Group 14 structural variations: Perhalo derivatives of the "dimetallenes": Dicarbenes, disilenes, digermenes, distannenes, and diplumbenes

Trends in the structural variations of all perhalo derivatives of dicarbenes and their Group 14 analogues have been studied. This included all M₂X₄ molecules, where M = C, Si, Ge, Sn, or Pb, and X = F, Cl, Br, or I. Mapping the potential energy surface of all molecules has uncovered several isomers. The stability of these isomers depends on both the Group 14 atoms and the halogen ligands. Several isomers were found stable; the ones that are global minima include (with their symmetries and an example in parenthesis): the typical ethene structure X₂M=MX₂ (D _2h, F₂C=CF₂), an X₃M-MX structure (C _S, F₃Si-SiF, a trifluorosilyl-silylene), another X ₃M-MX structure (C ₁, Cl₃Si-SiCl), one more X₃M-MX structure with a single halogen bridge (C ₁, I ₂Si-μI-SiI), a trans double halogen bridged structure (D _2h, FSn-μF₂-SnF), and another trans double-bridged structure with puckered ring (C _S, IPb-μI₂-PbI). Some of the other structures that are stable but are not the global minima include: a trans-bent structure X₂M-MX₂ (C _2h, all X ₂Si-SiX₂), cis double-bridged structure (C _2v with planar ring, FPb-μF₂-PbF, or with puckered ring, C _2v, IGe-μI₂-GeI), and even a square bipyramidal structure (D _4h, Sn-μF₄-Sn). The energy differences between some of the structures are small and the application of another computational method and using a different basis set might alter their relative stabilities. Reasons for the difference in the stabilities of isomers have been discussed.