Infrared And Raman Spectra Of Inorganic And Coordination Compounds Part B Applications In Coordination Organometallic -

Upon bridging, the CO bond order decreases further. A doubly bridging (μ₂) CO group appears 100–150 cm⁻¹ lower (typically 1750–1850 cm⁻¹), while a triply bridging (μ₃) CO can drop below 1700 cm⁻¹. The complex ( \text{Co} 4(\text{CO}) {12} ) provides a classic case: terminal CO stretches are observed at 2060 and 2025 cm⁻¹, while the edge-bridging COs produce a distinct band at 1855 cm⁻¹. This separation collapses upon heating or chemical reduction, signaling a fluxional process where bridges and terminals exchange on the vibrational timescale.

The CO stretching region (1850–2150 cm⁻¹) remains the most unambiguous probe for predicting carbonyl geometry. A purely terminal, linear M–C≡O group exhibits a strong, sharp IR band typically between 2050 and 2120 cm⁻¹ for neutral carbonyls (e.g., Ni(CO)₄ at 2057 cm⁻¹). Anionic or electron-rich metal centers lower this frequency due to increased π-backdonation into the CO π* orbital. Upon bridging, the CO bond order decreases further

The distinction between Fischer-type (electrophilic) and Schrock-type (nucleophilic) carbene complexes is elegantly captured by the C–X (X = O, N) stretching modes of the carbene substituent, rather than the M=C stretch itself. For a Fischer carbene ( (\text{CO})_5\text{Cr}=\text{C}(\text{OCH}_3)\text{CH}_3 ), the C–O(methoxy) stretch appears near 1200 cm⁻¹, significantly lower than that of a typical ether (~1270 cm⁻¹), reflecting partial double-bond character in the C–O bond due to resonance. In Schrock-type tantalum alkylidenes, this resonance is absent, and the C–O or C–N modes remain unperturbed. Anionic or electron-rich metal centers lower this frequency

The carbyne ligand (C≡M) is rarer but distinctive. Here, the M≡C stretch is often Raman-active and appears in the 1100–1300 cm⁻¹ region—a range devoid of most other metal-ligand vibrations. The complex ( \text{Cl}(\text{CO})_2\text{W}\equiv\text{C}-\text{CH}_2\text{CMe}_3 ) shows a strong, polarized Raman band at 1225 cm⁻¹ assigned to the W≡C stretch, with no corresponding IR absorption of comparable intensity, confirming the linear, symmetric nature of the moiety. confirming the linear