Vibrational spectroscopic studies of tetra-n-butylammonium trihalogenomercurates; crystal structures of [NBun4](HgCl3) and [NBun4]-(HgI3)

Abstract

The crystal structures of the title compounds have been determined. Crystals of [NBuⁿ₄](HgCl₃) are triclinic, space group P(no. 2) with a= 11.011 (3), b= 11.267(3), c= 19.341(5)Å, α= 87.33(2), β= 102.70(2), γ= 96.45(2)°, and Z= 4. The final R value was 0.034 for 2 187 observations. The structure is the first example established where (HgCl₃)^– is solely in the form of dimeric [Hg₂Cl₆]^2– doubly chloride-bridged anions with distorted tetrahedral geometry about the mercury atoms; these anions are not centrosymmetric and although the terminal Hg–Cl bond lengths are not significantly different from one another (2.38 ± 0.01 Å) the bridge is asymmetric with bond lengths 2.66₅± 0.01 and 2.57 ± 0.02 Å. Crystals of [NBuⁿ₄](HgI₃) are tetragonal, space group I2d(no. 122) with a= 13.790(5), c = 26.537(5)Å, and Z = 8. The final R value was 0.059 for 1 189 independent intensities measured at 200 K. The structure consists of discrete [NBuⁿ₄]⁺ cations and [HgI₃]^– anions. One Hg–I bond lies on a crystallographic two-fold axis and is of length 2.683(2)Å; the other two bonds are not significantly different [2.688(2)Å] but make an interbond angle of 115° giving the anion C_2v and not D_3h symmetry. The closest approach between atoms of different anions is ca. 7.4 Å precluding any significant polymeric interaction and making this the first example containing a truly monomeric trihalogenomercurate(II) anion. The [NBuⁿ₄]⁺ groups show the expected regular tetrahedral arrangement of bonds around the N atom, but each of the n-butyl chains has the C atoms approximately co-planar, and the ion as a whole is crystallographically constrained to C₂ symmetry. Vibrational spectra are rationalised in the light of the crystal structures and indicate that the anion of [NBuⁿ₄](HgBr₃) is dimeric [Hg₂Br₆]^2–. The spectral detail is greatly enhanced in measurements at low temperature (ca. 100 K), especially for the dimeric anions. The structural variety shown by trihalogenomercurates is discussed.