Stereospinomers of pentacoordinate iron porphyrin complexes: the case of the [Fe(porphyrinato)(CN)]− anions

Abstract

A computational study of the Fe^II porphyrinato complexes [Fe(tmp)CN]⁻ and [Fe(tmp)(CN)₂]²⁻ in the high- (S = 2) and low-spin (S = 0) states (tmp = tetramethylporphyrinato) unravels the reasons for a thermally accessible high-spin for the tetraphenylporphyrinato analogue of the former. It is shown that the different coordination geometry of the high-spin stereospinomer is in large part responsible for its stability, while a single strong-field cyanide ligand contributes to the stability of the low-spin stereospinomer. The results are compared with the isoelectronic complexes [Ru(tmp)CN]⁻ to evaluate the influence of the two-electron terms on the relative stability of the two stereospinomers. Shape and geometry ranges for the two spin states of [Fe^II(porphyrinato)(ligand)] complexes are established and structural data for the deoxygenated form of hemo- and myoglobins are analyzed from that point of view.