The number of CO bands for isomers from sets (i) and (ii) in their IR spectra

$$\text{Set (i): Trigonal bipyramidal isomers, axial-} \mathrm{Fe(CO)_4L} \text{ (A) and equatorial-} \mathrm{Fe(CO)_4L} \text{ (B)} \\\text{Set (ii): Octahedral isomers, } \mathit{fac}\text{-} \mathrm{Mo(CO)_3L_3} \text{ (C) and } \mathit{mer}\text{-} \mathrm{Mo(CO)_3L_3} \text{ (D) are}$$​

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Correct option is D

Metal carbonyls are coordination complexes of transition metals with carbon monoxide ligands. Metal carbonyls are useful in organic synthesis and as catalysts or catalyst precursors in homogeneous catalysis, such as hydroformylation.

​An important technique for characterizing metal carbonyls is infrared spectroscopy. The C–O vibration, typically denoted ν_CO, occurs at 2143 cm^-1 for carbon monoxide 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 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 nonzero direct products and are observed. The number of observable IR transitions (but not their energies) can thus be predicted.

In chemistry, a trigonal bipyramid formation is a molecular geometry with one atom at the center and 5 more atoms at the corners of a triangular bipyramid. The five atoms bonded to the central atom are not all equivalent, and two different types of position are defined. For phosphorus pentachloride as an example, the phosphorus atom shares a plane with three chlorine atoms at 120° angles to each other in equatorial positions, and two more chlorine atoms above and below the plane (axial or apical positions).

In chemistry, octahedral molecular geometry, also called square bipyramidal, describes the shape of compounds with six atoms or groups of atoms or ligands symmetrically arranged around a central atom, defining the vertices of an octahedron.

When three identical ligands occupy one face of an octahedron, the isomer is said to be facial, or fac. In a fac isomer, any two identical ligands are adjacent or cis to each other. If these three ligands and the metal ion are in one plane, the isomer is said to be meridional, or mer. A mer isomer can be considered as a combination of a trans and a cis, since it contains both trans and cis pairs of identical ligands.