Electronic Spectra of N-Heterocyclic Pentacyanoferrate(II) Complexes in Different Solvents, Studied by Multiconfigurational Perturbation Theory

Ligand-field and charge-transfer spectra of N-heterocyclic pentacyanoferrate(II) complexes [Fe(CN)(5)L]n- were investigated using multiconfigurational perturbation theory. The spectrum of [Fe(CN)(5)(py)](3)- was studied in detail under vacuum and in the following polarizable continuum model (PCM) simulated solvents: acetone, acetonitrile, dimethylsulfoxide (DMSO), ethanol, methanol, and water. The ligand-field states proved to be rather insensitive to the solvent environment, whereas much stronger solvent effects were observed for the charge-transfer (CT) transitions. The nature of the intense band was confirmed as a metal-to-ligand charge transfer originating from a 3d (L)-orbital transition. The difference between the calculated and experimental transition energy of this CT transition is minimal for aprotic solvents, but increases strongly with the solvent proton donor ability in the protic solvents. In an attempt to improve the description of this CT state, up to 14 solvent molecules were explicitly included in the quantum model. In DMSO, the spectra of complexes with ligands L (where L is pyridine, 4-picoline, 4-acetylpyridine, 4-cyanopyridine, pyrazine, and N-methylpyrazinium) correlate very well with the experiment.