Structure and bonding of the tetrameric cyclopentadienyliron carbonyl

Paramagnetic organometallic molecules. 2. Electrochemical investigation of the tricobalt carbon cluster. Alan M. Bond , Barrie M. Peake , Brian H. Rob...
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3389

Structure and Bonding of the Tetrameric Cyclopentadienyliron Carbonyl Monocation, [Fe,( h5-C,H,) (CO) 4 ] +. Stereochemical Effect Due to Oxidation of a Completely Bonding Tetrahedral Metal Cluster System Trinh-Toan, W. P. Fehlhammer, and Lawrence F. Dahl* Contribution from the Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706. Received August 24, 1971 Crystaltine [Fe4(h5-C5H5)4(C0)4][PF~] has been prepared and structurally characterized by three-dimensional X-ray diffraction techniques. The paramagnetic [Fer(h5-C5H5)4(C0)4]+ monocation, which retains the same molecular framework of the neutral parent analog, is composed of a bonding tetrahedron of iron atoms with triply bridging carbonyl ligands situated above the four triangular faces and with a pentahaptocyclopentadienylring coordinated to each iron atom. A detailed comparison of geometries shows only a slight change of the central Fer(CQ4core from cubic Tdgeometry in the neutral molecule toward a distorted tetrago@ DZdgeometry in the monpcation, together with a decrease of the iron-iron distances from a mean value of 2.520 A to a mean value of 2.484 A. The cubic-deformed geometry of the monocation may be ascribed to a Jahn-Telltr distortion upon which there :i superimposed crystal-packing effects. The average C-0 bond length of 1.188 A in the m:nocation is 0.015 A shorter than that in the neutral molecule, whereas the average Fe-CO bond length of 1.984 A is virtually identical with that in the neutral molecule. These observed changes in molecular parameters along with an increase of 80 cm-l in the carbonyl stretching frequency on oxidation of the parent tetramer to give the PFs- salt are rationalized with an MO correlation diagram. This energy-level diagram emphasizes for a completely bonding organometallic cluster system which has no electrons in the strongly antibonding metal symmetry orbitals that little change in the metal-metal distances may be expected on oxidation but that a large increase in metal-metal distances should be observed on reduction. Crystals of [Fe4(h5-C5H5)4(CO)2][PF6] are orthorhombic with symmetry Pnma and lattice parameters a = 18.717 (4), b = 9.721(2),c = 13.760(3)A; dabsd= 1.94vs.d,,1,~= 1.965gcm-abasedonfourformula species per unit cell. The structurewas solved with 1745independent nonzero diffraction maxima [Le,, I > 2.0, ~ ( l )collected ), with an automated diffractometer. The space group necessitates that each [Fe.(h5-C5H5)(C0)I4+ monocation and each PF6- anion in the unit cell possess C,-m crystallographic site symmetry with the octahedrallike PFs- anion randomly disordered in one of two crystal orientations. Evidence is presented that two of the three independent C5H5 rings are also distributed among two crystal orientations. Least-squares refinements by one model involving anisotropicthermal parameters for all nonhydrogen atoms and by a second model involving D5* idealized rigid pentagons for the disordered C5H5rings yielded unweighted R1values of 5.9 and 7.7 %, respectively, with all corresponding atomic coordinates and calculated molecular parameters for the central Fe4(C0)4core being equivalent for both models within statistical error (uiz., 9". Intensities were monitored by the 8-28 scan technique at a 2.0°/min rate with stationary crystalExperimental Section stationary counter background counts of 15 sec taken o n each side Preparation and Properties. The neutral Fe4(h6-C6Hs)4(CO)4 of the scan. Three standard reflections sampled at intervals of every 80 reflections revealed no indications of either electronic instacomplex was prepared by King's method.5 The cationic species was bility or crystal decay. formed by oxidation of Fe4(hS-C5Hs)4(CO)a