Crystal and molecular structure of [H3N(CH2)2NH2(CH2)2NH3]2HgCl8: a compound containing a discrete axially-compressed trigonal-bipyramidal pentachloromercurate anion

Abstract

The compound [NH₃(CH₂)₂NH₂(CH₂)₂NH₃]₂HgCl₈ has been prepared and investigated by an X-ray structural analysis and vibrational spectroscopy. The compound crystallizes with two formula units in space group P2/n of the monoclinic system in a cell of dimensions a= 11.530(2), b= 6.638(1), c= 15.028(3)Å, β= 93.00(2)°, and Z= 2. The crystal structure was determined by threedimensional X-ray diffraction and refined to R= 0.048, R′= 0.051. The structure consists of diethylenetriammonium cations, chloride anions and discrete [HgCl₅]^3– anions. Co-ordination around the metal, which lies on the two-fold axis, can be described as a compressed trigonal bipyramid, where the equatorial positions are occupied by three weakly bonded chlorine atoms, two of them related by the two-fold axis [Hg–Cl(2) 3.029(3)Å]. The apices of the bipyramid are occupied by two shortly bonded chlorine atoms [Hg–Cl(1) 2.327(4)Å]. This is an example of a trigonal-bipyramidal d¹⁰ metal ion in which the axial bonds are significantly shorter than the equatorial bonds. Therefore it may be considered to consist of a linear HgCl₂ molecule, with a bond angle [Cl(1)–Hg–Cl(1′) 178.8(1)°] perturbed by the interaction of three weakly bonded chlorine atoms. Packing is determined by a network of hydrogen bonds involving the diethylenetriammonium cation and co-ordinated and unco-ordinated chlorine atoms. The vibrational spectra of the [HgCl₅]^3– anion are rationalized in terms of D_3h symmetry and a correlation between the values of mercury–chlorine stretching frequencies and the related bond distances in HgCl₂ units is also discussed.