Group 6 Carbonyl Complexes of C‑Ylidic Cp Ligands

The synthesis and the analytical features of group 6 carbonyl complexes of electron-poor mono­(imidazolium)- and bis­(imidazolium)-substituted Cp ylides are presented. With the neutral ylidic imidazolium Cp and the monocationic 1,3-bis­(imidazolium) Cp ligand all of the group 6 carbonyl complexes were prepared, while in the case of the 1,2-disubstituted Cp ylide only the chromium Cp complex was accessible. The donor properties of the ligands were elucidated experimentally by means of the CO stretching frequencies of their group 6 metal complexes and theoretically by DFT calculations. Every imidazolium substituent and the planar conjugated π system of the imidazolium moieties annelated to the Cp ring increase the average CO stretching frequencies by about 20 cm–1. Overall, the donor properties of the C-ylidic Cp ligands are still stronger than those of the benzene ligand. Additionally, the protonation of the Cp ligands was investigated, and the basicity correlates qualitatively with the electron-donor character of the ylidic Cp ligands.