dissolved in CHzClz The treatment of intensity data including correction for backwith an equimolar amount of AgPF6 (Ozark-Mahoning Co.) in ground and Lorentz-polarization effects has been described preCHzC12 solution. The almost black precipitate was filtered off, viously. l 8 calculated transmission coefficients, based o n the linear washed with a CHzCl2-ether mixture, and extracted in a Soxhlet absorption coefficient for Mo K a radiation of 24.7 cm-l, ranged apparatus with methanol for several days. Since metallic silver is from 0.45 to 0.52 (corresponding to 3.6% fluctuation in F,'s), and also formed in the redox reaction, it was necessary to use glass extherefore absorption corrections were performed.18 N o extinction traction thimbles with fritted disks in order to keep the reduced corrections were made. After mergingz0of the equivalent reflecsilver powder from mixing with the extracted solution. Alternations for the three octants, 1745 independent reflections were obtively, a solution of the product in methanol or acetone-methanol tained (corresponding to one reciprocal lattice octant of intensities was first filtered through a fritte packed with cellulose powder and for Dzh symmetry) with IF,I > 447,). then evaporated to dryness followed by a n extraction in a Soxhlet Unit Cell and Space Group. The [Fe4(/~a-CsHj)4(CO),][PF6] salt apparatus with methanol for 3-4 days. After this period, dark crystallizes to give an orthorhombic unit cell of dimensions a = green crystals which were formed in the slowly cooled flask were 18.717 (4), b = 9.721 (2) and c = 13.760 (3) A;21 the cell volume is recrystallized from acetone solution to yield single crystals of size - ~ method) 2503.6 A3. The observed density of 1.94 g ~ m (flotation suitable for X-ray diffraction analysis. agrees well with the calculated value of 1.965 g c m P based o n four The above reactions were carried out under nitrogen and with formula species per cell. The total number o f electrons per unit nitrogen-saturated solvents in order to exclude moisture because, cell, F(OOO),is 1476. although the complexes are relatively stable to air, some reagents Systematic absences of { Okl] for k 1 odd and of ( hkO] for h odd such as AgPF6 are easily hydrolyzable. CHzC12 was first treated denote the probable space group as either PnZla (nonstandard axial consecutively (a) with a n aqueous solution of NaHC03, (b) with orientation of PnaZl (Czu9,no. 33)) or Pnma (DZh16,no. 62). The dilute H2S04, and (c) with distilled water; it was then predried with choice of the latter centrosymmetric space group was suggested by anhydrous NaZSO4and refluxed over PzObfor 2-3 days under a the distribution of the normalized structure factor magnitudes calnitrogen atmosphere before distillation. C H a O H was dried by its culated from the program FAME.^^ These statistical averages of ( E ) being refluxed over CaHzunder nitrogen. = 0.830, ( / E l z )= l.W(rescaled), and (/Elz- 1) = 0.935 with /El Anal. Calcd for CZ4HZOF6Fe4O4P: C, 38.91; H , 2.72; Fe, > 1, 31.1 %; \El > 2,4.4%; and lE\ > 3,0.2% are reasonably con30.15; P,4.18. Found: C, 38.95; H, 2.77; Fe, 29.94; P,4.03.l1 sistent with those expected from a centrosymmetric crystal strucFor the paramagnetic [Fep(hS-CjHj)P(CO)4][PF6] salt a magnetic t ~ r e .This ~ ~ choice was subsequently confirmed by the crystalmoment of Meff = 2.2 BM was obtained at 23" by the Faraday lographic analysis and successful refinement of the structure. The method,lZ while a magnetic moment of perf = 1.91 BM was despace group Pnma necessitates that each of the four cations and termined at 35" by the nmr method in acetone. These values are anions possess crystallographic site symmetry C,-m. For this space comparable with magnetic moments of 2.13 and 2.03 BM reported group the crystallographically independent atoms occupy either the by King5 for two samples of [Fe4(hS-CjH5)4(C0)4]Br3 at room temeightfold set of general positions ( 8 4 or the fourfold set of special