The complexes MH(eta(2)-H2BH2)(CO)(PPr3i)(2), M(C(2)Ph)(2)(CO)(PPr3i)(2) and M(eta(2)-C(2)Ph(2))(CO)(PPr3i)(2)(M=Ru, Os) are found to be very active and highly selective catalysts for the reaction of PhC=CH with the stoichiometric amount of HSiEt(3); in the presence of the ruthenium catalysts, the reactions lead to cis-PhCH=CH(SiEt(3)) with selectivities of about 95%.