Iron complexes vibrational spectra

The entropy difference A5tot between the HS and the LS states of an iron(II) SCO complex is the driving force for thermally induced spin transition [97], About one quarter of AStot is due to the multiplicity of the HS state, whereas the remaining three quarters are due to a shift of vibrational frequencies upon SCO. The part that arises from the spin multiplicity can easily be calculated. However, the vibrational contribution AS ib is less readily accessible, either experimentally or theoretically, because the vibrational spectrum of a SCO complex, such as [Fe(phen)2(NCS)2] (with 147 normal modes for the free molecule) is rather complex. Therefore, a reasonably complete assignment of modes can be achieved only by a combination of complementary spectroscopic techniques in conjunction with appropriate calculations. 

NRVS represents the ultimate limit in vibrational selectivity, because it reveals the vibrational spectrum of an individual probe atom even when embedded in a complex enviromnent such as a biomolecule containing thousands of other atoms. Fe NRVS is thus an exquisite probe for the stmcture and dynamics of the immediate coordination sphere of the iron, which is the heart of the reactivity of numerous important proteins. 

Fe[ ] = iron in complexes and metals by screening the s-elecuons). The isomeric shift decreases linearly with increasing s-electron density, i.e. an increasing s-electron density causes a shift of the resonance line toward negative velocity values. According to this the isomeric shifts of compounds with different oxidation states of the element in question fall into regions characteristic for these states, as is shown for iron in Fig. 2. The contribution of the temperature shift to the total line shift is generally small in relation to the isomeric shift. The temperature shift reflects the properties of the vibrational spectrum of the crystals. 

The carbonyl complex 33 is obtained in a clean displacement reaction when carbon monoxide is passed through a solution of 32 in methanol. Due to the increased charge on the cation, the product precipitates in the process. The rather low frequency of the C O stretching vibration in the IR spectrum of the solid (KBr disc) at 1960 cm-1 points to a strong Fe-CO interaction (100), as expected for the highly basic NN4 ligand environment. Similarly low values (1940-1960 cm-1) were reported for the pentaamine iron(II) complexes... 

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