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Preparation: BX3 are all planar monomeric species due to the extra B(pπ)-X(pπ) bonding (compare AlCl3, which dimerizes to give Al2Cl6, as the inter-molecular Lewis acid–Lewis base bonds are stronger than the intra-molecular bonds in BX3).
Many boron hydrides, BnHm, are known, and the most important is diborane, B2H6. All are endothermic in formation, and have a formal boron oxidation state of less than three: some contain B-B bonds.
Boron has electronic configuration [He]2s22p1, and main formal oxidation state +3. Its bonding is mainly covalent, and it forms compounds with three bonds, and therefore six electrons in its valence shell: It is electron deficient.