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Which one of the following molecular hydrides acts as a Lewis acid?
A
$CH_4$
B
$NH_3$
C
$H_2O$
D
$B_2H_6$
Detailed Solution
Molecular hydrides are classified as electron-deficient, electron-precise and electron-rich.
$B_2H_6$ is electron-deficient: it has too few electrons to write a conventional Lewis structure (it has 3-centre–2-electron bonds), so it accepts electron pairs and acts as a Lewis acid, e.g., $B_2H_6 + 2NH_3 \rightarrow 2BH_3\cdot NH_3$ type adducts and $B_2H_6 + 2CO \rightarrow 2BH_3\cdot CO$.
$CH_4$ is electron-precise; it is neither a Lewis acid nor a Lewis base.
$NH_3$ and $H_2O$ are electron-rich, with lone pairs on the central atom; they act as Lewis bases.
Hence the hydride that acts as a Lewis acid is $B_2H_6$.
$B_2H_6$ is electron-deficient: it has too few electrons to write a conventional Lewis structure (it has 3-centre–2-electron bonds), so it accepts electron pairs and acts as a Lewis acid, e.g., $B_2H_6 + 2NH_3 \rightarrow 2BH_3\cdot NH_3$ type adducts and $B_2H_6 + 2CO \rightarrow 2BH_3\cdot CO$.
$CH_4$ is electron-precise; it is neither a Lewis acid nor a Lewis base.
$NH_3$ and $H_2O$ are electron-rich, with lone pairs on the central atom; they act as Lewis bases.
Hence the hydride that acts as a Lewis acid is $B_2H_6$.