The binding of ethene to a metal (e.g., in Zeise’s salt, K[PtCl₃(C₂H₄)]) induces two key shifts. First, the ν(C=C) of free ethene at 1623 cm⁻¹ (Raman) drops to approximately 1515 cm⁻¹ in the complex—a direct measure of the population of the ethylene π* orbital via backdonation. Second, a new, weak IR band appears near 1200 cm⁻¹, assigned to the CH₂ wagging mode of the coordinated olefin; this mode is IR-forbidden in free ethene due to its center of inversion, but coordination breaks that symmetry, activating the band. The intensity of this “activation band” is proportional to the degree of metal-to-ligand backdonation and can distinguish between η²-olefin and metallacyclopropane extremes.
One of the most elegant applications of IR spectroscopy in coordination chemistry is the detection of the trans influence via CO probes. Consider the square-planar platinum(II) series ( trans)-([PtCl(CO)(L)_2]^+ ). As L varies from a strong σ-donor (e.g., CH₃⁻) to a weak donor (e.g., Cl⁻), the CO stretching frequency shifts inversely. With L = CH₃, the Pt–CO bond is strengthened (more π-backdonation), lowering ν(CO) to ~2030 cm⁻¹. With L = Cl⁻, ν(CO) rises to ~2080 cm⁻¹. This provides a direct, linear correlation with the trans ligand's Tolman electronic parameter, allowing spectroscopists to rank ligands without ever isolating a pure metal-hydride. The binding of ethene to a metal (e
The vibrational signature of the metal-carbon bond is the cornerstone of organometallic spectroscopy. While the M–C stretching mode itself often lies in the low-frequency region (usually below 600 cm⁻¹) where coupling with other metal-ligand modes is prevalent, the true power of IR and Raman lies in observing the perturbation of the ligand’s internal vibrations upon coordination. The intensity of this “activation band” is proportional
Thus, even in the age of X-ray crystallography and DFT, mid- and far-infrared Raman spectroscopy remains indispensable for mapping electron density flow in real time—particularly for solution-phase dynamics and fluxional organometallics where diffraction methods fail. As L varies from a strong σ-donor (e