The multiple bonding in heavier group 14 element alkene analogues is stabilized mainly by dispersion force effects

The structures and bonding in the heavier group 14 element olefin analogues [E{CH(SiMe₃)₂}₂]₂ and [E{N(SiMe₃)₂}₂]₂ (E = Ge, Sn, or Pb) and their dissociation into :E{CH(SiMe₃)₂}₂ and :E{N(SiMe₃)₂}₂ monomers were studied computationally using hybrid density functional theory (DFT) at the B3PW91 with basis set superposition error and zero point energy corrections. The structures were reoptimized with the dispersion-corrected B3PW91-D3 method to yield dispersion force effects. The calculations generally reproduced the experimental structural data for the tetraalkyls with a few angular exceptions. For the alkyls, without the dispersion corrections, dissociation energies of -2.3 (Ge), +2.1 (Sn), and -0.6 (Pb) kcal mol^-1 were calculated, indicating that the dimeric E-E bonded structure is favored only for tin. However, when dispersion force effects are included, much higher dissociation energies of 28.7 (Ge), 26.3 (Sn), and 15.2 (Pb) kcal mol^-1 were calculated, indicating that all three E-E bonded dimers are favored. Calculated thermodynamic data at 25 °C and 1 atm for the dissociation of the alkyls yield ΔG values of 9.4 (Ge), 7.1 (Sn), and -1.7 (Pb) kcal mol^-1, indicating that the dimers of Ge and Sn, but not Pb, are favored. These results are in harmony with experimental data. The dissociation energies for the putative isoelectronic tetraamido-substituted dimers [E{N(SiMe₃)₂}₂]₂ without dispersion correction are -7.0 (Ge), -7.4 (Sn), and -4.8 (Pb) kcal mol^-1, showing that the monomers are favored in all cases. Inclusion of the dispersion correction yields the values 3.6 (Ge), 11.7 (Sn), and 11.8 (Pb) kcal mol^-1, showing that dimerization is favored but less strongly so than in the alkyls. The calculated thermodynamic data for the amido germanium, tin, and lead dissociation yield ΔG values of -12.2, -3.7, and -3.6 kcal mol^-1 at 25 °C and 1 atm, consistent with the observation of monomeric structures. Overall, these data indicate that, in these sterically-encumbered molecules, dispersion force attraction between the ligands is of greater importance than group 14 element-element bonding, and is mainly responsible for the dimerization of the metallanediyls species to give the dimetallenes. In addition, calculations on the non-dissociating distannene [Sn{SiMe Bu₂}₂]₂ show that the attractive dispersion forces are key to its stability.