The molecular structure of fumaric acid and the deformation of carboxyl group geometry on crystallization

The molecular structure of fumaric acid was studied by means of gas electron diffraction at 260° C. The molecular parameters and their standard deviations obtained for a C_2h model are (r_a distances in Å, angles in degrees): CO: 1.202(0.002), C-O: 1.341(0.006), C-C: 1.486(0.004), CC: 1.356(0.016). C-C-O: 112.1(1.0), C-CO: 124.4(1.1), C-CC: 121.8(1.2). From the available data on carboxylic acids the weighted average deformation of the structure of a carboxylic group on crystallization was determined; a significant expansion of the O-H bond (0.040 Å ), the CO bond (0.010 Å ) and the C-C-O bond angle (1.5° ) and a shrinkage of the C-O bond (0.041 Å ), the C_α-C bond (0.012 Å ) and the C-CO bond angle (2.0° ) was found. The energy for these deformations was estimated to be 1.8 kcal mol^?1.     

Estimates for systematic empirical corrections of consistent 4–21G ab initio geometries and their correlations to total energy group increments

The structural parameters of the completely relaxed 4–21G ab initio geometries of more than 30 basic organic compounds are compared to experimental results. Some ranges for systematic empirical corrections, which relate 4–21G bond distances to experimental parameters, are associated with total energy increments. In general, for the currently feasible comparisons, the following corrections can be given which relate calculated distances to experimental r_g parameters and calculated angles to r_s-structures For CC single bond distances, deviations between calculated and observed parameters (r_g) are in the ranges of ?0.006(2) to ?0.010(2) Å for normal or unstrained hydrocarbons; ?0.011(3) to ?0.016(3) Å for cyclobutane type compounds; and +0.001(5) to +0.004(4) Å for CH₃ conjugated with CO. For CO single bonds the ranges are ?0.006(9) to +0.002(3) Å for CO conjugated with CO; and ?0.019(3) to ?0.027(9) Å for aliphatic and ether compounds. A very large and exceptional discrepancy exists for the highly strained ethylene oxide, r_s — r_e = ?0.049(5) Å and in CH₃OCH₃ and C₂H₅OCH₃ the r_s — r_e differences are ?0.029(5), ?0.040(10) and ?0.025(10) Å. Some of these discrepancies may also be due to deficiencies of the microwave substitution method caused by atomic coordinates close to inertial planes. For CN bonds, two types of NCH₃ corrections are from +0.005(6) to ?0.006(6) and from ?0.009(2) to ?0.014(6) Å; and the range for NCO is +0.012(3) to +0.028(4) Å. For isolated CC double bonds the range is + 0.025(2) to +0.028(2) Å. For conjugated CC double bonds the correction is less positive (+0.014(1) Å for benzene). For CO double bonds the corrections are ?0.004(3) to +0.003(3) Å. For bond angles of type HCH, CCH, CCC, CCO, CCO, OCO, NCO and CCC the corrections are of the order of magnitude about 1–2° (or better). Angles centered at heteroatoms are less accurate than that, when hydrogen atoms are involved. Differences in HOC and NHC angles were found in a range of ?2.3(5)° to ?6.2(4)°.     

A new approach to electron diffraction analysis of symmetric triatomic molecules with large-amplitude bending motion: The equilibrium geometry,force field and vibrational frequencies of BaI2 as determined by electron diffraction

Diffraction data on BaI₂, analyzed by a new approach, indicate an anharmonic potential with a barrier of 71(12) cm^?1 at a linear geometry. The structural and vibrational parameters were found to be r_e^h(Ba-I_o) = 3.150(7)Å, ∠_eIBaI = 148.0(9) °, f_q = 0.69(8) mdyn/Å,f_qq= 0.14(6) mdyn/Å, k₂ = ?0.0075(15) mdyn/Å, k₄ = 0.0025(9) mdyn/ų, v₁ = 106(12) cm^?1 and v₃ = 145(21) cm^?1. The bending frequency v₂ is predicted to be near 16 cm^?1.     

