Electron-Deficient Bonding in ◆ Rhomboid Rings

摘要

The bonding environment of boron is usually thought about in terms of localized 2c-2e/3c-2e bonding (as in diborane) or completely delocalized polyhedral bonding (as in B_(12)H_(12)~(2-)). Recently, a number of boron compounds having a rhomboidal B_4 framework have been synthesized; these show an amazing variation in their skeletal electron count, one that cannot be interpreted in familiar ways. In this report, we systematically explore the origin of the range of electron counts in these compounds. We find that four skeletal MOs are primarily responsible for keeping the B_4 skeleton together. As a subunit in a macropolyhedral environment, termed rhombo-B_4, such an arrangement of B atoms deviates from Wade's rule by three electron pairs (if treated as a distorted arachno system derived from B_6H_6~(2-)). Aided by this analysis, we examine the nature of bonding in Na_3B_(20), where the rhombo-B_4 unit forms linear chains fusing closo-B_7 units. Theory suggests that this structure requires one more electron per formula unit for optimal bonding. Finally, we study the nature of bonding in β-SiB_3, where silicon atoms also adopt the rhomboid framework.