Iodosilane and di-iodosilane react with NaCo(CO)4 in ether at low temperatures to give silyltetracarbonylcobalt, SiH3Co(CO)4, and silylenebis(tetracarbonylcobalt), SiH2[Co(CO)4]2, respectively. Physical properties of both compounds are reported, together with i.r., mass, and n.m.r. spectra of SiH3Co(CO)4. Pyrolysis of gaseous SiH3Co(CO)4 at 120° yields H2, CO, SiH4, and a dark-coloured inert solid; some HCo(CO)4 is also present in the early stages. The stoicheiometry and probable cross-linked polymeric nature of the solid product are inferred, and its metal coating properties are outlined. The Si–Co bond in SiH3Co(CO)4 is broken by HX (X = F, Cl, O/2) to give SiH3X and HCo(CO)4, also by HgX2(X = Cl, I) to give SiH3X and Hg[Co(CO)4]2. SiH2[Co(CO)4]2 behaves similarly. No insertion of CO into the Si–Co bond has been achieved; PPh3 reacts slowly with SiH3Co(CO)4 to displace CO and probably yields trans-SiH3Co(CO)3(PPh3). Evidence for dπ–dπ interaction in the silicon–metal bond is discussed.