MERCURY(II) SELENOLATES - CRYSTAL-STRUCTURES OF POLYMERIC HG(SEME)2 AND THE TETRAMERIC PYRIDINATES [(HGCI(PY)(SEET))4] AND [(HGCI(PY)0.5(SEBUT))4]

The crystal structures of the title compounds have been determined by single-crystal X-ray diffraction at 295 K and refined by least squares to residuals of 0.044, 0.046, 0.058 for 818, 1 678, 2 016 independent 'observed' reflections respectively. For all compounds, crystals are monoclinic, space group P21/c. For Hg(SeMe)2, a = 8.440(4), b = 10.732(3), c = 6.681(3) Å, β = 96.14(4)°, and Z = 4. For [{HgCl(py)(SeEt)}4] (py = pyridine), a = 8.044(5), b = 17.387(14), c = 15.585(21) Å, β = 101.75(2)°, and Z = 2. For [{HgCl(py)0.5(SeBu^t)}4] a = 12.151(5), b = 16.738(7), c = 10.138(6) Å, β = 90.93(4)°, and Z = 2. Crystals of Hg(SeMe)2 contain infinite one-dimensional chains along b, the pseudo-tetrahedral mercury atoms being bridged by pairs of selenium atoms; Hg-Se bond distances are in the range 2.614(2)-2.764(2) Å. The complexes [{HgCl(py)(SeEt)}4] and [{HgCl(py)0.5(SeBu^t)}4] were obtained from pyridine solutions containing Hg(SeR)2 and HgCl2, and both contain an eight-membered ring (-Hg-SeR-)4. The former contains two independent pseudo-tetrahedrally co-ordinated mercury atoms, 'Hg(μ-SeEt)2Cl(py)'; the latter contains two inversion related mercury atoms, 'Hg(μ-SeBu^t)2Cl(py)', and two other inversion related mercury atoms linked by a dichloro-bridge 'Hg(μ-SeBu^t)2(μ-Cl)2'. The structure of [{HgCl(py)0.5(SeBu^t)}4] is very similar to those of [{HgCl(L)0.5(SBu^t)}4] (L = py or 4-methylpyridine) and is isomorphous with the 4-methylpyridine analogue. Mercury-selenium bond distances are slightly shorter than expected by comparison with covalent radii of sulphur and selenium.