Subvalent Group 4B metal alkyls and amides. Part 8. Germanium and tin carbene analogues MR2[M = Ge or Sn, R = CH(SiMe3)2]: syntheses and structures in the gas phase (electron diffraction); molecular-orbital calculations for MH2 and GeMe2

Abstract

Bis[bis(trimethylsilyl)methyl]germanium, GeR₂[R = CH(SiMe₃)₂], is conveniently prepared from GeCl₂(diox)(diox = 1,4-dioxane)(for which an improved synthesis, from GeCl₄ and SnHBuⁿ₃, is reported) and 2MgCl(R)(OEt₂), MgR₂(diox)_0.5, or MgR₂(OEt₂) in OEt₂. The corresponding tin(II) alkyl is accessible from SnCl₄ and successively 2LiR (to yield SnCl₂R₂) and Li₂(cot)(cot = cyclooctatetraene) in OEt₂. Gas-phase electron diffraction (g.e.d.) patterns, recorded with nozzle temperatures of 155 °C for GeR₂ and 120 °C for SnR₂, show that the gas consists of V-shaped monomers. Least-squares refinements of models of C₂ symmetry yielded the bond distances Ge–C 203.8(15) and Sn–C 222(2) pm and the angles CGeC 107(2) and CSnC 97(2)°. In GeR₂ the –CH(SiMe₃)₂ ligands are oriented so that the HCⁱMCⁱH moiety (Cⁱ= inner, or methine, C) has a nearly planar syn,syn conformation but in SnR₂ the dihedral angles CⁱMCⁱH are ca. 15°. Ab initio molecular-orbital calculations with a better than DZ (double zeta) basis were carried out on the model compounds MH₂ and GeMe₂, and yielded the optimal bond distances Ge–H 158, Ge–C 202, and Sn–H 177 pm, and valence angles HGeH 93, CGeC 97, and HSnH 93°. Correlation of experimental and calculated structures shows that M^II–H and M^II–C bond distances are significantly larger (by 4–10 pm) than their M^IV–H and M^IV–C counterparts, and valence angles at M increase in the series GeH₂= SnH₂ < GeMe₂= SnMe₂ < GeR₂ < SnR₂. Both the long bond distances and the less-than-tetrahedral valence angles are rationalised by assuming the metal lone pair to occupy atomic orbitals of predominant s character and the M–H and M–C σ-bonding orbitals to be formed from metal atomic orbitals of predominant p character.