First-row transition metal bis(amidinate) complexes; Planar four-coordination of Fe-II enforced by sterically demanding aryl substituents

The sterically hindered benzamidinate ligand [PhC(NAr)2]– (Ar = 2,6-iPr2C6H3) has been employed to prepare bis(amidinate) complexes [{PhC(NAr)2}2M] of the divalent first-row transition metals Cr–Ni (1–5). For Cr (planar), Mn and Co (tetrahedral) the observed structures follow the electronic preference for the metal ion in its highest spin multiplicity, as determined by DFT calculations. Remarkably, the Fe derivative adopts a distorted planar structure while retaining the high-spin (S = 2) configuration. This rare combination is due to reduced interligand steric interactions in the planar vs. the tetrahedral structure, combined with a relatively small electronic preference of FeII for the tetrahedral environment. Thus, the simple bidentate ligand N,N'-diarylbenzamidinate provides a convenient means to make this unusual species accessible for further study. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)