An electron-diffraction study of the molecular structure of gaseous perbromyl fluoride and calculation of its force field and vibrational amplitudes

The molecular structure of FBrO₃ has been studied by gas-phase electron diffraction. Least-squares refinements of the molecular geometry using fixed spectroscopic amplitudes revealed two geometrical minima. Initially, the amplitudes employed were derived from diagonal force fields obtained by spectroscopic least-squares refinements to fit observed and calculated wave numbers; for each geometry there are two spectroscopic minima. In the lowest geometrical minimum the wave number agreement is poor, however, the introduction of the ∠OBrO/∠FBrO interaction force constant removed the discrepancies; the resulting force field is F(Br-O) = 6.92 ± 0.02 mdyn Å^?1F(Br-F) = 3.22 ± 0.03 mdyn Å^?1, F(∠OBrO) = 1.06 ± 0.02 mdyn Å, F(∠FBrO) = 0.81 ± 0.03 mdyn Å, F(∠OBrO/∠FBrO) = ?0.19 ± 0.02 mdyn Å. In the corresponding geometrical minimum r_g(Br-O) = 1.582 ± 0.001 Å, r_g(Br-F) = 1.708 ± 0.003 Å, r_α(∠OBrO) = 114.9 ± 0.3°, r_α(∠FBrO) = 103.3 ± 0.3°. Perpendicular amplitude correction coefficients, calculated for each force field employed, were used throughout to relate the interatomic distances through the r_α-structure. The geometries of the r_α^o- and r_e-structures are estimated.     

The molecular structures of cis-3-hexene and trans-3-hexene in the gas phase by electron diffraction and molecular mechanical calculations

The molecular structures of cis-3-hexene and of trans-3-hexene in the gas phase have been determined by electron diffraction combined with molecular mechanical calculations. For cis-3-hexene the data indicate the presence of the (+ac, +ac) and the (?ac, +ac) forms. In trans-3 -hexene three rotamers were observed, with an energy sequence E(+ac, +ac) ≈ E(?ac, +ac) < E(ac, sp). The refined r_α⁰-structural parameters are: cis-3-hexene: C-H = 1.073 Å, CC = 1.330 Å, C(sp²)-C(sp³) = 1.505 Å, ∠CCH(in CH₂) = 111.1°, ∠CCC = 111.4°, ∠(CC-C) = 129.1° trans-3-hexene: C-H = 1.078 Å, CC = 1.342 Å, C(sp²)-C(sp³) = 1.506 Å, ∠CCH(in CH₂) = 109.3°, ∠CCC = 112.8, ∠CC—C = 124.1°The agreement between calculated and experimental geometries and vibrational amplitudes is good.     

Molecular structure and conformation of cis-1,3- dichloro-1-propene as determined by gas phase electron diffraction

The molecular structure and conformation of cis-1,3-dichloro-1-propene have been determined by gas phase electron diffraction at a nozzle temperature of 90°C. The molecule exists in a form in which the chlorine atom of the methyl group and the carbon-carbon double bond are gauche to one another. The results for the distance (r_g) and angle (∠_α) parameters are: r(C-H) = 1.078(10)Å, r(CC) = 1.340(5)Å, r(C-C) = 1.508(7)Å, r( =C-Cl) = 1.762(3)Å, r(C-Cl) = 1.806(3)Å, ∠Cl-C-C = 111.7°(1.8), ∠(CC-C) = 125.5°(1.5), ∠Cl-CC = 124.6°(1.6) and ∠H-C-Cl = 111°(5). The torsion-sensitive distances close to the gauche form can be approximated using a dynamic model with a quartic double minimum potential function of the form V(Φ) = V₀[1 + ($\text{Φ}\text{Φ}{}_0$⁴ - 2($\text{Φ}\text{Φ}{}_0$)²], where V_o = 1.1(8) kcal mol^?1 and Φ₀ = 56°(5) (Φ = 0 corresponds to the anti form).     

Comparison of free and metal coordinated 1,4-disubstituted-1,4-diaza-1,3-butadienes : Crystal and molecular structures of 1,4-dicyclohexyl-1,4-diaza-1,3-butadiene and trans-[dichloro(triphenylphosphine)(1,4-di-tert-butyl- 1,4-diaza-1,3-butadiene)palladium(II)]

