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]

Abstract

X-Ray structures of the isomorphous centrosymmetric crystalline Ge₂R₄ and Sn₂R₄[R = CH(SiMe₃)₂] reveal a trans-folded C_2h 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.