Osmium(III) Halide Complexes with para-and meta-Substituted Benzeneseleninato Ligands

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New complexes of osmium(III) with para - and meta-substituted benzeneseleninic acids of the type XC6H4SeO2H (X = H, p- Cl , m- Cl , p-Br, m-Br, p-Me) are reported. The compounds, of the type Os(XC6H4SeO2)3, Os(XC6H4SeO2)2Y, Os(XC6H4SeO2)Y2 and Os2(XC6H4SeO2)3Y3 (Y = Cl , Br), have been studied through spectroscopic techniques ( i.r ., far- i.r .and electronic spectra), magnetic susceptibility measurements, thermogravimetric studies and conductivity measurements. The wavelengths of the principal electronic absorption peaks have been accounted for quantitatively in terms of the crystal-field theory; the nephelauxetic parameter is indicative of an appreciable metal- ligand covalency. It is worth noting that among the present complexes the highest q values are related to the Os(XC6H4SeO2)3 derivatives in which OsO6 chromophores are present; the Dq values decrease on passing to the 1 : 2, 1 : 1.5 and 1 : 1 metal/ ligand molar ratio complexes according to the presence of chlorine- and bromine-containing chromophores . The i.r . data point to a seleninato -O,O′ coordination for all the complexes; in particular, the presence of three SeO bands with the irreducible representation A2 + 2E in the i.r . spectra of the tris derivatives suggests an octahedral configuration with D3 symmetry. All the halo complexes are polymeric octahedral with bridging halide atoms. The magnetic moment values lie in the expected range for the trisbenzeneseleninato derivatives, and they decrease on passing to the halo complexes.