The crystal and molecular structures of c-Hex-DAB (c-hexyl-NC(H)C(H)N-c-hexyl; DAB = 1,4-diaza-1,3-butadiene) and of trans-[PdCl₂(PPh₃)(t-Bu-DAB)] are reported. Crystals of c-Hex-DAB are monoclinic with space group C_2/c and cell constants: a = 24.70(1), b = 4.660(2), c = 12.268(3)Å, β = 107.66(4)°, Z = 4. The molecule has a flat E-s-trans-E structure with bond lengths of 1.258(3)Å for the CN double bond and 1.457(3)Å for the central CC′ bond. These bond lengths and the NC-C′ angle of 120.8(2)° indicate that the C- and N-atoms are purely sp²-hybridized and that there is little or no conjugation within the central DAB skeleton. Crystals of trans-[PdCl₂(PPh₃)(t-Bu-DAB)] are triclinic with space group P-1 and cell constants: a = 17.122(3), b = 18.279(3), c = 10.008(5)Å, α = 96.77(2), β = 95.29(3), γ = 109.79(2). Z = 4. The t-Bu-DAB ligand is coordinated to the metal via one lone pair only. In this 2e; σ-N coordination mode the E-s-trans-E conformation of the free DAB-ligand is still present and the bonding distances within the DAB-ligand are hardly affected. [CN: 1.261(10)Å; CC′: 1.479(10)Å (mean).] The PdN-, NC- and central CC′-bond lengths are compared with those found in other metal -R-DAB complexes.     

Spectra and structure of organophosphorus compounds—XXX. Vibrational and conformational study of methoxy difluorophosphinoxide-d0 and -d3

The i.r. (4000-40 cm⁻¹) and Raman (4000-10 cm⁻¹) spectra of gaseous, liquid and solid methoxy difluorophosphinoxide, CH₃OP(O)F₂, and the deuterated analog have been recorded. Results obtained from variable solvent and matrix isolation studies are consistent with the existence of both trans (CO bond trans to the PO bond) and gauche (dihedral angle approximately 120° from the trans form) conformers in the fluid phases. From simulations of observed gas phase i.r. band profiles, it was possible for assignments to be made to the individual conformers for a number of the fundamentals. Variable temperature studies carried out for the gaseous and liquid phases give energy differences between the gauche and trans conformers of 451 ± 100 cm⁻¹ (1.29 ± 0.3 kcal/mol) and 69 ± 20 cm⁻¹ (197 ± 57 cal/mol), respectively. Furthermore, these data are consistent with the gauche form being the thermodynamically preferred conformer for the gas phase whereas the trans conformer is preferred in the liquid phase and the only conformer present in the annealed solid. The methoxy torsional mode of the gauche conformer has been assigned to a very strong band observed in the far i.r. spectrum of the gas phase at 42 cm⁻¹. The matrix isolation spectra of the normal compound in Ar, CO and N₂ matrices indicated no changes in the conformational equilibrium among these different matrices and this equilibrium remains unchanged upon annealing the matrices.     

On the geometry and internal rotation of XSCF3 compounds (X = F,Cl and CF3)

The molecular structures (r_α⁰ values) for XSCF₃ with X = F, Cl and CF₃ have been determined by electron diffraction of gases. While the geometry (C-F bond length and FCF angle) of the CF₃ groups and the bond angle at the sulfur atom depend very little on the substituent X, the S-C bond length increases with decreasing electronegativity of X from 1.805 (3) Å for X = F to 1.824 (6) Å for X = Cl. Torsional force constants for the CF₃ groups were derived from vibrational amplitudes. A strong increase of this force constant is observed between FSCF₃ (f_τ = 0.09 (2) mdyn Å) and CISCF₃ (f_τ = 0.18 (5) mdyn Å). The torsional frequencies derived from the electron diffraction experiment agree very well with the values observed in the far IR spectra for CISCF₃, and CF₃SCF₃. A force field for CF₃SCF₃ has been derived from IR and Raman data.     

Triferrocenyl-komplexe von wolfram(VI): Molekülstruktur im festkörper und dynamisches verhalten in Lösung von triferrocenyl-ferrocenoxy-oxo-wolfram,WO(OFc)Fc3

Triferrocenyltungsten complexes of the type WO(X)Fc₃ (X = Cl, (1), OMe (2), OFc (3) and OⁿBu (4)) were obtained by treating WOCl₄ with ferrocenyllithium, FcLi, in tetrahydrofuran solution¹. Reaction of WOCl₄ with a threefold excess of FcLi gives 1, which may be converted into 2 using KOCH₃. Reaction of WOCl₄ with a sixfold excess of FcLi gives a mixture containing 3 und 4 in addition to ferrocene and biferrocene. According to the X-ray crystallographic analysis, WO(OFc)Fc₃ (3) has a trigonal-bipyramidal structure with three ferrocenyl ligands occupying the equatorial positions and an axial ferrocenoxy group coordinated trans to the oxo ligand. The three WC(ferrocenyl) (average 2.092 Å) and the OC(ferrocenyl) (1.33(1) Å) bond distances are remarkably short. The axial tungsten—oxygen distances correspond to a WO double and a WO single bond (1.705(5) and 1.945(5) Å), respectively. The ¹H and ¹³C NMR spectra of WO(OFc)Fc₃ (3) are temperature-dependent. This is ascribed to a hindered rotation of the ferrocenyl ligands around the WC(ferrocenyl) bonds; the free activation enthalpy ΔG^≠(T_c) of this intramolecular dynamic process is 62.5 ± 0.5 kJ mol⁻¹.     

