Quantum Chemical Calculations (ab initio, semi-empirical)

"Low-spin" Ni-porphine pyridine complex ("expanded-core structure")

Immediately after the dz2 orbital becomes doubly occupied following photoexcitation.

The antibonding character of this doubly occupied orbital photoejects the ligand.

Strong mixing of dz2 with the a2u orbital occurs as can be seen in the image below.

a2u orbital (HOMO) is mixed with dz2 and is antibonding with respect to ligand binding.