Treatment of [{Ru(P(OCH3)(3))(2)(CH3CN)(3)}(2)(mu-Q(2))](CF3SO3)(4) (1, Q = S; 2, Q = Se) with haloalkenes resulted in the formation of complexes carrying unsaturated C(3)Q(2) five-membered or C(4)Q(2) six-membered rings via elimination of HX (X = Cl, Br). The reactions of 1 and 2 with allyl bromide gave the corresponding addition products, [{Ru(P(OCH3)(3))(2)(CH3CN)(3)}(2)(mu-QCH=CHCH(2)Q)](CF3SO3)(4) (3, 0 = S; 4, Q = Se), via elimination of HBr. The elimination process seems to be thermodynamically controlled and takes place at the final stage of the reaction. The steric effect of the halogen atoms seems more operative than the electronic one.