Selenoketene substitution structure

Microwave studies (26.5–40 GHz) of further isotopic species of selenoketene formed by pyrolysis of 1,2,3-selenodiazole (¹²CH₂¹²C^76,77,82Se, ¹²CH₂¹³C⁸⁰Se and ¹³CH₂¹²C⁸⁰Se) and by pyrolysis of 5-deuterio-1,2,3-selenodiazole (¹²CHD¹²C^78,80Se) are reported. In conjunction with earlier results for ¹²CH¹²C^78,80Se an r_s structure has been derived with distances SeC (1.706 Å), CC (1.303 Å), CH (1.0908 A) and a HCH bond angle of 119.7°. The geometry of the CH₂C moiety of selenoketene is closer to allene, CH₂CCH₂, than to ketene, CH₂CO.     

Substituent and coupling effects in the IR spectra of arylidenecyclanones

By studying the IR spectra of arylidenecyclanones possessing exocyclic CC double bonds, a correlation has been found between the wave number and intensity values of the ν CO and ν CC vibrations and the substituent constants. Transmissive factors of the CC double bond have been calculated and these indicated that the most pronounced conjugation occurs in the case of the indanones and the smallest for 1-thiochromanones. It has also been established that the substituent effect transmitted by the CC double bond is mainly of a resonant character. Coupling of the ν CO and ν CC vibrations has been studied by the method of Taylor and Smith. It has been concluded that in the case of arylidene-indanones possessing strong coupling, of the two intense bands between 1600 and 1800 cm⁻¹ the one found at higher wave number is a result of an out-of-phase (ν_as) coupling of the ν CO and ν CC vibrations while the other originates from an in-phase (ν_s) coupling. Coupling was less in the case of the other compounds investigated. Our studies prove that for the evaluation of the substituent effects the coupling phenomena should be taken into consideration.     

Microwave spectra of isotopic glycolaldehydes,substitution structure,intramolecular hydrogen bond and dipole moment

The microwave spectra of ¹³CH₂OH-CHO, CH₂OH-¹³CHO, and CH₂OH-CH¹⁸O are reported and have been used in combination with previously published data on other monosubstituted glycolaldehydes to determine the substitution structure of the molecule as r(CO) = 1.209 Å, r(C-O) = 1.437 Å, r(C-C) = 1.499 Å, r(O-H) = 1.051 Å, r(C-H_ald) = 1.102 Å, r(C-H_alc) = 1.093 Å, r(O β H) = 2.007 Å, r(O β O) = 2.697 Å, ∠(C-CO) = 122°44', ∠(C-C-H_ald) = 115°16', ∠(C-C-O) = 111°28', ∠(C-O-H) = 101°34', ∠(C-C-H_alc) = 109°13', ∠(H-C-H) = 107°34', ∠(O-H β O) = 120°33', ∠(H β OC) = 83°41', and ∠(O-H, C0) = 24°14'. The intramolecular hydrogen bond and the other structural parameters are discussed and compared to related molecules. The dipole moment is redetermined to be μ_a = 0.262 ±0.002 D, μ_b = 2.33 ± 0.01 D, and μ_tot = 2.34 ± 0.01 D. Relative intensity measurements yielded 195 ± 30 cm^?1 for the C-C torsional fundamental and 260±40 cm^?1 for the lowest in-plane skeletal bending mode. Computations performed by the CNDO/2 method correctly predict the observed cis hydrogen-bonded conformer to be the energetically favoured one and in addition yield some indication of the existence of at least two other non-hydrogen-bonded forms of higher energy.     

