Metallocene derivatives of early transition metals. Part 4. Chemistry of the complexes [M(η-C5H5)2RR′][M = Ti, Zr, or Hf; R = CH2M′Me3(M′= C, Si, Ge, or Sn) or CH(SiMe3)2; R′= Cl or alkyl] and the X-ray structures of [Zr(η-C5H5)2(CH2M′Me3)2](M′= C or Si)