Anomalous Properties of Boron Hydrides

Introduction to Boron

Group 13 elements show certain patterns in their chemical conduct. Boron, the first element in this group, shows some specific properties that are completely different from its other members. The reasons why Boron is different from the other members are,

• Its smaller size
• Higher electronegativity
• Its ionization enthalpy is higher
• d shell is absent in it
• It can attain a maximum covalency of only 4 due to the absence of d orbital

Properties of Boron

• Boron is a non-metal or a metalloid
• Boron is a bad conductor of electricity
• Boron is allotropic
• It is available in crystalline and amorphous forms
• Boron has higher boiling and melting points
• Boron forms only covalent bonds
• Boron oxides, as well as hydroxides, are acidic
• It forms inert hydrides
• While the other members of the Boron family have p orbital, it is absent in boron

Group 13 Elements

Elements in this group show a trend in their physical and chemical properties:-

• They form covalent bonds with halogens to form chlorides, bromides, or iodides which are easily hydrolyzed with water
• They usually form tetrahedral or octahedral structured compounds
• These elements are metals whose metallic nature increases going down the group
• Oxides of the group 13 elements are generally amphoteric in nature with the exception of boron which shows acidic nature
• Being amphoteric, they react with bases but boron is an exception
• These elements are reactive to acids but boron does not react with acids
• Boron family members form both ionic and covalent bonds

Hydrides of group 13 elements

Generally, the hydrides of group 13 elements are polymeric in nature with their stability decreasing as they go down the group. Boron forms the most number of hydrides and shows some anomalous properties. Boron hydrides are volatile in nature and are called boranes. There are other boron hydrides with sodium and aluminium ( NaBH₄ and Al(BH₄)₃) that form an octet to form stable compounds, It is because of the presence of tetrahydroborate ion- BH₄. Lithium boron hydride is a boron hydride which is significant as a reducing agent for aldehydes, esters, etc.

Anomalous Properties of Boron Hydrides

• Boron hydrides are isolated only through the Lewis acid-base complex method
• Diborane is formed in the balance of a Lewis base
• Diborane is spontaneously flammable and is highly exothermic
• Sodium borohydride is more stable and is present in crystalline form
• Borohydride anion is less reactive than other hydrides of group 13 elements
• The volatile boron hydrides are easily hydrolyzed and can burn instantly in presence of oxygen