A re-investigation of the molecular structure of trifluoroacetic acid by means of gas phase electron diffraction

Analysis of the electron diffraction patterns of trifluoroacetic acid at 140°C indicates the existence of one conformation with the CF₃-group rotated 17.3± 0.9° from a position with a fluorine atom eclipsed with respect to the CO bond. The data does not exclude the possibility of free internal rotation but it seems improbable.The important bond lengths, r_g(1), and bond angles with their standard deviations in parentheses, are: C-F: 1.325 (0.003), C-C: 1.546 (0.005), CO: 1.192 (0.003), C-O: 1.353 (0.014) Å, C-C-F: 109.5 (0.3), C-CO: 126.8 (0.8), C-C-O: 111.1 (0.9)°.     

The molecular structure of gaseous tetrachloro-p-benzoquinone and tetrachloro-o-benzoquinone by electron diffraction

The structures of tetrachloro-p-benzoquinone and tetrachloro-o-benzoquinone (p- and o-chloranil) have been investigated by gas electron diffraction. The ring distances are slightly larger and the carbonyl bonds slightly smaller than in the corresponding unsubstituted quinones. The molecules are planar to within experimental error, but small deviations from planarity such as those found for the para compound in the crystal are completely compatible with the data. Values for the geometrical parameters (r_a distances and bond angles) and for some of the more important amplitudes (l) with parenthesized uncertainties of 2σ including estimated systematic error and correlation effects are as follows. Tetrachloro-p-benzoquinone: D_2h symmetry (assumed); r(CO) = 1.216 Å(4), r(CC) = 1.353 Å(6), r(C-C) = 1.492 Å(3), r(C-Cl) = 1.701 Å(3), ∠C-C-C = 117.1° (7), ∠CC-C1 = 122.7° (2), l(CO)= 0.037 Å(5), l(CC) = l(C-C) - 0.008 Å(assumed) = 0.049 Å(7), and l(C-Cl) = 0.054 Å(3). Tetrachloro-o-benzoquinone: C_2v symmetry (assumed); r(CO) = 1.205 Å(5), r(CC) = 1.354 Å(9), r(C_cl-C_cl) = 1.478 Å(28), r(Co-C_cl) = 1.483 Å(24), r(C_o-C_o) = 1.526 Å(2), r(C-Cl)= 1.705 Å(3), <C_o-CO = 121.0° (22), ∠C-C-C = 117.2° (9), ∠C_co, ClC-Cl = 118.9° (22), ∠C_ccl, ClC-Cl = 122.2°(12), l(CO) = 0.039 Å(5), and l(C_cl-C_cl) = l(C_o-C_cl) = l( Co-Co) = l(CC) + 0.060 Å(equalities assumed) = 0.055 Å(9). Vibrational'shortenings (shrinkages) of a few of the long non-bond distances have also been measured.     

Untersuchung von Eisen(III)-phosphiten im Hinblick auf die Wasserstoffbindungen

Iron(III) phosphites, vic. Fe₂(HPO₃)₃·9 H₂O, FeH₃P₂O₆·3 H₂O, FeH₆P₃O₉·H₂O and Fe₄H₃₃P₁₅O₄₅·6 H₂O were studied by means of powder X-ray, thermographic, IR and UV spectroscopy methods and by measurement of magnetic susceptibility. From the results obtained, and from analogy with phosphites studied earlier, the following structural model can be proposed: in the compounds studied, every iron atom is surrounded by six oxygen atoms of the water molecules and phosphite or, polyorthophosphite anions which form a weak ligand field of approximately octahedral symmetry. In Fe₂(HPO₃)₃·9 H₂O, symmetry of the anion is decreased from the point group C_3v to the C_s group. This anion is characterised by two bonding distances between phosphorus and oxygen atoms,r _PO=1,46 Å andr _{PO 2}=1,50 Å, the respective force constants beingK _PO=8.7 mdyn/Å andK _PO2=7.1 mdyn/Å. Three types of hydrogen bonds occur in the crystal lattices of the compounds studied. The weakest bond (bond lengthr=2.86–2.88 Å, bond energyE=4.6–5.0 kcal/bond) is formed between molecules of hydrate water, its energy approaching that of the hydrogen bond in liquid water. The stronger hydrogen bond (r=2.67–2.70 Å,E=5.7 to 8.0 kcal/bond) is found between water molecules and phosphite or polyorthophosphite anions. The strongest hydrogen bond (r=2.55–2.64 Å) is formed by polyorthophosphite anions, linking hydroxyl groups to oxygen atoms bound to different phosphorus atoms.     

