How do you explained the bonding of the carbonyl groups in the structure of Fe2(CO)9 through IR-spectra ?

How do you explained the bonding of the carbonyl groups in the structure of  Fe₂(CO)₉ through IR-spectra ?

The sixth member of 3d transition series is iron (Fe). Few properties of elemental iron are shown below pictorially,

Iron formed few carbonyl compounds like Fe(CO)₅ , Fe₂(CO)₉  etc. The most important carbonyl compound of iron is diiron nona carbonyl ,Fe₂(CO)₉ .

Infrared and  x-ray studies have shown that in this organometallic compound each Fe atom is directly linked with the other Fe atom by a sigma bond  ( bond distance 2.46 Å ) to three bridging carbonyl ( CO ) groups  by a sigma bond and for three terminal carbonyl groups by a coordinated bond. 

Presence of both type of carbonyl group ,that is,bridging and terminal can be easily proved with the help of I.R-spectra of the molecule. 

In bridging or ketonic  carbonyl groups , there is no possibility or necessity of back bonding , C – O bond order is purely 2.  The stretching frequency  of such group is in the order of 1700 – 1850 cm ^-1 .

 In terminal carbonyl  groups , there is a back bonding due to which the bond order of terminal carbonyl group is reduced to some extent from the value of three , which is the bond order of  free CO molecule.

Due to this back bonding stretching frequency of free CO molecule ( 2143 cm ^-1 ) is reduced, but obviously remains  above 1850 cm ^-1. 
The presence of two absorbance peaks one at around 1830 cm -1 for bridging CO group and other at 2020 cm -1 for terminal  CO group . 

This two value confirms the above structure of di-iron nona carbonyl ,Fe₂(CO)₉ .

The absorbance curve of stretching frequency of bridging and terminal carbonyl groups  in the structure of Fe₂(CO)₉  are given below.

Practice problem:

Discuss the structure and bonding of Fe₂(CO)₉ . 

Write down the structure of iron pentacarbonyl .

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