Subvalent Group 4B metal alkyls and amides. Part 9. Germanium and tin alkene analogues, the dimetallenes M2R4[M = Ge or Sn, R = CH(SiMe3)2]: X-ray structures, molecular orbital calculations for M2H4, and trends in the series M2R′4[M = C, Si, Ge, or Sn; R′= R, Ph, C6H2Me3-2,4,6, or C6H3Et2-2,6]
X-Ray structures of the isomorphous centrosymmetric crystalline Ge2R4 and Sn2R4[R = CH(SiMe3)2] reveal a trans-folded C2h M2C4 framework, with a fold angle θ of 32° for M = Ge and 41° for M = Sn, but no twist of the MC2 planes about the M–M axis. The M–M distance [2.347(2)Å for Ge and 2.768(1)Å for Sn] is slightly shorter (4% for M = Ge, 1.5% for M = Sn) than in the tetrahedral element, M∞. The conformation of each MR2 moiety in M2R4 approximates to planar syn,anti(cf. Ca. syn,syn in gaseous MR2), and the four ligands R– are oriented in a ‘paddle-wheel’ fashion. There is consequently an asymmetry in the M–C bonding in M2R4 as shown (data for M = Sn in square brackets) by variations in M–C and M–C′(Å), 1.979(9) and 2.042(8)[2.207(5) and 2.225(6)]; M′MC and M′MC′(°),113.7(3) and 122.3(2)[112.0(1) and 119.4(1)°]; individual MCSi angles (°), 110.0(4)[110.2(3)], 113.9(4)[109.3(3)], 119.1(4)[119.1(2)], and 121.8(4)[118.9(2)]. The average M–C bond lengths are comparable but the CMC angles are wider in M2R4 than those previously found for gaseous MR2. Ab initio molecular orbital calculations with better than double zeta basis on the model compounds M2H4 show that (i)trans-folded equilibrium structures are more stable than planar by 13 kJ mol–1(θ= 40°) for M = Ge and 26 kJ mol–1(θ= 46°) for Sn; (ii) the M–M bond distance is 2.30 Å for Ge and 2.71 Å for Sn; and (iii) the M–M dissociation energy is 130 kJ mol–1 for Ge and 90 kJ mol–1 for Sn. These energies are about half the experimental M–M single-bond dissociation energies of H3GeGeH3 or Me3MMMe3. The decreasing strength of M–M bonding in the series M2R′4[M = C, Si, Ge, or Sn; R′= R, Ph, C6H2Me3-2,4,6, or C6H3Et2-2,6] or, more generally, M2X4(X = R′ or H) with increasing atomic number of M, as well as the increasing stability of the trans-folded relative to planar structures, is attributed to the increasing inertness of the electron lone pair in the MX2(X = R′ or H) monomer, which in turn is reflected in an increasing singles → triplet excitation energy.
- This article is part of the themed collection: A collection of papers in memory of Professor Michael Lappert