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 P6(3)/m [a/angstrom = 7.401(1), c/angstrom = 18.098(4)], Z = 2, conferring strict D3h symmetry on the complex anion. The internal geometry of the 10-e (5d5/5d5) binuclear complex [beta = 66.54(5)-degrees; Os-Os = 2.779(1), Os-Br(t) = 2.499(1), Os-Br(br) = 2.532(2) angstrom] is consistent with significant net sigma bonding [sigma2delta(pi)4delta(pi)*4]. Comparative SCF-Xalpha-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 sigma --> sigma* (6a1’ --> 5a2") and ligand-field (6a1’ --> 9e’) transitions in the visible/near-UV envelope and, at higher energy, to intense Br(br/t)-to-M2 charge transfer to pi(delta)* and pi(delta) (7e", 9e’) arising from the single-ion e(g) levels. The sigma --> sigma* 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.