Nitridoborate

The nitridoborates are chemical compounds of boron and nitrogen with metals. These compounds are typically produced at high temperature by reacting hexagonal boron nitride (α -BN) with metal nitrides or by metathesis reactions involving nitridoborates. A wide range of these compounds have been made involving lithium, alkaline earth metals and lanthanides, and their structures determined using crystallographic techniques such as X-ray crystallography. 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 Li₃BN₂, Mg₃BN₃, La₃B₃N₆, La₅B₄N₉.[1] However there are examples of compounds with more than one metal, for example La₃Ni₂B₂N₃[2] and compounds containing anions such as Cl⁻, for example Mg₂BN₂Cl.[3]

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 Ca₃[BN₂]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.[4] The simplest ion BNⁿ⁻ is comparable to the C²⁻
₂ ion, but attempts to prepare the compound CaBN analogous to CaC₂ calcium carbide 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 BN²⁻ to 2 (double bond) in BN⁴⁻.[5]

Some nitridoborates are salt-like such as Li₃BN₂, LiCa₄[BN₂]₃ others have a metallic lustre, such as LiEu₄[BN₂]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.[4]

anion	geometry	Typical B–N bond length (pm)	B-B bond length (pm)	isoelectronic with	Examples of compounds
BN^n−	linear	138–202[5][6][7]			CaNiBN,(Ca^2+Ni^2+BN^4−); LaNiBN, (La^3+Ni^2+BN^4−(e^−))
BN^3− 2	linear	132–137[4][8]		[CN2]^2−, CO2	Ca3(BN2)2
BN^6− 3	trigonal planar	145–149[4][7]		BO^3− 3, CO^2− 3	La6(BN3)O6
B 2N^8− 4	planar	147–150[4]	177–182[4]	C 2O^2− 4	La3B2N4, ((La^3+)3(B 2N^8− 4)(e^−))
B 3N^9− 6	planar or chair form	144–151[4]			La3B3N6[9]

For comparison purposes the following are considered to be typical BN bond lengths[1]

Compound	B–N (pm)	Bond type
Me3N·BBr3	160.2	single
Me3N·BCl3	157.5	single
Cubic BN	157	Single
Hexagonal BN	144.6	intra-layer distance some π-bonding
B(NMe2)3	143.9	some π-bonding
Mes2BNBMes2	134.5	double bond
(t-Bu)BN(t-Bu)	125.8	triple bond

References

1. Housecroft, Catherine E; Sharpe, Alan G (2005). Inorganic Chemistry (2nd ed.). Pearson education. p. 318. ISBN 978-0-13-039913-7.
2. Blaschkowski, Björn; Meyer, H.-Jürgen (2003). "X-Ray Single Crystal Refinement and Superconductivity of La₃Ni₂B₂N₃". Zeitschrift für anorganische und allgemeine Chemie. 629 (1): 129–132. doi:10.1002/zaac.200390004. ISSN 0044-2313.
3. Somer, Mehmet; Kütükcü, Mehmet Nuri; Gil, Raul Cardoso; Borrmann, Horst; Carrillo-Cabrera, Wilder (2004). "Mg₂[BN₂]Cl and Mg₈[BN₂]₅I: Novel Magnesium Nitridoborate Halides — Syntheses, Crystal Structures, and Vibrational Spectra". Zeitschrift für anorganische und allgemeine Chemie. 630 (7): 1015–1021. doi:10.1002/zaac.200400055. ISSN 0044-2313.
4. Meyer, H. Jurgen (2006). "Chapter 8: Current State on (B,C,N) compounds of Calcium and Lanthanum". In Meyer, Gerd; Naumann, Dieter; Wesemann, Lars (eds.). Inorganic Chemistry in Focus III. Wiley-VCH. pp. 121–138. ISBN 978-3-527-31510-9.
5. Blaschkowski, Björn; Meyer, H.-Jürgen (2002). "Electronic Conditions of Diatomic (BN) Anions in the Structure of CaNiBN". Zeitschrift für anorganische und allgemeine Chemie. 628 (6): 1249. doi:10.1002/1521-3749(200206)628:6<1249::AID-ZAAC1249>3.0.CO;2-S. ISSN 0044-2313.
6. Cava, R. J.; Zandbergen, H. W.; Batlogg, B.; Eisaki, H.; Takagi, H.; Krajewski, J. J.; Peck, W. F.; Gyorgy, E. M.; Uchida, S. (1994). "Superconductivity in lanthanum nickel boro-nitride". Nature. 372 (6503): 245–247. doi:10.1038/372245a0. ISSN 0028-0836.
7. Blaschkowski, Björn; Jing, Haipeng; Meyer, H.-Jürgen (2002). "Nitridoborates of the Lanthanides: Synthesis, Structure Principles, and Properties of a New Class of Compounds". Angewandte Chemie International Edition. 41 (18): 3322–3336. doi:10.1002/1521-3773(20020916)41:18<3322::AID-ANIE3322>3.0.CO;2-8. ISSN 1433-7851. PMID 12298029.
8. Somer, Mehmet; Herterich, Uwe; Čurda, Jan; Carrillo-Cabrera, Wilder; Zürn, Anke; Peters, Karl; Schnering, Hans Georg von (2000). "Darstellung, Kristallstrukturen und Schwingungsspektren neuer ternärer Verbindungen mit dem Anion [N–B–N]³⁻". Zeitschrift für anorganische und allgemeine Chemie. 626 (3): 625–633. doi:10.1002/(SICI)1521-3749(200003)626:3<625::AID-ZAAC625>3.0.CO;2-4. ISSN 0044-2313.
9. Reckeweg, Olaf; Meyer, H.-Jürgen (1999). "Lanthanoidnitridoborate mit sechsgliedrigen B₃N₆-Ringen: Ln₃B₃N₆". Angewandte Chemie. 111 (11): 1714–1716. doi:10.1002/(SICI)1521-3757(19990601)111:11<1714::AID-ANGE1714>3.0.CO;2-X. ISSN 0044-8249.