The carbosilane dendrimers Si[CH(2)CH(2)SiMe(2)C=CH](4), 1, Si[CH(2)CH(2)SiMe(C=CH)(2)](4), 2, and Si[CH2CH2Si(CH(2)CH(2)SiMe(2)C=CH)(3)](4), 5, have been prepared. Reaction of Co-2(CO)(8) with all the C=CH groups in 1 and 5 gave the corresponding acetylenedicobalt hexacarbonyl complexes. The structure of Si[CH(2)CH(2)SiMe(2)-C2HCo2(CO)(6)](4) was determined in an X-ray diffraction study.