. . . "where M is Si, C, Ge or Sn, preferably Si or C, and most preferably C, X' is a halogen, preferably Cl or Br, and most preferably Cl, n is 0, 1, 2 or 3, and p is the valence of M. Suitable halogenating agents are methylene chloride, chloroform, carbon tetrachloride, dichlorosilane, trichlorosilane and silicon tetrachloride." . .