Group frequencies alicyclic

The most direct evidence for back coordination in iron(II) methine complexes comes from the observation of IR C=N stretching frequencies in complexes of ligands containing aliphatic or alicyclic imine or cizine groupings (see Table 5). The substantial decrease of v (C=N) (ca. 100 cm i) on complexation with iron(II) can, with some confidence, be taken as a qualitative indication of back coordination. However, at present it is difficult to assess the extent of r-electron delocalization in the iron(II) diimine ring from the observed IR shifts. 

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