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 pi 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.