Redetermination of (acetonitrile-κN)dicarbonyl(η5-cyclopentadienyl)iron(II) tetrafluoridoborate

The crystal structure of the title compound, [Fe(C5H5)(CH3CN)(CO)2]BF4, of which only the coordinates of the non-H atoms of the cation have previously been reported [Fadel et al. (1979 ▶). Z. Anorg. Allg. Chem. 453, 98–106] has been redetermined. The FeII atom in the complex cation is coordinated by a cyclopentadienyl ring, two carbonyl ligands and an acetonitrile molecule displaying a three-legged piano stool structure. Three of the four F atoms of the BF4 − anion are disordered over two sets of sites, with a site-occupancy factor of 0.709 (10) for the major occupied site.

The crystal structure of the title compound, [Fe(C 5 H 5 )-(CH 3 CN)(CO) 2 ]BF 4 , of which only the coordinates of the non-H atoms of the cation have previously been reported [Fadel et al. (1979). Z. Anorg. Allg. Chem. 453, 98-106] has been redetermined. The Fe II atom in the complex cation is coordinated by a cyclopentadienyl ring, two carbonyl ligands and an acetonitrile molecule displaying a three-legged piano stool structure. Three of the four F atoms of the BF 4 À anion are disordered over two sets of sites, with a site-occupancy factor of 0.709 (10) for the major occupied site.

Experimental
Crystal data [Fe(C 5  The crystal structure of the title compound ( Fig. 1), of which only the coordinates of the non-H atoms of the cation have previously been determined (Fadel et al., 1979) has been reinvestigated, now with the location of the tetrafluorido borate anion and improved R factors for the present model.
The Fe II atom in the cation is coordinated by a cyclopentadienyl ring, two carbonyl ligands and an acetonitrile molecule displaying a three-legged piano-stool structure. Three of the four F atoms of the BF 4anion are disordered over two sites with a site occupancy factor of 0.709 (10) for the major occupied site.
It is noteworthy, that Callan et al. (1987) determined the closely related structure of dicarbonyl-(η 5 -cyclopentadienyl)-(N-methyl cyanido)-iron(II) tetrafluoridoborate which has a methylisocyanide molecule bonded to the iron(II) atom instead of an acetonitrile molecule as in the title compound.

Refinement
H atoms bonded to C were refined using a riding model, with C-H = 0.95 Å and with U iso (H) = 1.2U eq (C) or with C-H = 0.98 Å and with U iso (H) = 1.5U eq (C methyl ). The methyl group was allowed to rotate but not to tip. Three of the F atoms of the BF 4anion are disordered over two sites with a site occupancy factor of 0.709 (10) for the major occupied site and were refined with isotropic displacement parameters. The B-F bond lengths and the F···F distances were restrained to be equal.

Figure 1
Reaction scheme for obtaining the title compound.

Figure 2
Perspective view of the title compound with the atom numbering scheme; displacement ellipsoids are at the 50% probability level. Only the major occupied site of the disordered BF 4anion is shown.

Crystal data
[Fe(C 5 H 5 )(C 2 H 3 N)(CO) 2 ]BF 4 M r = 304.82 Monoclinic, P2 1 /c Hall symbol: -P 2ybc a = 6.8842 (7)  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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (