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JEE Advanced levelSingle correct optionDifficulty 3 of 3cb048

Diborane, $B X_{2} H X_{6}$ , has 12 valence electrons in total. The fraction of these electrons that is held in the multicentre (bridging) bonds is

A $\frac{1}{3}$
B $\frac{1}{2}$
C $\frac{2}{3}$
D $\frac{1}{4}$

Answer: A

Total valence electrons =

2 \times 3 + 6 \times 1 = 12

. Four terminal B-H 2c-2e bonds use 8 electrons. The two B-H-B 3c-2e bonds use the remaining 4 electrons. Fraction =

4/12 = 1/3

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