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 (X2Π) electronic ground state which differ from C2H2, which is bound through interactions between CH in the a4Σ- excited state. Bonding between two (X2Π) fragments of the heavier EH hydrides is favored over the bonding in the a4Σ- excited state because the X2Π → a4Σ- excitation energy of EH (E = Si−Pb) is signific...