The structure of octachloropentafulvalene

The structure of octachloropentafulvalene has been redetermined by a 3-dimensional X-ray crystallographic analysis. The space group is monoclinic, C2/c, and cell parameters are a = 14·998, b= 7·9110, c= 11·8068 Å, β= 103·38°. The X-ray intensity data were measured on a computer-controlled diffractometer using Mo radiation. The least-squares structure refinement used anisotropic temperature factors for C and Cl and gave a final R of 0·036. Corrections were applied for absorption, Cl dispersion and secondary isotropic extinction. The central CC is twisted by 37° as a result of Cl?Cl repulsions from one C₅Cl₄ half to the other. The central CC length, 1·365 Å, is only slightly larger than the standard 1·34 Å ethylenic value. The bond distance increase can be explained in terms of a decrease in π-bond character accompanying the CC twist. Carbon-carbon distances in the 5-ring have typical values for the cyclopentadienylidene moiety; there is no evidence for single-double bond delocalization.     

Darstellung,Kristallstruktur, Schwingungsspektren und Normalkoordinatenanalyse von K2[OsCl5(CO)] · H2O

Synthesis, Crystal Structure, Vibrational Spectra, and Normal Coordinate Analysis of K₂[OsCl₅(CO)] · H₂O The X-ray structure determination of K₂[OsCl₅(CO)] · H₂O (monoclinic, space group P2₁/c a = 13.600(2), b = 7.122(1), c = 22.186(11) Å, β = 98.66(3)°, Z = 8) revealed two crystallographic independent bat very similar complex anions [OsCl₅(CO)]^2? with rough C_4v point symmetry. Due to the stronger trans influence of the carbonyl group the bond lengths in the Cl? Os? CO axis Os? Cl = 2.449(2), 2.430(2) Å are langer as compared with the octahedron basis Os? Cl = 2.340-2.370 Å. The water of crystallization is coordinated to potassium (K? OH₂ = 2.625-2.815 Å). Using the molecular parameters the IR and Raman spectra are assigned by normal coordinate analysis. The valence force constants are f_d(CO) = 15.30, f_d(OsC) = 3.88, f_d(OsCl) = 1.81, f_d(OsCl) = 1.36, f_d(OH) = 7.65, 7.82, 7.79 mdyn/Å. The strengthening of the Os? C bond by stronger back donation of the Os^III(d⁵) complex in comparison with the isostructural Os^IV (d⁴) compound is discussed.     

An electron diffraction investigation of the molecular structure of dichloromaleic anhydride

The molecular structure of gaseous dichloromaleic anhydride has been investigated by electron diffraction at a nozzle-tip temperature of 164–170°C. The molecule is planar to within experimental error, but small deviations from planarity corresponding to torsion up to about 10° around the carbon-carbon single bonds cannot be ruled out. Values of the more important r_α distances and angles with estimated 2σ uncertainties are r(CO) = 1.188(2) Å, r(CC) = 1.332(5) Å, r(C-O) = 1.389(3) Å, r(C—C) = 1.495(3) Å, r(C—Cl) = 1.685(2) Å, ∠CC-Cl = 129.4(2)°, ∠C-CO = 128.5(4)° and ∠CC—C = 107.9(2)°. The shortening of the carbonyl bond relative to that in maleic anhydride itself is discussed in terms of a possible general effect of vicinal substitution.     

Preparation and some reactions of organogermylmercuryplatinum complexes and related compounds

A single-crystal X-ray diffraction study of tetracarbonyl-ferra-3-cyclopentene-2,5-dione has been made. Formally the compound can be derived from maleic anhydride by substitution of the bridging oxygen by Fe(CO)₄. Accordingly the bonding character is similar to that of maleic anhydride. The ironcarbon distances in the ring indicate partial double bonds. The octahedrally coordinated iron atom is linked to four terminal carbon monoxide ligands, with a longer bond distance to the equatorial than to the axial ones (FeC_ax 1.809 Å, FeC_eq 1.854 Å). The axial CO groups are strongly inclined towards the ring (C_axFeC_ax 164°). The latter effect is explained by electronic repulsion of the CO groups.IR, ¹H NMR, and ¹³C NMR data are reported. Crystal data: space groupPnama:α  12.708(10),b  10.058(7),c  7.527(5) Å;Z  4. With 625 reflections [F_o > 3o(F_o)] the structure has been refined anisotropically (hydrogen isotropically) to R0.022.