perature by the Faraday method. A KBr pellet infrared spectrum of [Fe,(h6-CsHs)c(CO)4][PF6] positions (4c) on mirror planes. These positions are as follows: X, '12 - Y , '12 - Z ; 2 , '/z Y , 5; '/z - X, F, 8d, + ( x , Y , Z ; '/z gives one single carbonyl band at 1700 (vs) cm-l, in close agreement VZ z ) ; 4c, i ( x , V4,z ; VZ - x , 3 / 4 , lh with the single peak reported6 at 1695 cm-1 for both the chloride Determination and Refinement of the Structure. Initial coordiand tribromide salts13 and at 1700 cm-1 for the pentaiodide salt. nates for three independent iron atoms under Pnma symmetry werc Most of the other absorption bands for the PFB- salt at 3120 (w), obtained from a vector analysis of a three-dimensional sharpened 1425 (m), 1355 (w), 1010 (w), 945 (w), 855 (s), 840 (s), 820 (s), and Patterson functionz6by the use of the computer program PHASE.^^ 550 (m) cm-1 are in accord with those of the KBr pellet spectrum obtained by Kings for the tribromide salt. One or more peaks in the 820-855-~m-~region and the one at 550 cm-l may be ascribed (16) A. S. Foust, Ph.D. Thesis (Appendix), University of Wisconsin at least partially to the vibrational modes of the PF6- anion, since (Madison), 1970. the KPF6 salt as a mineral oil mull shows ir-active bands at 830 (s) (17) Local version of Argonne National Laboratory "Orientation and 558 (m) cm-1 attributedl4 to normal vibrational modes of the and Angle Setting Generation Program," Program B-101,1965. (18) CY. V. A. Uchtman and L. F. Dahl, J . Amer. Chem. Soc., 91, PF6- anion. 3756 (1969). In the calculation of ui*(F), the so-called "ignorance" Single-Crystal X-Ray Data Collection. For the collection of infactor, E, was empirically assigned a value of 0.0016. tensity data, a n approximately cubic-shaped crystal of dimensions (19) J. F. Blount, DEAR,an absorption correction program based 0.35 X 0.32 X 0.34 mm was selected by optical examination from on a method of W. R. Busing and H. A. Levy, Acta Crystalbgr., 10, the available crystals and mounted on a glass fiber with epoxy ce180(1957). ment. Precession and Weissenberg photographs displayed &h-2/ (20) J. C. Calabrese, SORTMERGE, a Fortran program for sorting and m2/m2/m Laue symmetry characteristic of the orthorhombic sysmerging crystallographic intensity data, University of Wisconsin (Maditem. The crystal was optically aligned about the rotation axis son), 1968. (21) The uncertainties of the lattice parameters obtained from least (corresponding to the a axis) on a General Electric full-circle, Datexsquares16 are estimates of precision and are unrealistically small due controlled diffractometer and then centered in the X-ray beam.16 to the large number ofreflections used in the determination. After 26 representative diffraction maxima were carefully centered, (22) R. B. K. Dewar and A . L. Stone, "Fame and Magic, Fortran Computer Programs for Use in the Symbolic Addition Method," University of Chicago, 1966; Cf.E. B. Fleischer, R. B. K. Dewar, and A. L. (1 1) Analyses were performed by Galbraith Laboratories, Inc., Stone, Abstracts of Papers, American Crystallographic Association, Knoxville, Tenn. 37921. Winter Meeting, Atlanta, Ga., 1967, p 20. (12) We wish to acknowledge Dr. Michael Camp for carrying out (23) (a) Cf,G. H. Stout and L. H. Jensen, "X-Ray Structure Dethis measurement for us. termination," Macmillan, New York, N. Y., 1968, p 321; (b) I. L. (13) A somewhat lower carbonyl frequency of 1675 (s) cm-l is reKarle, K. S. Dragonette, and S. A. Brenner, Acta Crystallogr., 19, 713 corded by Kings from an infrared KBr pellet spectrum of the tribromide (1965). salt. (24) "International Tables for X-Ray Crystallography," Vol. I, (14) G. M. Begun and A. C. Rutenberg, Inorg. Chem., 6,2212 (1967). Kynoch Press, Birmingham, England, 1965, p 151. (15) T. C. Furnas, Jr., "Single Crystal Orienter Instruction Manual," (25) The Patterson and Fourier maps were computed with the Blount General Electric Co., Milwaukee, Wis., 1966.

