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

Complex formation leads to several changes in the ligand. These may manifest through changes in the electronic structure, the state of energy or symmetry of the ligand. These changes affect the vibrations of the ligand and this in turn will cause a change in its vibrational spectrum compared to that of the free ligand.

The vibrational spectrum of a complex compound is influenced by the structure, the symmetry of the complex, the strength of its co-ordination bonds and its interaction with the environment. Based on the changes in the spectrum of the complex (compared to the free ligand) many properties of the compound may be deduced.

Study of changes in the spectra of donor molecules

When a donor molecule is involved in co-ordination, the IR spectrum of the molecule will show some changes as in the following case.

N,N-Dimethylacetamide

The molecule can be involved in co-ordination with a metal atom, through either oxygen or nitrogen atom. In most cases co-ordination occurs through oxygen. The carbonyl stretching frequency for this molecule is at 1662 cm^-1.

i) When oxygen is involved in co-ordination with a metal atom, it has been found that the absorption due to CO stretch is less than 1662 cm^-1. Since oxygen is attached to the Lewis acid, the electron density around the CO group is pulled or drained towards the metal atom due to which there is decrease in the electron density around the CO group. This in turn decreases the stiffness of CO bond and therefore decrease in its force constant as well, resulting in lowering of the CO stretching frequency with concomitant increase in the C-N stretching frequency.

ii) If Nitrogen atom is involved in co-ordination with the metal atom then the lone pair on it would be involved, the force constant of the carbonyl bond increases due to draining of electron density around oxygen into the CO group towards the nitrogen, which in turn will result in a decrease in the C-N stretching frequency (see fig.).

This will create a partial triple bond character for the carbonyl group leading to enhanced force constant and thereby increase in its stretching frequency.

Thus from comparison of absorption pattern in the IR spectra of the complexes it is possible to get information about the nature of the ligands. It is possible to predict which donor atom in the ligand is involved in co-ordination.