IR spectrum of an octahedral complex

In summary, bond vibrations in molecules are only IR active if they cause a change in dipole moment. In the case of chromium hexacarbonyl, the significant IR absorption at 1900 cm-1 is produced by an asymmetrical stretch of the C-O bond, which temporarily changes the dipole moment. Bending of the C-O bond can also change the dipole moment, but it is not as significant as the stretching motion. Jahn-Teller distortions do not cause a change in dipole moment in this molecule due to its full octahedral symmetry. Only the T1u modes, such as the carbonyl stretch mode, will be infrared active in this complex. Other modes, such as bending and asymmetric
  • #1
mycotheology
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I read that bond vibrations are only IR active if they produce a change in dipole moment. I'm trying to visualise how this works with chromium hexacarbonyl:
150px-Cr%28CO%296.png

If I'm not mistaken, this compound has one significant IR absorption at around 1900 cm-1, which is a C-O stretching absorption. Is this produced by an asymmetrical stretch? For example, if the ligand at the top is stretched, but the ligand at the bottom is compressed (as in the bond length is temporarily shorter)? Thats the only way I can see the dipole changing. Then again, what about bending? If one of the C-O bonds bends so that it is no longer colinear with the M-C bond axis, that would change the dipole moment, wouldn't it?

Also, what about Jahn-Taller distortions? Would they cause a change in dipole moment? Because all the ligands are identical and the molecule is completely, I'm guessing Jahn-Taller distortions don't change the dipole moment of the molecule. Am I right?
 
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  • #2
The carbonyl stretch mode that you describe will indeed give a strong signal. It is a triply degenerate T1u mode in the Oh symmetry of the complex.

As a low-spin Cr(0) complex, Cr(CO)6 will have full octahedral symmetry, and there will be no question of Jahn-Teller distortion.

In full octahedral symmetry, only the T1u modes will be infrared active. There are only 4 such modes for this complex; the others are both at much lower frequency and intensity. Two are bending modes, roughly equivalent to an umbrella type motion of the four equatorial ligands (out-of-plane co-ordinated L-M-L motion), and the other equivalent to an arm-waving motion of these ligands (out-of-plane co-ordinated O-C-M bending). The other is an asymmetric metal-ligand stretching mode for the pair of axial ligands. My guess (and it is a very rough guess) is that you will find them represented by very weak bands around 150, 550, and 250 cm–1. Remember that CO is hardly polar at all, neither as a free molecule nor as a ligand.
 

What is an IR spectrum of an octahedral complex?

An IR spectrum of an octahedral complex is a graph that shows the absorption of infrared radiation by the compound as a function of frequency or wavelength. It provides information about the vibrations and bonds present in the complex.

What are the main features of an IR spectrum of an octahedral complex?

The main features of an IR spectrum of an octahedral complex include the number of peaks, their positions on the graph, and their relative intensities. These features can be used to identify the type of ligands present in the complex, as well as the metal ion and its oxidation state.

What factors can affect the IR spectrum of an octahedral complex?

The IR spectrum of an octahedral complex can be affected by various factors such as the type and number of ligands, the metal ion, and the geometry of the complex. The presence of different functional groups, such as C-H, O-H, and N-H bonds, can also influence the spectrum.

How is the IR spectrum of an octahedral complex analyzed?

The IR spectrum of an octahedral complex is analyzed by comparing the peaks on the spectrum to known values for different functional groups. The positions and intensities of the peaks can provide information about the types of bonds and ligands present in the complex.

What are the applications of studying the IR spectrum of an octahedral complex?

Studying the IR spectrum of an octahedral complex can provide valuable information for various fields such as inorganic chemistry, biochemistry, and material science. It can be used to identify and characterize compounds, determine their purity, and study their chemical and physical properties.

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