THE FIRST CP ASTERISK OS HALF-SANDWICH COMPLEXES CONTAINING CHALCOGENIDO LIGANDS

The reactions of the pentamethylcyclopentadienyl-osmium compound CpOs (NO)Br-2 (1) with Na2S4 and (NEt(4))(2)Se-6 in dimethylformamide solut ion lead to mononuclear tetrachalcogenido complexes CpOs(NO)(E(4)) (E = S (2a), Se (2b)), whereas binuclear products Cp(2)Os-2(NO)(2)(mu-E )(2) (E = S (3a), Se (3b), Te (3c)) are obtained from the reactions of 1 with Li(2)E in tetrahydrofuran solution. Dechalcogenation of 2a,b u sing tri(n-butyl)phosphane gives 3a,b. The molecular geometry of CpOs (NO)(Se-4) (2b) has been determined by an X-ray crystal structure anal ysis which confirms the presence of a non-planar cyclotetraselenido ch elate ligand.