which of the following chemical compound does not exist? why B₂H₂(CH₃)₄does not exist??
it is a question in topic test of p-block, first concept?the explanation given is not sufficient for me?
how they find it is of B₂H₆group?

Dear Student,

Please find below the solution to the asked query:

Diborane(B₂H₆) and similar structures form a banana bond or a 3 center 2 electron bond. The bond between the two boron atoms and hydrogen is held together with 2electrons.
Electronic configuration of B is 1s²2s²2p¹. It has 3 valence electrons and three covalent bonds with three hydrogen atoms in BH₃ gives B a total of 6 electrons. Thus after bonding it has an incomplete octet. BH₃ has an empty 2p orbital. This orbital overlaps with the existing B-H sigma bond cloud of a neighbouring BH₃ molecule forming a 3center-2-electron or banana bond. This gives both B atoms a full octet and therefore stability. BH₃ therefore exists as a dimer, B₂H₆.
The requirement of forming the banana bond in boranes is to have one atom as H. Boranes in which the hydrogen atoms are replaced by methyl groups show similar dimeric structure provided they have a single B-H bond. Tetramethyldiborane has hydrogen atoms to form the banana bond, trimethylborane does not contain any hydrogen atom so exists as a monomer. The structures are given in the figure below.


 All diboranes have at least two hydrogen atoms have the banana bond.
The infrared spectra of  tetramethyldiborane shows strong absorption band at 1602 cm^-1 which is a characteristic feature of bridging hydrogen.
 

Hope this information will clear your doubts about tetramethyldiborane.

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