Comparison of Thionitrosyl and Nitrosyl Bonding in Dicarbonyl(η5-cyclopentadienyl)chromium Complexes by Gas-Phase Ultraviolet and X-ray Photoelectron Spectroscopy

The gas-phase He I/He II UPS and Mg Ka XPS data for the compounds (η5-C5H5)Cr(CO)₂NO and (η5-C5H5)Cr(CO)₂-NS are reported. The first ionization potentials for the two complexes are nearly the same (nitrosyl 7.56 eV; thionitrosyl 7.47 eV), contrary to indications from mass spectrometry appearance potentials. A short C-O stretching progression (ca. 2000 cm-1) in the first ionization band of the nitrosyl compound shows that it is predominantly associated with metal d character and is symmetrically π delocalized into the carbonyls. This orbital is primarily of δ symmetry with respect to the NO or NS ligand. The next two ionizations for both complexes, which complete the formal d6 configuration at the metal, are sensitive to the π symmetry interaction with the NO or NS group. These ionizations are approximately degenerate for the NO complex and are stabilized by nearly 1 eV relative to the first band as a consequence of the stronger Cr-NO π-back-bonding relative to Cr-CO back-bonding. These ionizations are slightly destabilized and are sharper for the NS complex relative to the NO complex, showing stronger metal π interaction with both the NS π bond and π∗ orbitals. The dramatic loss of intensity of these ionizations in the He II spectrum of the thionitrosyl complex experimentally demonstrates significant derealization with both the NS π-donor and π-acceptor levels to give large NS (sulfur) character for these ionizations. The nitrogen Is binding energy for the NS complex is almost 2 eV lower than that for the NO complex, suggesting a much more negative charge on the NS nitrogen. In contrast, the chromium, carbon, and oxygen core binding energies do not differ significantly between the two complexes. A formal potential model analysis of the (Cr-N-O) and (Cr-N-S) XPS shifts indicates a slightly more positive metal in the NS complex. These results are in agreement with Fenske-Hall approximate calculations and with other physical properties of the two compounds.