Diborane molecule has six hydrogen atoms, but all atoms cannot be substituted in methylation. Why?

Explanation

Diborane has two planar `-BH_2` groups. Thus it has only four terminal hydrogen atoms. The other two hydrogen atoms are involved in the hydrogen bridge bonds. Hence they cannot be replaced in mcthylation.
Methylation of diborane is limited to tetramethyl derivative. Diborane does not form pentamethyl or hexamethyl derivatives.