Electronic Structures and Spectra of Nitrido Complexes of Osmium(VI)

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 C_4v, 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 C_4v 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.