When liquid Fe(CO)5 was first isolated, two structures were postulated, a C4v sq

Question

When liquid Fe(CO)5 was first isolated, two structures were postulated, a C4v square pyramidal structure and a D3h trigonal bipyramidal structure. Using the 5 CO stretching vibrations as generating vectors in each of these point groups, generate the corresponding reducible representation, reduce each down to their respective irreps, and determine the symmetries of the IR and Raman active stretching vibrations. Using this information with knowledge of the experimental data (IR: 2002, 1989 cm-1; Raman: 2116, 2030, 1989 cm-1), indicate the correct liquid phase structure of Fe(CO)5.

Explanation / Answer

Given that the point group for the square pyramidal Fe(CO)5 = C4v

Here, the ligand field strength is higher.

And the point group for the trigonal bipyramidal Fe(CO)5 = D3h

Here, the ligand field strength is lower.

In case of square pyramidal Fe(CO)5, the CO stretching frequencies are higher, i.e. 2013 and 2034 cm-1.

In case of trigonal bipyramidal Fe(CO)5, the CO stretching frequencies are comparatively lower, i.e. 1989 and 2002 cm-1.

Hence, according to the given data and as explained above, the correct liquid phase structure of Fe(CO)5 = trigonal bipyramidal (Point group = D3h)

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