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
- 1 January 1986
- journal article
- research article
- Published by Royal Society of Chemistry (RSC) in J. Chem. Soc., Dalton Trans.
- No. 8,p. 1551-1556
- https://doi.org/10.1039/dt9860001551
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 SnHBun 3, 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 MII–H and MII–C bond distances are significantly larger (by 4–10 pm) than their MIV–H and MIV–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.Keywords
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