boranes (boron hydrides)

If you imagine boron's chemistry as a cousin of carbon's, you might expect boron to build hydrocarbon-like chains: BH3, B2H6 mimicking ethane, and so on. Nature plays a stranger game. Boron has one fewer valence electron than carbon, so it cannot make the simple electron-rich chains that give us alkanes. Instead boron and hydrogen build a whole family of cage-like and basket-like molecules, the boranes, that look nothing like organic chemistry and were a genuine puzzle for decades.

Boranes are binary compounds of boron and hydrogen. The simplest stable one is diborane, B2H6, not the elusive BH3 (which exists only fleetingly and immediately dimerizes). Larger boranes come in series with names like closo, nido, and arachno that describe how complete their cage is: closo boranes such as the borane anion B6H6^2- are closed polyhedra (here an octahedron of borons), nido (nest-like) boranes such as B5H9 are a closed cage with one vertex missing, and arachno (web-like) boranes such as B4H10 have two vertices missing. They hold themselves together using a mixture of ordinary 2c-2e B-H and B-B bonds plus electron-deficient 3c-2e bonds (B-H-B bridges and central B-B-B bonds), and the number of skeletal electron pairs is what fixes the shape.

Boranes matter far beyond curiosity. Counting their cluster electrons is exactly what Wade's rules were invented to do, and that same counting later unified the structures of metal clusters and carboranes. Practically, boranes and their salts are reducing agents and the starting point for boron-based reagents; the borohydride ion BH4-, related to this family, is the workhorse reducing agent sodium borohydride. They are also pyrophoric and toxic, a reminder that beautiful structures are not the same as friendly ones.