Effect of Pressure on the Electronic Structure of Phthalocyanine and Iron Phthalocyanine Derivatives

Abstract

The electronic structures of phthalocyanine, ferrous phthalocyanine, and the adducts with pyridine, 3‐picoline, 4‐picoline, and piperidine have been studied to 175 kbar pressure using optical absorption and Mössbauer resonance. One observes shifts of the optical absorption peaks to lower energy and changes in relative intensity of peaks which can be explained in terms of changes in the configuration interaction. Fe(II) in ferrous phthalocyanine is in an intermediate spin configuration at all pressures. In the axially coordinated adducts the Fe(II) is low spin at 1 atm. For the pyridine and picoline adducts this is caused by the increased backbonding to the axial ligands in addition to the phthalocyanine and by repulsion due to $\sigma$ bonding. These factors both tend to separate the $d_{z}2$ orbitals from the $d_{\pi }$ orbitals. In the piperidine adduct only the latter factor is operative. With increasing pressure the low spin compounds convert partially to a new spin state, believed to be intermediate spin. The picolines show the most conversion, the pyridine adduct less, and the piperidine complex still less. The general explanation lies in the decrease in backbonding at high pressure caused by the tendency for occupation of the ligand $\mathrm{\pi *}$ orbitals by ligand $\pi$ electrons. A similar effect has been observed in ferrocyanides and in phenanthroline complexes. The differences among the various adducts can be explained in terms of differences in the degree of $\pi$ backbonding and $\sigma$ bonding.