Mercury(II) Selenolates. Crystal Structures of Polymeric Hg(SeMe)2 and the Tetrameric Pyridinates [{HgCl(py)(SeEt)}4] and [{HgCl(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 818,1678,2016 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) A, beta = 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) A, beta = 101.75(2), Z = 2. For [(HgCl(py)0.5(SeBut)}4] a = 12.151 (5), b = 16.738(7), c = 10.138(6) A, beta = 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) A. The complexes [{HgCl(py)(SeEt)}4] and [{HgCl(py)0.5(SeBut)}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(mu-SeEt)2Cl(py)'; the latter contains two inversion related mercury atoms, 'Hg(mu-SeBut)2Cl(py)', and two other inversion related mercury atoms linked by a dichloro-bridge 'Hg(mu-SeBut)2(mu-Cl)2'. The structure of [{HgCl(py)0.5(SeBut)}4] is very similar to those of [{HgCI(L)0.5(SBut)}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.