page_title: Digermyne
section_title: Molecular orbital (MO) treatment
hierarchical_section_title: Digermyne / Bonding / Molecular orbital (MO) treatment
caption_attribution_description: English: Second order Jahn-Teller effect-digermyne
context_page_description: Digermynes are a class of compounds that are regarded as the heavier digermanium analogues of alkynes. The parent member of this entire class is HGeGeH, which has only been characterized computationally, but has revealed key features of the whole class. Because of the large interatomic repulsion between two Ge atoms, only kinetically stabilized digermyne molecules can be synthesized and characterized by utilizing bulky protecting groups and appropriate synthetic methods, for example, reductive coupling of germanium(II) halides.
  The bonding between two Ge atoms in digermyne is different from C≡C bond in alkynes, which results in the trans-bent structure of digermyne. Trans-bent structure is quite common in heavier Group 14 element analogues of alkynes. The second order Jahn-Teller effect of digermynes gives rise to slipped π-bond and large molecular geometrical distortion.
context_section_description: In a molecular orbital (MO) description, the geometrical distortion (trans-bent structure) of digermynes is the consequence of the second order Jahn-Teller (SOJT) effect, which is the symmetry allowed interaction between filled bonding MO (generally the HOMO in digermynes) and empty nonbonding or antibonding MOs (usually the latter one) that are close in energy and can lead to large molecular distortion. If constraining the digermyne molecule in D∞h point group, two Ge atoms will form one low-lying σ-bonding orbital, two degenerate π-bonding orbitals and π-antibonding orbitals, and one high-lying σ-antibonding orbital, which are the same as alkynes. SOJT mixing of in-plain π-bonding orbital (πₓ, HOMO) and σ-antibonding orbital, which have the same bᵤ symmetry in the trans-bent C₂ₕ point group, give rise to a slipped π-bond with significant non-bonded electron lone pair character which is lower in energy, as well as a σ-antibonding orbital with higher energy. This second order mixing of MOs leads to the molecular distortion in geometry from linear D∞h to trans-bent C₂ₕ. The mixing of σ-bonding orbital and in-plain π-antibonding orbital (πₓ*, LUMO) is also symmetrically (both are in bg symmetry) and energetically allowed. Noticing that large SOJT effect occurs between two orbitals with energy difference of 2 eV or even larger, for example, 4 eV, the extent of mixing of orbitals is neglectable in alkynes, but is maximized in heavier elements, like Ge atoms in digermynes.
caption: Second order Jahn-Teller effect of the bonding between two Ge atoms in digermynes leads to the geometrical distortion of digermynes.
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