An alternative approach was reported by Romanov et al. who prepared linear Cu I complexes stabilized by strongly binding cyclic (alkyl)(amino)carbene (CAAC) ligands trans to halide, pseudohalide, aryloxide, or amide ligands. With a linear geometry, the ligand field is strongest along the Cu-ligand axis, and the HOMO in the Cu I 3d 10 system is the dz2 orbital. Therefore, after photoexcitation there is no longer a thermodynamic driving force for structural distortion since the HOMO of the electronic ground state, and hence the singly occupied molecular orbital (SOMO) of the photoexcited Franck-Condon state, is non-degenerate.