Complexes with sulphate (SO4)2-
The IR spectra of metal complexes containing tetrahedral anions such as (SO4)2-, (ClO4)-, (PO4)3-have been extensively investigated. The different modes of coordination of (SO4)2- are given below.
The correlation chart connecting the vibrational modes for the above symmetries is shown below.
From the above chart we observe that when the symmetry changes from Td to C3v,
v3 and v4 split into two bands and when it changes to C2v, v3 and v4 split into three bands. Also we find that v1 and v2, which are IR inactive for Td symmetry, are active for C3v and C2v symmetry.
Vibrational frequencies for sulfato complexes
For the complex [Co(NH3)5SO4]Br, we see that v1 and v2 have medium intensities and v3 and v4 each are split into two bands which is the splitting pattern observed for compounds of C3v symmetry. Thus it is possible to identify the complex belonging to that point group. The complex,
Both show similar splitting pattern as observed for compounds of C2v symmetry, this enables identification based on the splitting pattern. But from the pattern it is not possible to identify the two forms viz., bidentate and bridged bidentate complexes
Nitrogen complexes
Nitrogen molecule is IR inactive, because there is no change in dipole moment during the vibration. But on complexation bands due to N-N and M-N2 vibrations can be seen in the IR range, which implies on coordination N2 is IR active and bands due to various modes of vibration are seen as in the following example.
[Ru(N2(NH3)5]Br2 with C2v symmetry shows N-N stretching between 2170-2115 cm-1 and Ru-N2 stretching between 510-470 cm-1. Conclusion: Thus through the study of the IR spectra of donor molecules in their free and complexed states it is possible to determine important characteristics of the complex, like the donor atom of the ligand involved in coordination, the symmetry of the complex. The effects of change in co-ordination number, of electro negativity and size of metal ions on the ligands in the complex can also be studied. IR studies of complexes and the free donor molecules can be used to determine whether complex formation has occurred or not, thus it becomes an important analytical method in synthetic inorganic chemistry of complexes.