Tetrakis(μ-pivalato-κ2 O:O′)bis[(2-methylpyridine-κN)iron(II)](Fe—Fe)

The asymmetric unit of the title compound, [Fe2(C5H9O2)4(C6H7N)2], contains one unique Fe-atom site located close to a centre of symmetry which generates the molecular dimer. The two Fe atoms are bridged by four carboxylate groups and are each coordinated by a molecule of 2-picoline. Electron counting and the 18-electron rule suggest that a chemical single bond is likely to exist between the two Fe atoms, which are separated by a distance of 2.8576 (4) Å. This bond completes an approximately octahedral coordination environment around each Fe atom.

The asymmetric unit of the title compound, [Fe 2 (C 5 H 9 O 2 ) 4 -(C 6 H 7 N) 2 ], contains one unique Fe-atom site located close to a centre of symmetry which generates the molecular dimer. The two Fe atoms are bridged by four carboxylate groups and are each coordinated by a molecule of 2-picoline. Electron counting and the 18-electron rule suggest that a chemical single bond is likely to exist between the two Fe atoms, which are separated by a distance of 2.8576 (4) Å . This bond completes an approximately octahedral coordination environment around each Fe atom.

S1. Comment
In a systematic study of the reactions of iron powder with simple carboxylates and aromatic amines to prepare iron(II) carboxylates, the title compound, (1) (Fig. 1) resulted, under ambient reaction conditions. Both monomeric molecular complexes (Celengil-Cetin et al., 2000) and extended iron(II) carboxylates have previously been prepared (Weber, 1980) using similar methods.
In the present study, the iron atoms are only coordinated to five ligands each, with a total donation of 10 electrons. This gives a total of 16 electrons on both Fe, thus a Fe-Fe bond needs to be present to fill the outer orbitals on Fe. The relatively short Fe(1)-Fe(1) interaction distance (d(Fe(1)-Fe(1)) = 2.8576 (4) Å) is likely evidence for such iron-iron bond.

S2. Experimental
Iron powder (1.0 g) was refluxed in a solution of 2-picoline (C 6 H 7 N, 10.0 ml), pivalic acid (4.0 g) and water (1.0 ml) for 5 h under an inert atmosphere. The obtained yellow solution was filtered while hot under an inert atmosphere and the filtrate afforded green crystals upon cooling slowly at room temperature.

S3. Refinement
The methyl hydrogen atoms were constrained to tetrahedral geometry with C-H distances of 0.98 Å and U iso (H) = 1.5U eq (C). The positions of each set of three of the H atoms of the methyl groups constrained to tetrahedral geometry were refined so as to optimize the overlap with the observed electron density (AFIX 137). The H atoms bonded to the aromatic C atoms were constrained to ride on their parent atom in a distance of 0.95 Å in an ideal geometry and with

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.