Why do the heavy-atom analogues of acetylene E₂H₂ (E = Si-Pb) exhibit unusual structures?

DFT calculations at BP86/QZ4P have been carried out for different structures of E₂H₂ (E = C, Si, Ge, Sn, Pb) with the goal to explain the unusual equilibrium geometries of the heavier group 14 homologues where E = Si-Pb. The global energy minima of the latter molecules have a nonplanar doubly bridged structure A followed by the singly bridged planar form B, the vinylidene-type structure C, and the trans-bent isomer D1. The energetically high-lying trans-bent structure D2 possessing an electron sextet at E and the linear form HE≡EH, which are not minima on the PES, have also been studied. The unusual structures of E₂H₂ (E = Si-Pb) are explained with the interactions between the EH moieties in the (X ²Π) electronic ground state which differ from C₂H ₂, which is bound through interactions between CH in the a ⁴Σ^- excited state. Bonding between two (X ²Π) fragments of the heavier EH hydrides is favored over the bonding in the a⁴Σ^- excited state because the X ²Π → a⁴Σ^- excitation energy of EH (E = Si-Pb) is significantly higher than for CH. The doubly bridged structure A of E²H² has three bonding orbital contributions: one σ bond and two E-H donor-acceptor bonds. The singly bridged isomer B also has three bonding orbital contributions: one π bond, one E-H donor-acceptor bond, and one lone-pair donor-acceptor bond. The trans-bent form D1 has one π bond and two lone-pair donor-acceptor bonds, while D2 has only one σ bond. The strength of the stabilizing orbital contributions has been estimated with an energy decomposition analysis, which also gives the bonding contributions of the quasi-classical electrostatic interactions.