Consideration of the well-known very stable icosahedral B12H12(2-) as a three-dimensional analogue of benzene was extended by the recent synthesis of the biicosahedral B21H18(-) as a three-dimensional analogue of naphthalene. The preferred structures of metallaboranes derived from B21H18(-) have now been examined by density functional theory. The isoelectronic species CpNiB20H17 and CpCoCB19H17 have the 46 skeletal electrons expected by the Wade-Mingos and Jemmis rules for a structure consisting of two face-sharing fused icosahedra. The CpM units in these structures energetically prefer to be located at a meta vertex of the biicosahedron. The analogous ferraboranes CpFeB20H17 with only 44 skeletal electrons also have related biicosahedral structures. The presence of an agostic hydrogen atom bridging an Fe-B edge compensates for the two-electron deficiency in CpFeB20H17 relative to CpNiB20H17. The nucleus-independent chemical shift (NICS) values of these systems indicate them to be strongly aromatic.
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