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, Sri, 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 equivalent to 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-2 Pi) electronic ground state which differ from C2H2, which is bound through interactions between CH in the a(4)Sigma(-) excited state. Bonding between two (X-2 Pi) fragments of the heavier EH hydrides is favored over the bonding in the a a(4)Sigma(-) excited state because the X-2 Pi -> a(4)Sigma(-) 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 a bond and two E-H donor-acceptor bonds. The singly bridged isomer B also has three bonding orbital contributions: one pi bond, one E-H donor-acceptor bond, and one lone-pair donor-acceptor bond. The trans-bent form D1 has one pi bond and two lone-pair donor-acceptor bonds, while D2 has only one a 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.