| Abstract
| - The bonding environment of boron is usually thought about in terms of localized 2c−2e/3c−2ebonding (as in diborane) or completely delocalized polyhedral bonding (as in B12H122-). Recently, a numberof boron compounds having a rhomboidal B4 framework have been synthesized; these show an amazingvariation in their skeletal electron count, one that cannot be interpreted in familiar ways. In this report, wesystematically explore the origin of the range of electron counts in these compounds. We find that fourskeletal MOs are primarily responsible for keeping the B4 skeleton together. As a subunit in amacropolyhedral environment, termed rhombo-B4, such an arrangement of B atoms deviates from Wade'srule by three electron pairs (if treated as a distorted arachno system derived from B6H62-). Aided by thisanalysis, we examine the nature of bonding in Na3B20, where the rhombo-B4 unit forms linear chains fusingcloso-B7 units. Theory suggests that this structure requires one more electron per formula unit for optimalbonding. Finally, we study the nature of bonding in β-SiB3, where silicon atoms also adopt the rhomboidframework.
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