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Journal of the American Chemical Society

94:lO

May 17, 1972

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3391

(a)

(a) Figure 1. Each octahedral-like PFe- anion is randomly distributed in one of two orientations about a crystallographic site of C,-m symmetry such that only the P, F(2), and F(4) atoms (which lie on a crystallographic mirror plane) are whole-weighted (owing to the superposition of the two orientations for these three atoms). The other four fluorine atoms are statistically distributed in one of two orientations with F(l), F(3), F(5), and F(5’) corresponding to halfweighted atoms in the orientation a and F(6), F(6’), F(7), and F(7’) representing the half-weighted atoms in the other orientation (b). The 20% probability thermal ellipsoids of the atoms are shown for both PFs- orientations which are approximately related to each other by a 45” rotation about the F(2)-F(4) axis. This program revealed a tetrahedron of four iron atoms, of which two (uiz., Fe(1) and Fe(2)) reside on the crystallographic mirror plane at y = 1/4, while the other two (uiz., Fe(3) and Fe(3’)) are related to each other by the mirror plane. The Fe-Fe distances ~$1culated from these initial positions ranged from 2.44 t o 2.55 A. The program PHASE also correctly assigned the position of the phosphorus atom o n the mirror plane, but this position was not used in the initial refinement process but instead was obtained from the subsequent Fourier synthesis. The coordinates and assigned isotropic temperature factors of the three independent iron atoms were refined by one least-squares c y ~ l e 2 ~to- R1 ~ ~and Rt values30 of 39.8 and 42.1 %, respectively. Further determination and refinement of this structure was not straightforward owing to both a crystal disorder of the PFs- anion (Figure 1) and a crystal disorder of two of the three independent CSHSrings. Successive Fourier syntheses coupled with least squares eventually led to the assignment of coordinates for all nonhydrogen atoms (with each of the C s H j rings initially assumed to be ordered). After several cycles of least-squares refinement with anisotropic thermal parameters utilized for all nonhydrogen atoms, convergence was obtained with R1 = 5.9 and R P = 6.8%. During the last cycle, n o individual positional parameter changed by more than 0.20-except for one fluorine parameter which changed by 0.440-and several cyclopentadienyl carbon parameters which still varied by 1 . 4 ~ - 1 . 9 ~ . At the termination of this anisotropic refinement, bond lengths salt were completely and angles within the [Fe4(h5-CsHs)a(CO)4][PF6] consistent with expected values except for those of the CbHsrings 2 and 3 (where rings 1,2, and 3 denote thepentahaptocyclopentadienyl ligands attached to Fe(l), Fe(2), and Fe(3), respectively). In this anisotropic refinement each C j H s ring was assumed to be ordered with rings 1 and 2, which are bisected by a crystallographic mirror plane, each comprised of three crystallographically independent car-

(b)

(C)

