Subvalent Group 4B metal alkyls and amides. Part I. The synthesis and physical properties of kinetically stable bis[bis(trimethysilyl)methyl]-germanium(II), -tin(II), and -lead(II)

Abstract

Two methods are described for the synthesis of the unusual bivalent Group 4B metal alkyls M[CH(SiMe₃)₂]₂(M = Ge, Sn,or Pb)from Li[CH(SiMe₃)₂] indiethyl ether at 0 to –20°C and (a) the metal(II) chloride (M = Sn or Pb) or (b) M[N(SiMe₃)₂]₂(M = Ge or Sn). At ambient temperature in cyclohexane or benzene the solutions are yellow (Ge), red (Sn), or purple (Pb), and the compounds are monomeric and in a singlet electronic ground state. There are colour changes between the solid and the melt, and the compounds tend to become colourless at –196 °C. The crystal structure of the tin(II) alkyl shows a centrosymmetric dimer with a Sn–Sn bond (2.76 Å) similar in length to that in Sn₂Ph₆, and the two pairs of geminal alkyl groups in a mutually trans arrangement. The solid germanium compound is inferred to be structurally similar because of the presence of a strong polarised Raman line at 300 cm^–1. The monomer is believed to be angular with three approximately sp² hybridised orbitals at the metal, one of which is non-bonding; the dimer. with a SnSn bent double bond, is formed by overlap of the non-bonding orbital of each monomer unit with the orthogonal vacant p_z orbital of the other.