Spin coupling in Cu porphyrin π-cation radicals

We examine the magnetic coupling that exists between the unpaired electrons on copper and on porphyrin in porphinato copper (II) cations. We find that the electrons are ferromagnetically coupled, and argue that this must always be so at the molecular orbital (SCF) level of theory, regardless of the geometry of the system. We find that the a1u hole state (3B1u) is lowest lying for systems such as etio-porphyrins, but argue that the a2u hole state (3B2u) should be lowest for the tetraphenyl systems, as is observed. The closed shell 1A1g porphinato copper(III) state is sensitive to geometric considerations, but is never the lowest lying in the systems examined here.