Question

44 2) On the following page you will find illustrations of the v(CO) region of the IR spectra of (n-C5H5)Fe(CO)21 and [(1-C5H5)Fe(CO) 2]2 obtained using cyclohexane solutions of the two complexes. 2000 1800 2000 (n-CSH5)Fe(CO)21 [(n-CSH5)Fe(CO)2]2 Compare the solution IR spectrum of (n-CSH5) Fe (CO) 21 with the KBr disk spectrum that you obtained and note the higher resolution (narrower IR line-width) obtainable for solution spectra in non- polar solvents compared to spectra recorded in the solid state as well as any differences in the positions of the v (CO) bands. Compare the solution IR spectrum of (n- C5H5)Fe(CO)2I with that of [(n-CSH5)Fe(CO)2]2 and calculate v(cO) av for each complex. (Use the two highest wave-number bands in the spectrum of the dimer). How can the differences in v(CO)av be explained? 6S02

3) The IR spectrum of [(-C5H5)Fe(CO)2]2 shows two bands in two different areas of the (CO) region - close to 1800 cm-1 and close to 2000 cm-1 . Why is this so? (A general answer will suffice; it is not necessary to assign all the bands in the IR spectrum of the dimer).

Answer 1

Solution:

There are two types of stretching frequencies in metal-carbonyls.These are terminal C=O stretching and bridged C=O stretching frequencies.The frequency for terminal free carbonyl group is higher than the bridged carbonyl group.

Hence,

1)The frequency close to 1800 cm-1 is due to bridging (-C=O-) group stretching.

2) Frequency near 2000 cm-1 is due to the terminal carbonyl. (-C=O) stretching.