Figure 2. View normal to the mean plane of each of the three crystallographically independent CsHs rings showing the cyclopent adienyl carbon atoms, refined by a crystal-ordered anisotropic leastsquares model, as 20% probability thermal ellipsoids: (a) ring 1 (bisected by a crystallographic mirror plane) attached to Fe(l), (b) ring 2 (bisected by the same mirror plane) attached to Fe(2), and (c) ring 3 (whose atoms are all located in general positions) attached to Fe(3). Whereas the thermal ellipsoids of ring 1 are “normal” in accord with expected bond lengths and angles, the “abnormal” thermal ellipsoids (and apparent nonplanarity) of rings 2 and 3 have been interpreted in terms of a crystal-disordered model involving two orientations of Dsh-idealized rigid C ~ H pentagons. S bon atoms with one atom located on the mirror plane (Figure 2). Ring 1 showed almost perfect planarity, with the largest perpendicular displacement of the individual carbon atoms from the idealized plane being only 0.004 A. Furthermore, for ring 1 the three independent intraring angles of range 105.9 (18)-109.4 (13)’ compare favorably with those in other cyclopentadienyl metal complexes. However, for both rings 2 and 3 wide ranges of C-C distances and intraring angles were observed together with pronounced deviations of the rings from planarity. Definite evidence of crystal disorder rather than high librational ring motion for rings 2 and 3 is shown in Figure 2, which pictures the thermal ellipsoids, calculated from the anisotropic refinement, for three independent CjHs rings. In contrast to the normal thermal ellipsoids in ring 1 , the thermal ellipsoids of three carbon atoms in each of the rings 2 and 3 are markedly elongated relative to those of the other two carbon atoms. It would be expected that a librational motion per se would uniformly spread the electron density of each carbon atom such to give rise to elongated thermal ellipsoids approximately within the mean plane of the five carbon atoms. In addition, a three-dimensional Fourier difference map showed some residual electron-density peaks between adjacent carbon atoms for rings 2 and 3 but not for ring 1 . These observations were interpreted as a n indication of crystal disorder for rings 2 and 3. Despite the large variations in C-C distances, the individual Fe-C(cyclopentadienyl) ring distances for rings 2 and 3 as wel) as for ring 1 were all within 0.04 A of the average value of 2.09 A. The distinct nonplanarity of rings 2 and 3 together with the small range of Fe-C(cyclopentadieny1) distances implies that these rings have preferred crystal orientations which are tilted but not noticeably skewed31 with respect to the Fe-(ring centroid) axis. T o the extent that this ring tilting (which most likely is due t o the molecular packing in the crystal) actually occurs, the crystal-ordered anisotropic model would less adequately represent the actual situation. In an attempt to provide a better electron-density representation of rings 2 and 3, idealized rigid-body CsHjarings of Djh symmetry (withoassumedC-C bond lengths of 1.405 A and C-H distances of 1.08 A) were placed in two orientations (which appeared to best fit the anisotropic thermal ellipsoids) and refined by least squares.a2 For this crystal-disordered CsHs ring model a statistical disorder was assumed (corresponding t o half-weighted carbon and hydrogen atoms in the two orientations of each disordered ring), with variable isotropic temperature factors for the carbon atoms and fixed ones for the hydrogen atoms. This rigid-body refinement was terminated at R1 = 7.7 and R2 = 9 . 7 x . A final Fourier difference map showed the highest three peaks of 1.0-1.4 e/A3 to be in the vicinity of the ring carbon atoms. Nevertheless, a n examination showed the individual isotropic thermal parameters of the carbon atoms (Table I) to be reasonable. Most importantly, a comparison between the corresponding determined atomic coordinates and calculated bond

program: J. F. Blount, Ph.D. Thesis (Appendix), University of Wisconsin (Madison), 1965. (26) J. C. Calabrese, Ph.D. Thesis (Appendix), University of Wisconsin (Madison), 1971. (27) W. R. Busing, K. 0. Martin, and H. A. Levy, “ORFLS, A Fortran Crystallographic Least-Squares Program,” ORNL-TM-305, Oak Ridge National Laboratory, Oak Ridge, Tenn., 1962. (28) The atomic scattering factors used for all atoms except hydrogen are those based on Hartree-Fock-Slater calculations (H. P. Hanson, F. Herman, J. D. Lea, and S. Skillman, Acfa Crystallogr., 17, 1040 (1964)); for hydrogen the scattering factors are those of R. F. Stewart, E. R. Davidson, and W. T. Simpson, J . Chem. Phys., 42, 3175 (1965). Real and imaginary anomalous dispersion corrections were made for theironatoms(uiz., Af’ = 0.4andAf” = l.0for MoKaradiation).28 (29) D. H. Templeton in ref 24, Vol. 111, 1962, p 215. (31) Cf.J. L. Calderon, F. A. Cotton, and P. Legzdins, J . Amer. Chem. (30) RI = [Z/lFo/ - lFcll/ZIFoll~OO and R Z = [ZwillFol lFol12/ Soc., 91, 2528 (1969); F. A. Cotton and G. A. Rusholme, ibid., 94, ZwilF,l 21’/2100. All least-squares refinements were based on the 402 (1972). minimization of Z w i A F i 2 ; the weights were assigned according to the (32) “DBCGHW, A Fortran Crystallographic Least-Squares Rigidrelationship wi = l/ui2(Fo),1* Body Program,” University of Wisconsin (Madison), 1965.

