Matrix-isolation experiments give evidence that broad-band UV-vis irradiation (200 less than or equal to lambda less than or equal to 800 nm) of an Ar matrix doped with SiH4 and a group 12 metal atom M (M = Zn, Cd, or Hg) induces metal insertion into an Si-H bond to give the silyl metal hydride molecule HMSiH3 as the primary product. Si2H6 is a second product, irrespective of the identity of M, while the binary hydride MH2 is also formed when M = Zn or Cd. The products have been identified by their IR spectra and experiments with SiD4, together with the results of quantum chemical calculations, have provided the means of authentication. The properties of the HMSiH3 molecules are compared with those of related species, and consideration is given to how the products come to be formed.