The interconversion of model compounds {[(NH3)(3)Cu](2)(mu-eta(2):eta(2)-O-2)}(2+) (1) and {[(NH3)(3)Cu](2)(mu-O)(2)}(2+) (2) has been examined using multireference second-order perturbation theory with an 8-electron/8-orbital active space. At this level of theory, 1 and 2 are separated by only 0.3 kcal/mol, and the barrier to isomerization is predicted to be very low based on single-point energy calculations for intermediate structures. The flat nature of the potential energy surface along the interconversion coordinate derives from a balancing of Coulomb forces and nondynamic electron correlation. The latter effect depends critically on the significant energy change experienced by the 13a(u) sigma(00)* virtual orbital on passing from one isomer to the other. In addition, solvation electrostatics favor 2 over 1.