Question

What does the MO diagram look like for a CO ligand. Why is CO such a good ligand for IR spectroscopy? What are the types of vibration a ligand can have?

Answer 1

the MO diagram look like traingular bypyramid for a CO ligand

An important technique for characterizing metal carbonyls is infra-red spectroscopy.^[12] The C-O vibration, typically denoted ν_CO, occurs at 2143 cm⁻¹ for CO gas. The energies of the ν_CO band for the metal carbonyls correlates with the strength of the carbon-oxygen bond, and inversely correlated with the strength of the π-backbonding between the metal and the carbon. The π basicity of the metal center depends on a lot of factors; in the isoelectronic series (Ti to Fe) at the bottom of this section, the hexacarbonyls show decreasing π-backbonding as one increases (makes more positive) the charge on the metal. π-Basic ligands increase π-electron density at the metal, and improved backbonding reduces ν_CO. The Tolman electronic parameter uses the Ni(CO)₃ fragment to order ligands by their π-donating abilities.^[13][14]

The number of vibrational modes of a metal carbonyl complex can be determined by group theory. Only vibrational modes that transform as the electric dipole operator will have non-zero direct products and are observed. The number of observable IR transitions (but not the energies) can thus be predicted.^[15][16][17] For example, the CO ligands of octahedral complexes, e.g. Cr(CO)₆, transform as a_1g, e_g, and t_1u, but only the t_1u mode (anti-symmetric stretch of the apical carbonyl ligands) is IR-allowed. Thus, only a single ν_CO band is observed in the IR spectra of the octahedral metal hexacarbonyls. Spectra for complexes of lower symmetry are more complex. For example, the IR spectrum of Fe₂(CO)₉ displays CO bands at 2082, 2019, 1829 cm⁻¹. The number of IR-observable vibrational modes for some metal carbonyls are shown in the table. Exhaustive tabulations are available.^[12] These rules apply to metal carbonyls in solution or the gas phase. Low polarity solvents are ideal for high resolution For measurements on solid samples of metal carbonyls, the number of bands can increase owing in part to site symmetry.