Q.Diborane, $\text{B}_2\text{H}_6$, has only 12 valence electrons but must account for 8 B–H connections. Describe its structure, distinguishing the four terminal hydrogens from the two bridging hydrogens, and explain what a three-centre two-electron (3c–2e) bond is.

[!TLDR] Diborane has 4 ordinary terminal B-H bonds and 2 bridging hydrogens, each held between both borons by a 3-centre-2-electron bond, because the molecule has too few electrons for 8 normal bonds. [!ANSWER] $\text{B}_2\text{H}_6$'s 4 terminal H's use ordinary 2c-2e bonds, while its 2 bridging H's use 3c-2e bonds — a si …

Diborane, $\text{B}_2\text{H}_6$, has a total of $2(3)+6(1)=12$ valence electrons, i.e. only 6 electron pairs. If all 8 of its B–H connections were ordinary two-centre-two-electron (2c–2e) bonds, 16 electrons (8 pairs) would be required — two more pairs than are actually available. Diborane resolves this shortfall with a structure in which the six hydrogens play two distinct roles. Four hydrogens are terminal: two bonded to each boron by conventional 2c–2e $\sigma$ bonds lying in the plane containing both boron atoms, exactly as in a simple $\text{BH}_2$ fragment. The remaining two hydrogens are bridging, positioned above and below the B···B axis; each bridging hydrogen is simultaneously bonded to both boron atoms at once through a single, delocalised three-centre two-electron (3c–2e) bond — one pair of electrons spread across three nuclei (the bridging H and both B atoms) rather than confined between just two. These two 3c–2e bridge bonds, often nicknamed 'banana bonds' for their curved shape, are what hold the two $\text{BH}_2$ halves of the molecule together and account for diborane's remaining electron count exactly ($4$ terminal 2c-2e bonds $=4$ pairs, plus $2$ bridging 3c-2e bonds $=2$ pairs, totalling $6$ pairs $=12$ electrons). Each boron atom is best described as roughly $sp^3$ hybridised, using two hybrid orbit …

Count total valence electrons in B2H6, compare against the electrons needed for 8 ordinary bonds, then describe the terminal vs brid …

Describing all six hydrogens as equivalent/terminal, ignoring the bridging pair entirely; describing a 3c-2e bond as simply 'two bonds sharing an elect …