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Spin fluctuations in Fe(II)-porphyrins are at the heart of heme-proteinsfunctionality. Despite significant progress in porphyrin chemistry, themechanisms that rule spin state stabilisation remain elusive. Here, it isdemonstrated by using multiconfigurational quantum chemical approaches,including the novel Stochastic-CASSCF method, that electron delocalizationbetween the metal centre and the pi system of the macrocycle differentiallystabilises the triplet spin states over the quintet. This delocalisation takesplace via charge-transfer excitations, involving the out-of-plane iron dorbitals, key linking orbitals between metal and macrocycle. Through acorrelated breathing mechanism, the 3d electrons can make transitions towardsthe pi orbitals of the macrocycle. This guarantees a strong coupling betweenthe on-site radial correlation on the metal and electron delocalization.Opposite-spin 3d electrons of the triplet can effectively reduce electronrepulsion in this manner. Constraining the out-of-plane orbitals from breathinghinders delocalization and reverses the spin ordering. Our results find aqualitative analogue in Kekul\'e resonance structures involving also the metalcentre.

Understanding the mechanism stabilizing intermediate spin states in Fe(II)-Porphyrin