Bis[bis(trimethylsilyl)methyl]germanium, GeR, [ R = CH (SiMe,),], is conveniently prepared from GeCl,(diox) (diox = 1,4-dioxane) (for which an improved synthesis, from GeCI, and SnHBu",, is reported) and 2MgCI( R)(OEt,), MgR,(diox),.,, or MgR,(OEt,) in OEt,. The corresponding tin(ii) alkyl is accessible from SnCI, and successively 2LiR (to yield SnCI,R,) and Li,(cot) (cot = cyclooctatetraene) in OEt,. Gas-phase electron diffraction (g.e.d.) patterns, recorded with nozzle temperatures of 155 "C for GeR, and 1 2 0 "C for SnR,, show that the gas consists of V-shaped monomers. Least-squares refinements of models of C, symmetry yielded the bond distances Ge-C 203.8(15) and Sn-C 222(2) p m and the angles CGeC 107(2) and CSnC 97(2)". In GeR, the -CH(SiMe,), ligands are oriented so that the HCIMCIH moiety (Cl = inner, or methine, C) has a nearly planar syn,syn conformation but in SnR, the dihedral angles C'MC'H are ca. 15". Ab initio molecular-orbital calculations with a better than DZ (double zeta) basis were carried out on the model compounds MH, and GeMe,, 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'l-H and M"-C bond distances are significantly larger (by 4-1 0 pm) than their MIV-H and MIV-C counterparts, and valence angles a t M increase in the series GeH, = SnH, < GeMe, = SnMe, < GeR, < SnR,. Both the long bond distances and the less-thantetrahedral 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 o-bonding orbitals to be formed from metal atomic orbitals of predominant p character.
