Influence of counter-ions on structure and vibrational states of <Emphasis Type="Italic">d</Emphasis>-anionic forms of metalloporphyrins

The structure and vibrational states of complexes of Co- and Ni-porphine monoanions with Na⁺, K⁺, and Rb⁺ cations are examined on the basis of density functional theory calculations. It is found that anion×cation pairs exist in a conformation with the cation situated along the axis connecting opposite carbon atoms in meso-positions. Interaction with the counter-ion assists localization of a major portion of the excess of charge on the metal and not on the macrocycle as in the case of free anions. The formation of anion–cation pairs, in spite of a decrease in the anion charge and its localization on the metal, is accompanied by larger structural rearrangements due to the asymmetric localization of the counter-ion and its influence on the π-electron density distribution. The possibility of forming sandwich-type dimers of anion–cation pairs with D_2h-symmetry is demonstrated. It is shown that such a model for anionic forms of Co- and Ni-porphine, as compared with those for free anions and anion–cation pairs, is in better accordance with data of IR and resonance Raman spectroscopy.