The complexes [(n-Bu)4N] [OsNX4], [(n-Pr)4N] [OSNX4], and [(n-Pr)4N] [OsNX4(OH2)] (X- = Cl-, Br-) have been prepared and characterized. Infrared spectral data suggest that the OSNX4- units in all six complexes possess C4v, symmetry. The [(n-Bu)4N] [OsNCl4] complex exhibits relatively weak electronic absorption spectral bands at 20 300 (ε 93), 22 700 (ε 77), and 25 600 cm-1 (ε 50) in dichloromethane solution at 298 K. Low-temperature (77 and 5 K) spectral measurements on solid samples show that the lowest band system consists of two components. Both components are perpendicularly polarized and exhibit similar vibrational structure (a1(Os≡N), ∼800-cm-1 spacing). Assignment of the components to transitions to a Jahn-Teller split E(3E) excited state is proposed. It is estimated that the Os≡N bond is lengthened by about 0.09 Å in the E(3E) state. Measurements on a single crystal at 5 K show that the band centered at 22810 cm-1 is parallel polarized and exhibits a 390-cm-1 progression attributable to a1(Os—Cl). Assignment of this band to 1A1 → A1(3A2) is suggested. The band that peaks at 25 490 cm-1 in the 5-K crystal spectrum appears to be only vibronically allowed and is attributed to 1A1 → A2(1A2). A progression in quanta of about 410 cm-1 is observed on 1A1 → A2(1A2) at low temperature. Additional bands are observed at 5 K and are assigned as follows: 30340 cm-1 (a1(Os≡N) progression, ∼840 cm-1), 1A1 → E(3B2); 34 710 cm-1 (a1(Os≡N) progression, ∼ 1000 cm-1), 1A1 → E(1E). The estimated Os≡N bond lengthening in E(3B2) is 0.17 Å, which is consistent with a large degree of axial σ-antibonding character in that excited state. A spin-orbit crystal field calculation of the d-orbital energy levels of C4v OsNCl4- has been performed. Satisfactory agreement with the band positions in the spectrum was obtained for the following parameter values: Δ1(b1-b2) = 26000 cm-1; Δ2(e-b2) = 36000 cm-1; Δ3(a1-b2) = 49000 cm-1; F2 = 1400 cm-1; F4 = 55 cm-1. The unusually high energies of the ai and e levels are taken to indicate that both the σ and π bonds in the Os≡N unit of OsNCl4- are very strong. The electronic spectra of crystals containing related tetrahalonitrido complexes of osmium(VI) have also been measured at 5 K. The spectral features in each case are similar to those of OsNCl4-, and analogous assignments have been made.