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

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 D3h symmetry and a correlation between the values of mercury-chlorine stretching frequencies and the related bond distances in HgCl2 units is also discussed.