Condensation of chalcogenide vapours (S, Se, Te) onto a THF matrix containing the species (C5R5)2M2(CO)4(MM) (R = H, CH3; M = Cr, Mo, W) in a metal vapour apparatus (temperature of liquid nitrogen, 10-4 bar) upon warming (S 10, Se 15, Te < 25°C), gave the compounds (C5(CH3)5W2(CO)4E3 (E = S (2), Se (13)) and (C5(CH3)5)2W2(CO)4E2 (Se, (13a), Te (17)). The species, which are primary adducts of the chalcogenides to the WW bond, could not be synthesized by the "thermic" reactions reported in the literature. The species 2 and 13 are readily converted into the chalcogenide complexes such as (C5(CH3)5)2Mo2O2Se (16) liberating CO. 17 was protonated to give the complex salt [(C5(CH3)5)2W2(CO)4(Te2H2)]2+ [BF4]2- (18) by HBF4. The compounds were characterized by elemental analysis and their spectroscopic data, X-ray diffraction studies of 13a, 16 and 18 are described. © 1988.