PENTAMETHYLCYCLOPENTADIENYLSELENIUM DERIVATIVES - SYNTHESIS AND X-RAYCRYSTAL-STRUCTURES OF [M(CO)(5)(SE-2(SIGMA-C(5)ME(5))(2))] (M=CR, W)

Bis(pentamethylcyclopentadienyl)selenium reacts with sources of M(CO)( 5)(M = Cr, W) fragments, to yield complexes of the diselenide Se-2(sig ma-C5H5)(2), which are characterised spectroscopically and by X-ray cr ystallography: these are the first examples of selenide to diselenide reduction within the coordination sphere of a transition metal, and th e first structures of complexes containing a terminal diselenide ligan d to be reported.