157 T INTERNAL MAGNETIC FIELD IN Fe[C(SiMe 3 ) 3 ] 2 COMPOUND AT 20K Ernő Kuzmann, 1,2 Roland Szalay, 2 Attila Vértes, 1,2 Zoltán Homonnay, 2 Imre Pápai, 3 Peter de Châtel, 1 László Szepes 2 1 Laboratory of Nuclear Chemistry, Chemical Research Center, Hungarian Academy of Sciences, Budapest, Hungary 2 Chemical Institute, Eötvös Loránd University, Budapest, Hungary 1 Department of Theoretical Chemistry, Chemical Research Center, Hungarian Academy of Sciences, Budapest, Hungary

Iron coordination compounds with coordination number smaller than 4 are in the focus of interest, since they often show unusual features or interesting properties. iron is coordinated by two carbons Fe[C(SiMe 3 ) 3 ] 2 LaPointe, A. M., Inorganica Chimica Acta 345 (2003) 359. Reiff, W.M., LaPointe, A.M., Witten, E.H., J. Am. Chem. Soc. 126(2004 ) 10206

CHECK BY MÖSSBAUER SPECTROSCOPY: Hyperfine field of iron valence state of iron

Mössbauer spectrum of Fe[C(SiMe 3 ) 3 ] 2 dark red crystalline compound prepared from the reaction between FeCl 2 and (Me 3 Si) 3 CLi The sextet has an extremely high (±0.8) T magnetic field at the site of the 57 Fe nucleus. This is somewhat higher than that reported by Reiff et al, J. Am. Chem. Soc. 126(2004) at 4 K

PXRD was applied to determine the structure of the compound

predicted equilibrium geometry, obtained from the DFT calculations, is in line with the XRD data The predicted equilibrium geometry, obtained from the DFT calculations, is in line with the XRD data, as the C-Fe-C unit of the ground state molecule is found to be linear with a nearly staggered arrangement of the trimethylsilyl groups and the calculated interatomic distances are reasonably close to the experimental values. For instance, the Fe-C bond length is predicted to be 2.03 Ǻ, which compares well with the experimental value of 2.05 Ǻ. Spin density distribution in Fe[C(SiMe 3 ) 3 ] 2 molecule (isodensity surface cutoff is 0.05 au).

The electron configuration of the Fe atom is characterized by 4s d 5.83 The spin density distribution is fully localized on the central Fe atom The NBO analysis carried out at the equilibrium geometry of Fe[C(SiMe 3 ) 3 ] 2 reveal a substantial charge transfer from Fe to the C(SiMe 3 ) 3 ligands (Q(Fe) = +1.45). The electron configuration of the Fe atom is characterized by 4s d 5.83, where the 4s electrons are only slightly polarized and most of the unpaired electrons have 3d character (see in the Table). The spin density distribution is fully localized on the central Fe atom, as shown in the Figure.

Mössbauer parameters of Fe[C(SiMe 3 ) 3 ] 2 sample recorded at 20 K: observed isomer shift can be well understood since the considerable amount (0.7) of the 4s contribution decreases the isomer shiftappropriately The observed isomer shift value, which is unusually low for the Fe(II) complexes, can be well understood by taking into consideration the result of DFT calculation, since the considerable amount (0.7) of the 4s contribution decreases the isomer shift appropriately in this compound. extreme high hyperfine fieldexplained the orbital contribution from the 3.94 unpaired electrons associated with the 3d orbitals The main contribution of the extreme high hyperfine field can be explained by the orbital contribution from the 3.94 unpaired electrons associated with the 3d orbitals as well as by the slightly modified Fermi contact contribution due to the effect of 4s electrons.

A hyperfine magnetic field of T was found A hyperfine magnetic field of T was found at Fe[C(SiMe 3 ) 3 ] 2 compoundiron is coordinated by two carbons 20 K at the site of the iron nucleus of the Fe[C(SiMe 3 ) 3 ] 2 compound, in which iron is coordinated by two carbons, prepared using the reaction of FeCl 2 with (Me 3 Si) 3 CLi. the quintet statesclearly favoredenergetically considerable 4s large unpaired 3d electron contributions responsible for the extremely high hyperfine field DFT calculations predict that the quintet states are clearly favored energetically over the lower spin states. The population analysis reveals considerable 4s as well as large unpaired 3d electron contributions, which can be responsible for the extremely high hyperfine field. E Kuzmann, R Szalay, A Vértes, Z Homonnay, I Pápai, P de Châtel, L Szepes, Hyperfine Interact. (2008)