Comparative X-ray absorption analysis of the spectrum of vacant electronic states in cobalt and nickel tetraphenylporphyrin complexes

The energy distributions and the properties of the lower vacant electronic states in cobalt and nickel tetraphenylporphyrin complexes CoTPP and NiTPP are studied by X-ray absorption spectroscopy. Quasimolecular analysis of the experimental absorption spectra measured in the region of the 2 $p$ and 1 $s$ ionization thresholds of complexing metal atoms, as well as the 1 $s$ thresholds of ligand atoms (nitrogen and carbon), is based on the comparison of the corresponding spectra with each other and with the spectra of the simplest nickel porphyrin NiP. It has been established that, despite a general similarity of the spectra of nitrogen and carbon in CoTPP and NiTPP, the fine structure of the 2 $p$ and 1 $s$ absorption spectra of cobalt and nickel atoms are radically different. The observed differences in the spectra of cobalt and nickel are associated with the features of the energy distribution of vacant 3 $d$ electron states. The presence in CoTPP of the partially filled valence $3db_2 g$ molecular orbital (MO) results in the appearance in the cobalt spectra of a low-energy band, which is absent in the spectrum of nickel in NiTPP and leads to a doublet structure of transitions to $b_1 g$ and $e_ g$ MOs due to the exchange interaction between 3 $d$ electrons in partially filled $3db_{2g}$ and $3db_{1g}$ or $3de_{g}$ МОs. The spectrum of vacant states in CoTPP differs from that in NiTPP also due to the smaller energy distance between $3db_{1g}$-МО and $e_{g}$ MOs and the different positions of nonbonding MOs with the C2 $p$ character of the porphine ligand.