Effect of Coordination and Symmetry on the IR spectra of Donor Molecules

Spectral changes accompanying change in Symmetry upon coordination

When the symmetry of the ligand is altered due to co-ordination, changes in the IR spectrum are anticipated due to changes in the selection rule. This feature could be illustrated by considering a group, which can function as both unidentate and bidentate ligand.
Carbonato complexes (CO3)2-

Carbonate ion can coordinate to the metal atom both as a unidentate and a bidentate ligand as well.

Coordination of Carbonate complex

Due to differences shown above the selection rule changes as shown in the following correlation charts connecting the vibrational modes ( IR active ) and symmetry. Note: Both unidentate carbanato groups have the same C2v symmetry if the metal atom is ignored.( Nakamoto; IR spectra of coordination compounds)

point group of carbonate complex

When the coordination of the carbonate ion changes from unidentate to bidentate its point group changes from Cs to C2v as a result the selection rule changes and it is seen that fundamental frequencies v3 and v4 which were degenerate split into two bands (see chart). Also the fundamental frequencies, which are IR inactive in case of D3h system (free ion) are IR active when symmetry changes.

In carbonate complexes, though, v3 and v4 split into two bands both in Cs and C2v groups, it was found that the splitting was larger in the bidentate than in the monodentate complex.

carbonate Complex

The IR spectral behaviour of NO3-and its complexes is analogous to those of carbonate ion and metal carbonates.

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Comparision of Raman and IR
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