Why UO22+ is linear and isoelectronic ThO2 is bent

The isoelectronic species UO22+ and ThO2 possess very different geometries, namely, UO22+ is linear while ThO2 is strongly bent (theta/sub exptl/ = 122 +- 20). Relativistic effective core potential (RECP) calculations using Hartree-Fock wave functions and double-zeta-plus-polarization quality basis sets were performed to determine the origin of this difference. The RECP calculations correctly predict the linear and bent geometries of UO22+ and ThO2 (theta/calcd/ = 1180). The Th-O bond length, which is not known experimentally, is calculated to be 1.91 A. Analysis of the results shows that the difference in geometries for UO22+ and ThO2 has its origin in the relative ordering of the 5f and 6d levels. For uranium the 5f levels are lower and dominathe te back-bonding from the oxygen in UO22+, while for thorium the 6d levels are lower and dominate the back-bonding in ThO2. Finally, the 5f levels prefer linear geometries, while the 6d prefer bent geometries, hence, the difference between UO22+ and ThO2. The relative ordering of the 5f and 6d levels has a profound effect