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Trinh-Toan, Fehlhammer, Dah1

Structure and Bonding of [Fec(h5-C5H~>4(C0)4]+

3392 Table I. Final Positional and Thermal Parameters of [Fe,(h5-C5H5),(Co)4][PF6] (Model 11)”~~ Atom

Y

X

Z

0.47497 (9) 0.25000 0,34439 (10) 0.25000 0.38864 (7) 0.12224 (12) 0.35868 (20) 0.25000 0.3018 (13) 0.2500 0.2960 (8) 0,2500 0.4129 (23) 0.2500 0.4198 (6) 0.2500 0.3554 (20) 0.0972 (23) 0.3966 (1 5) 0.1416 (31) 0.3267 (15) 0.1368 (24) 0.2486 (5) 0.2500 0.5046 (6) 0.2500 -0.0037 (12) 0.4216 (5) 0.2500 0.3079 (7) 0.2500 0.4635 (7) 0.0979 (15) 0.4149 (6) 0.5407 (12) 0.2500 0.3673 (17) 0.5566 (7) 0.3238 (14) 0.5796 (5)

(i) Individual Atomic Parameters B~~( x 104) B~~(x 104) B~~( x 104) B~~(x 104) B~~( x 104)

0.18960 (14) 21.2 (5) 0.14929 (13) 23.3 (6) 0.29201 (10) 30.8 (4) 0.70775 (29) 34.0 (13) 0.7839 (16) 49 (9) 0.6362 (11) 84 (6) 69 (13) 0.6230 (36) 0.7798 (8) 57 (5) 0.6995 (27) 199 (25) 0.6470 (21) 112 (16) 0.7722 (17) 111 (13) 0.3181 (7) 22 (3) 0.4004 i9j 43 (4) 0.1026 (9) 73 (4) 0.2843 (9) 28 (4) 0.3352 (11) 30 (4) 0.1485 (10) 45 (4) 0.0621 (20) 44 (8) 0.1254 (15) 29 (4) 0.2164 (13) 17 (3)

140.8 (25) 280.8 (42) 96.6 (15) 143.6 (50) 425 (66) 493 (35) 466 (146) 494 (33) 146 (25) 352 (83) 283 (45) 380 (23) 329 (24) 365 (21) 153 (18) 176 (20) 263 (23) 526 (83) 299 (31) 225 (20)

71.6 (13) 45.9 (11) 80.5 (10) 70.0 (25) 70 (14) 163 (13) 175 (32) 115 (9) 255 (40) 159 (26) 132 (18) 77 (7) 108 (9) 225 (12) 66 (9) 83 (10) 115 (10) 161 (24) 191 (18) 209 (16)

0.oc 0.oc

- 6 . 3 (7) 0.oc 0,oc 0.oc 0.oc 0.oc 22 (19) 68 (28) - 56 (18) 0.oc

0.oc

- 37 (8) 0.oc 0.oc -24 (8) 0.oc -9 (10) - 13 (6)

823

( x 104)

5 . 4 (7) 0.0c - 4 . 4 (6) 0.0c - 4 . 1 (5) 14.0 (10) 9 . 1 (14) 0.0c 0.0c 25 (8) 0.0c -48 (7) 43 (15) 0.0c - 14 (5) 0.p 85 (22) 20 (29) 40 (14) -73 (35) 16 (11) 73 (23) 0.0c 13 (3) -42 (5) 0.0c -240 (15) 36 (6) 0.0c -1 (5) 0.0c - 4 (6) -54 (13) 15 (5) 61 (12) 0.0c 77 (18) 34 (7) - 2 (15) 14 (6)

