Why is Si2H2 Not Linear? An Intrinsic Quasi-Atomic Bonding Analysis.

The molecular energy of Si2H2 geometric structures increases in the order dibridged < trans-bent < linear, in contrast to acetylene, C2H2, for which the linear structure is the global minimum. In this study, the intra-atomic (antibonding) and bonding contributions to the total molecular energy of these valence isoelectronic molecules are computed by expressing the density matrices of the full valence space multiconfiguration self-consistent field wave function in terms of quasi-atomic orbitals. The analysis shows that the intra-atomic contributions to the molecular energy become less favorable in the order dibridged → trans-bent → linear for both C2H2 and Si2H2. By contrast, the inter-atomic bonding contributions become energetically more favorable in that order for both C2H2 and Si2H2. The two systems differ as follows. For Si2H2, the antibonding intra-atomic energy changes that occur when the dibridged molecule reconstructs into the trans-bent and linear structures prevail over the interatomic interactions that induce bond formation. In contrast, for C2H2, the interatomic interactions that create bonds prevail over the intra-atomic energy changes that occur when the dibridged molecule reconstructs into the trans-bent and linear structures. The intra-atomic energy changes that occur in these systems are related to the hybridization of the heavy atoms in an analogous manner to the hybridization of C in CH4 from (2s)2(2p)2 to sp3 hybrid orbitals.