4304233

B3H5 and B3H6+, isoelectronic to cyclopropenyl cation, form the basis for structural chemistry of boron

Date
August 18, 2025


Structural chemistry of carbon is largely explained by sp, sp2, and sp3 hybridisation; all of organic chemistry, graphite, graphene, fullerenes and nanotubes follow from this. An equivalent structural chemistry of boron is only evolving. We present here an Extended Rudolph Diagram that explains most of the structural chemistry of boron. Chemistry of molecules with a few boron atoms mostly follow the rules of carbon chemistry. The polyhedral boranes and related molecules call for a different model of bonding. Several years ago we had extended the Wade’s Rule to mno Rule, so that polyhedral boranes represented in the Rudolph Diagram can be related to 3D allotropes of boron, similar to the relationship between condensed benzenoid aromatics and graphene. Here we present an Extended Rudolph Diagram that includes the nido molecules B3H5 and B3H6+ derived from the closo-B4H4 (Td). Sequential replacements of the bridging hydrogens by BH units in the C3V B3H6+ could generate all structures of the original Rudolf diagram. If we start from the flat B3H5 and replace the H by BH groups we end up in borophenes. Traditional van der Waals interactions and localized 2c-2e bonds between stacks of borophenes allow potentially infinite variety of “borophites”. The Extended Rudolph Diagram relates polyhedral boranes to 3D-allotropes such as alpha- and beta-rhombohedral boron and to 2D-allotropes such as mono-, bi- and multilayer borophenes and borophites. My students over the years are gratefully acknowledged.

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