Applications of Calcium and Potassium, Rev Ed., Academic Studio, New York, NY, 2016, p. 23.

2. G.V. Samsonov, B. Paaderno. Borides of Rare Earth Metals, Academy of the Sciences, SSR Kiev, 1961, p. 10-11.

3. K. Schmitt, C. Stückl, H. Ripplinger, B. Albert. Crystal and electronic structure of BaB₆ in comparison with CaB₆ and molecular [B₆H₆]^{2 -}. Solid State Sci. 2001, vol. 3, pp. 321-32.

4.  S. J. Blanksby, G.B. Ellison. Bond Dissociation Energies of Organic Molecules. Acc. Chem. Res. 2003, vol. 36, no. 4, pp. 255–2637.

Conclusions:

Calcium hexaboride is insoluble in water.

Executive summary:

Based on a review of literature and the crystal structure of calcium hexaboride, water solubility is not a relevant property. The structure of calcium hexaboride is a covalently linked lattice of hexaboride unit interspersed with calcium ions [Ca(II)]. The bond strength of the boron-boron connections is too great for dissolution or hydrolysis in aqueous solution. After ligand-exchange by protons, the crystalline lattice may be disrupted more easily. Acid-mediated hydrolysis can then occur in non-oxidizing mineral acid solution. This study is based on a review of the scientific literature and sound scientific concepts. It is deemed reliable with restrictions, and is suitable for Risk Assessment, Classification & Labelling, and PBT Analysis.