The first cyclopentadienylselenium derivative, Se(C 5 Me 5 ) 2 (1), has been prepared by the reaction of LiC 5 Me 5 with selenium bis(diethyldithiocarbamate). Steric crowding about the selenium atom in 1 results in long Se-C bonds and an expanded C-Se-C angle, giving rise to unprecedented reactivity. Treatment of 1 with sources of M(CO) 5 fragments yields the terminal diselenide complexes [M(CO) 5 {Se 2 (C 5 Me 5 ) 2 }] (M ) Cr, W) via reduction within the transition-metal coordination sphere. Reaction of 1 with [Fe 2 (CO) 9 ] leads to loss of both C 5 Me 5 groups and formation of [Fe 3 Se 2 (CO) 9 ].

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Bis(pentamethylcyclopentadienyl)selenium reacts with sources of M(C0)5 (M = Cr, W) fragments, to yield complexes of the diselenide Se2(a-CSH&, which are characterised spectroscopically and by X-ray crystallography: these are the first examples of selenide t o diselenide reduction within the coordination sphere of a transition metal, and the first structures of complexes containing a terminal diselenide ligand t o be reported.

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