Tetragermacyclobutadiene: energetically disfavored with respect to its structural isomers

Germanium has been a central feature in the renaissance of main-group inorganic chemistry. Herein, we present the stationary-point geometries of tetragermacyclobutadiene and its related isomers on the singlet potential energy surface at the CCSD(T)/cc-pVTZ level of theory. Three of these 12 structures are reported for the first time and one of them is predicted to lie only 0.4 kcal mol(-1) above the previously reported global minimum. Focal-point analyses has provided electronic energies at the CCSD(T) level of theory, which are extrapolated to the complete basis-set limit and demonstrate the convergence behavior of the electronic energies with improving levels of theory and increasing basis-set size. The lowest-energy structure is the bicyclic structure, which lies 35 kcal mol(-1) below the "all-Ge" cyclobutadiene structure. The reaction energies for the association of known Ge hydrides (e.g., digermene) to form Ge4H4 indicate that Ge4H4 could be observed experimentally. We investigate the bonding patterns by examining the frontier molecular orbitals. Our results demonstrate that: 1) the cyclic isomers of (GeH)4 distort to maximize the mixing of the p orbitals that are involved in the π system of tetragermacyclobutadiene and 2) the lowest-energy isomers exhibit unusual bonding arrangements (e.g., bridging H bonds) that maximize the nonbonding electron density at the Ge centers.