Resonance raman study of (η2-TCNE)M(CO)5 (M = Cr, W; TCNE = tetracyanoethene). Evidence for an olefin-complex → metallacyclopropane transition upon low-energy metal-to-ligand charge-transfer excitation

Abstract A solid state resonance Raman investigation of the non-hydrogen-containing title complexes was performed with long-wavelength laser excitation (514.5-611 nm). The spectra confirmed the η 2 -coordination of the π-acceptor olefin tetracyanoethene (TCNE) and showed that the absorption bands near 700 nm arise from transitions between the metal and TCNE. The metal-to-ligand charge-transfer (MLCT) character of these transitions is much more pronounced for the tungsten complex. These observations confirm the higher degree of metal ( d )/ligand(π ∗ ) orbital mixing in the chromium complex and also provide evidence for a structural change from that of an alkene π-complex towards a metallacyclopropane arrangement upon MLCT excitation