Geometric Structure And Electronic Configuration Of Dnic In Solution And Crystalline State

About forty years after their discovery, the structure of the iron center in DNIC is still a subject of controversy. Accurate structural data were obtained from diffraction experiments on crystallized DNIC, which are usually EPR silent. These diffraction studies showed that the iron was held in the center of a tetrahedral configuration [37,41,90,100-109]. In contrast, the available information on the electronic configuration was provided by EPR spectroscopy on isolated paramagnetic complexes in solution. The analysis of the EPR parameters suggested that dissolved DNIC complexes have the iron in a square plane geometry [39]. The sets of data suggest that the geometry and electronic configuration change significantly upon dissolution of the crystals in a solvent.

We will first discuss the paramagnetic DNIC with thiol-containing ligands in solution. At room temperature, the EPR spectra of DNIC show resolved HFS interactions with the ligands in coordination to the iron. The spectra of the DNIC identify four ligands: two anionic compounds and two nitrosonium ions. The DNIC with thiol-containing ligands, and thiosulfate or xanthogenate ligands all have very similar EPR spectra with axial symmetry of the g-tensor. The g-factors are g± = 2.04-2.45 and gH = 2.014 [39,40,56]. We noted above that DNIC in solution shows oblate axial symmetry with g± > g||. This indicates that the iron atom can be placed in plane-square structure (Scheme 7). The crystal field is dominated by four strong ligands in the plane, with small perturbation from weakly binding solvent molecules at the axial positions. In this structure, the ligand fields of the four in-plane ligands lift the ^x2-y2 to a high-energy level, sufficiently high to preclude the high spin state as expected from Hund's rule. The seven electrons of the {Fe(NO)2}7 center distribute over the four remaining lower orbitals with a total spin S = 1/2. This low spin configuration is compatible with three possible orbital arrangements as indicated in Scheme 8(A-C):

Scheme 7.

yx2-y2 yx2-y2 yx2-y2

Scheme 8. Electronic configuration of iron in a square planar ligand field as expected for DNIC with {3d}7 in solution.

The three configurations may be distinguished from the EPR properties. According to Griffith [110], the g-factor values for these orbital arrangements are

Scheme A : g|| = 2.0, g± = 2 + 6a2ß2Ç/AEZ2-(xz,yz)

SchemeB : g||= 2.0 - 8a2ß2Ç/AE(x2-y2)-(xz,yz) g± = 2 + 2a2ß2£/AExy-(xz,yz), where f is a one-electron constant of spin-orbital interaction, a2fi2 are covalency coefficients and AEz2-(xz,yz) is the splitting between the respective levels.

Both the schemes A and B have oblate axiality with gx > g||and are compatible with experimental values. For scheme C, Griffith predicts g||> g^ = 0 which is incompatible with experiment. Schemes A and B are clearly distinguished by the position of the ^z2 orbital of the iron. This orbital has high density in axial directions and is not strongly disturbed by the ligands in the xy plane. In contrast, the tyxz,yz,xy orbitals have strong interaction with the ligands, in particular with the low-lying orbitals of NO+ ligands. This interaction can significantly decrease the energy of the ^xz,yz,xy orbitals to below that of the ^z2 orbital and lead to the realization of the scheme A for DNIC in solution.

Isotopic substitution of iron with the 57Fe (I = 1/2) supports this interpretation. For 57Fe-DNIC with ethylxanthogenate (Fig. 11, curve c [56]), we find experimental values for the HF couplings |Ax|(57Fe) = 1.7 ± 0.2 mT, |A|||(57Fe) = 0.3 ± 0.3 mT, |Aiso|(57Fe) = 1.20 ± 0.05 mT [39,56]. The signs of the couplings can be deduced from the following considerations. Transition metals have negative values for Aiso = (2Ax + A||)/3 [51,111]. This negative average is only compatible with negative signs for both Ax and A| |. Therefore, A|| = -0.3 mT and Ax = -1.7 mT. The HF coupling appears as the sum of an isotropic Fermi contact term and an anisotropic dipolar interaction described by the traceless tensor T = diagfe, ty, tz):

Since ti = Ai — Aiso, we find a positive value for tz = A|| — Aiso = +0.9 ± 0.3 mT. Since tz is the spatial average of the dipolar interaction between the nuclear moment of iron and the unpaired electron wavefunction we have the usual relation r2 — 3z2 tz = —g^gNPn(^e |-r5-|^e)

The positive value of this quantity shows that the unpaired electron must have strong overlap with the orbital of the iron atom.

The proposed square-planar structure of thiol-DNIC is fully compatible with the formation of diamagnetic dimeric DNIC by cofacial binding as indicated in Scheme 9. Optical and GR (Mossbauer) spectroscopy [43,44] have confirmed the dimeric nature of this complex. We propose that the dimer be composed of two cofacial monomers rotated relative to each other through 180°. In this geometry, the oppositely charged RS— and NO+ ligands are positioned above each other, and provide electrostatic stabilization of the dimer.

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