Boron Hydride formation properties and Structures

Aug 26, 2022, 16:45 IST

Hydrides:

The IIIA group elements donot directly react with, however, a number of hydrides are known.

Element	B	Al	Ga	In	Tl
General Formulae of Hydride	&				-
Name of hydride	boranes	allanes	(di) gallanes	(poly) indanes	-
Stability	✔	✔	Unstable	Unstable	does not exist
Anionic hydride complex*	borohydride	aluminium	gallium hydride	-	-

Physics Wallah Chemistry Doubts page consist of more questions for reference.

* Powerful reducing agents

Boron forms a number of stable hydrides (boranes) of two series:

Nido-boranes (nido = nest)		Arachno borane (arachno = web)
Formula	Name	Formula	Name
	diborane
	pentaborane - 9		tetraborane
	hexaborane – 10		pentaborane – 11
	octaborane – 12		hexaborane – 12
	decaborane		octaborane – 14

* Afred Stock (1912 – 36) pioneered the boron chemistry and suggested various methods of preparation.

They are electron deficient compounds. The expected boron hydride is however not reported. Because the hydrogen, with which boron is bonded doesnot have free electrons available for back bonding so there is high scarcity or deficiency of an electron pair., as it would have existed, because of lack of complete octet dimerises to form.

Electron deficient compounds:

The compounds having less than eight electrons in their valence shell are called electron deficient.

They are very reactive and tend to stabilize by accepting two or more electrons and making their octet complete e.g., , , etc.

Anomalous behaviour of boron:

Like Li & Be in s–block, B, the first member of IIIA or 13 group exhibits some properties, which are not in conformity with the rest elements of the group. It is due to high nuclear charge/size ratio (i.e., high charge density), high eletronegativity and non-availability of d-orbitals.

In fact the charge/ratio of B becomes almost comparable to Si that is why it resembles Si in its properties.