The nitridoborates are chemical compounds of boron and nitrogen with metals. These compounds are typically produced at high temperature by reacting hexagonal
boron nitride
Boron nitride is a thermally and chemically resistant refractory compound of boron and nitrogen with the chemical formula B N. It exists in various crystalline forms that are isoelectronic to a similarly structured carbon lattice. The hexago ...
(α -BN) with metal
nitride
In chemistry, a nitride is a chemical compound of nitrogen. Nitrides can be inorganic or organic, ionic or covalent. The nitride anion, N3−, is very elusive but compounds of nitride are numerous, although rarely naturally occurring. Some nitr ...
s or by
metathesis reactions involving nitridoborates. A wide range of these compounds have been made involving
lithium
Lithium (from , , ) is a chemical element; it has chemical symbol, symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard temperature and pressure, standard conditions, it is the least dense metal and the ...
,
alkaline earth
The alkaline earth metals are six chemical elements in group 2 of the periodic table. They are beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).. The elements have very similar properties: they are al ...
metals and
lanthanide
The lanthanide () or lanthanoid () series of chemical elements comprises at least the 14 metallic chemical elements with atomic numbers 57–70, from lanthanum through ytterbium. In the periodic table, they fill the 4f orbitals. Lutetium (el ...
s, and their structures determined using crystallographic techniques such as
X-ray crystallography
X-ray crystallography is the experimental science of determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to Diffraction, diffract in specific directions. By measuring th ...
. Structurally one of their interesting features is the presence of
polyatomic anions of boron and nitrogen where the geometry and the B–N bond length have been interpreted in terms of
π-bonding.
Many of the compounds produced can be described as ternary compounds of metal boron and nitrogen and examples of these are Li
3BN
2, Mg
3BN
3, La
3B
3N
6, La
5B
4N
9.
However, there are examples of compounds with more than one metal, for example La
3Ni
2B
2N
3 and compounds containing anions such as
Cl−, for example Mg
2BN
2Cl.
Structures and bonding
Examination of the crystallographic data shows the presence of polyatomic units consisting of boron and nitrogen. These units have structures similar to those of isoelectronic anions, which have π-bonded structures. The bonding in some of these compounds is ionic in character, such as Ca
3 2">N2sub>2, other compounds have metallic characteristics, where the bonding has been described in terms of π-bonded anions with extra electrons in anti-bonding orbitals that not only cause a lengthening of the B–N bonds but also form part of the conduction band of the solid.
The simplest ion BN
''n''− is comparable to the ion, but attempts to prepare the compound CaBN analogous to CaC
2 calcium carbide
Calcium carbide, also known as calcium acetylide, is a chemical compound with the chemical formula of . Its main use industrially is in the production of acetylene and calcium cyanamide.
The pure material is colorless, while pieces of technica ...
failed. The bonding of compounds containing the diatomic BN anion have been explained in terms of electrons entering anti-bonding orbitals and reducing the B–N bond order from 3 (triple bond) in BN
2− to 2 (double bond) in BN
4−.
Some nitridoborates are salt-like such as Li
3BN
2, LiCa
4 2">N2sub>3 others have a metallic lustre, such as LiEu
4 2">N2sub>3. Bonding calculations show that the energy of the valence orbitals of metal atoms of group 2 and lanthanide elements are higher than those of the bonding orbitals in BN
''x'' ions which indicates an ionic like interaction between a metal atom and a BN
''x'' ion. With lanthanide compounds where extra electrons enter the anti-bonding orbitals of an ion there can be a smaller band gap giving the compounds metal like properties such as lustre. With transition metals the d orbitals can be similar in energy to bonding orbitals in the BN anions suggesting covalent interactions.
For comparison purposes the following are considered to be typical BN bond lengths
References
{{Reflist
Boron–nitrogen compounds
Anions