Structural and spectroscopic characterization of complexes containing the mononuclear trihalogenometalates [CuI3]2–, [CuBr3]2–, and [AgI3]2–. Crystal structure of [PMePh3]2[CuI3]

Abstract

The new compounds [PMePh₃]₂[CuI₃], [AsMePh₃]₂[CuI₃], [PMePh₃]₂[CuBr₃], and [PMePh₃]₂[AgI₃] have been prepared. Unlike previously reported compounds of stoicheiometry [cation]₂[MX₃](M = Cu^I or Ag^I) which contain infinite polymeric anions with bridging halide ions and tetrahedrally co-ordinated metal atoms, the present compounds contain mononuclear [MX₃]^2– ions with essentially trigonal planar co-ordination of the metal atoms. This has been verified in the case of [PMePh₃]₂[CuI₃] by conventional X-ray diffraction techniques. Crystals are Monoclinic of space group P2₁ with a= 9.580(1), b= 12.476(2), c= 16.410(1), and Z= 2. The [CuI₃]^2– anion in this compound is not perfectly symmetrical, as the Cu atom is displaced by 0.015(1)Å from the plane of the three iodine atoms and the Cu–I bond lengths show significant differences. The far-i.r. spectra of these compounds show a strong band at 163, [CuI₃]^2–, 133, [AgI₃]^2–, or 190 cm^–1, [CuBr₃]^2–, which can be assigned to the antisymmetric metal–halogen stretching mode of the [MX₃]^2– units. Approximate normal co-ordinate calculations yield metal-halogen force constants for [MX₃]^2– which, as expected, are lower than those for the corresponding [MX₂]^– species. ¹²⁷I nuclear quadrupole resonance measurements on [PMePh₃]₂[MI₃](M = Cu^I or Ag^I) show the presence of three crystallographically inequivalent iodine atoms. The ⁶³Cu and ⁶⁵Cu n.q.r. frequencies in the copper complex are almost identical to those found previously in other complexes containing copper(I) bound to three equivalent ligands in a trigonal planar environment. [PMePh₃]₂[CuBr₃] can be prepared in a second isomeric form. This is shown by means of vibrational spectroscopy, and copper and bromine n.q.r. spectroscopy, to contain non-interacting [CuBr₂]^– and Br^– ions, and is therefore formulated as [PMePh₃]₂[CuBr₂]Br. Attempts to prepare complexes containing [MX₃]^2– ions with cations such as [NEt₄]⁺, [NPr₄]⁺, [AsPh₄]⁺, and [N(PPh₃)₂]⁺ resulted either in the formation of complexes of different stoicheiometry or compounds of the desired stoicheiometry which, however, do not appear to contain mononuclear, planar [MX₃]^2– species.