Q.In diborane, the number of electrons that accounts for banana bonds is

Diborane's structure is the classic textbook illustration of electron-deficient, three-centre bonding. Two BH₂ units are joined by two bridging hydrogen atoms, giving eight B-H bonds in total -- but diborane's 12 total valence electrons are not enough to form eight ordinary two-electron covalent bonds (which would need 16 electrons).

The four terminal B-H bonds (two per boron, pointing outward) are ordinary two-centre-two-electron (2c-2e) bonds, using 8 of the 12 valence electrons. The remaining four electrons are shared between the two bridging B-H-B bonds; because each bridge bond spans three atoms (one bridging H plus both borons) using only two electrons, each is a three-centre-two-electron (3c-2e) bond -- popularly called a 'banana bond' for the curved shape of its electron density. The two bridging hydrogens both lie in the same plane. …