Silylpentacarbonylmanganese, SiH3Mn(CO)5, has been prepared from NaMn(CO)5 and iodosilane. Its physical properties, including u.v., mass, n.m.r., i.r., and Raman spectra are reported and discussed. It decomposes on heating above about 115° to give H2, CO, SiH4, and HMn(CO)5 as chief volatile products, together with a dark solid that probably has a cross-linked skeleton of Si–Mn bonds. The Si–Mn bond in SiH3Mn(CO)5 is essentially unaffected by HCl and by HgX2(X = Cl or I) and is cleaved slowly by water. On heating, HCl gives the chlorosilyl derivatives SiH3–xClxMn(CO)5(X= 1–3). Attempts at insertion reactions with CO or SO2 were unsuccessful.