The single-crystal X-ray structure of Rb3Os2Br9 provides the first crystallographic definition of a diosmium confacial nonahalide. Rb3Os2Br9 crystallizes in the hexagonal space group P63/m [a/Å = 7.401(1), c/Å = 18.098(4)], Z = 2, conferring strict D3h symmetry on the complex anion. The internal geometry of the 10-e (5d5/5d5) binuclear complex [β = 66.54(5)°; Os-Os = 2.779(1), Os-Brt = 2.499(1), Os-Brbr = 2.532(2) Å] is consistent with significant net σ bonding [σ2 δπ4δπ*4]. Comparative SCF-Xα-SW calculations on [Os2Br9]3− and [Ru2Br9]3− reinforce the picture of greater metal-metal interaction in the heavier (third-row) species. The optical spectra of [Os2Br9]3− and [Ru2Br9]3− bear a qualitative resemblance; the main features are attributed to superposition of metal-based σ → σ* (6a1′ → 5a2″) and ligand-field (6a1′ → 9e′) transitions in the visible/near-UV envelope and, at higher energy, to intense Brbr/t-to-M2 charge transfer to πδ* and πδ (7e″, 9e′) arising from the single-ion eg levels. The σ → σ* band is located near 24 000 cm−1 in [Os2Br9]3− and near 20 000 cm−1 in [Ru2Br9]3−. An electron-exchange term (K) of the order 8000 cm−1 is implied in both cases. © 1994, American Chemical Society. All rights reserved.