(ii) Rigid-Body Cyclopentadienyl Ring Parametersd Rigid-bod y group

Basis x

Basis y

Basis z

Ring 2 Ring 3 Ring 3P

0.2789 (5) 0.3757 (8) 0.3795 (8)

0.2954 (12) -0.0182 (18) - 0.0372 (14)

0.0594 (7) 0.3690 (11) 0.3502 (8)

X

C( 10) C(11) C( 12) C(13) ~(14) ~(15) C(16) C(17) C( 18) C(19) C( 15P) C(16P) C(17P) C(18P) C(19P) W10) HU1) H(12) H(13) ~(14) ~(15) H( 16) H(17) H(18) ~(19)

0.2415 0.2414 0.2931 0.3252 0.2933 0.3370 0.4110 0.4362 0,3777 0.3164 0.3727 0,4376 0.4222 0.3478 0.3172 0.2077 0.2075 0,3060 0.3670 0,3063 0.3021 0.4430 0,4908 0.3796 0.2629 0.3666 0.4900 0.4607 0.3192 0.2610

H( 16P) H(17P) H(18P) H( 19P)

0

P

- 1.018 (13)

2.515 (10) 2.533 (20) 3.008 (17)

-0.498 (14) 0.780 (25) 1.082 (14)

1.890 (26) 1.546 (17)

(iii) Atoms of Disordered Cyclopentadienyl Rings Y

Atom

H(15P)

$e

0.3536 0.2117 0.1854 0.3111 0.4151 0.0492 0.0328 - 0.0541 - 0.0914 -0.0275 0.0209 - 0.0222 - 0.0861 -0.0825 -0.0164 0.4063 0.1360 0.0860 0.3253 0.5233 0.1102 0.0788 - 0.0866 -0.1575 - 0.0359 0,0734 - 0,0086 -0.1302 -0.1234 0.0025

Z

B, A z

0.1165 0.0974 0.0258 0.0006 0,0567 0.4191 0.4317 0.3577 0,2992 0.3372 0.4262 0,3847 0.2956 0.2819 0,3627 0.1681 0.1317 -0.0046 - 0.0526 0.0542 0.4644 0.4884 0.3474 0.2362 0.3085 0.4949 0.4159 0.2462 0.2202 0.3739

7 . 1 (5) 7 . 3 (6) 8 . 6 (7) 6 . 3 (5) 8 . 3 (6) 8 . 3 (9) 6.6 (5) 7 . 9 (6) 21.6 (24) 8 . 4 (9) 7 . 0 (5) 10.5 (11) 6 . 0 (4) 5 . 3 (4) 8 . 2 (8) 10.0, 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0

Anisotropic temperature factors of the form a Estimated standard deviations of the last significant figure are given in parentheses. exp( - [Bllh2 B 2 2 k 2 B 3 J 2 2B12hk 2B13hl + 2Bz3k[I)were used. c The location of some atoms on a mirror plane ( y = l/4) requires the anisotropic thermal coefficients B12and & to be zero by symmetry. d The rigid-group atomic positions are defined relative to an orthonormal coordinate system x’,y ’ , Z’ as follows: the origin of the coordinate system lies on the centroid of the ring, x passes through one atom of the ring, and all atoms lie in the xy plane. The rigid-group angles 0, p define the orientation of the rigid-group coordinate system with respect to the crystallographic axis system. The hydrogen atoms of the cyclopentadienyl rings lie ?long vectors extended from the ring centroid through the carbon atoms such that the rings conform to D6h symmetry with C-C = 1.405 A and C-H = 1.08 A. 4, 0, and p are reported in radians. f Isotropic thermal parameters of hydrogen atoms were not varied in the course of refinement.

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lengths and angles for both the central Fe4(CO)( cluster and PF6anion showed all of them to be equivalent for both models within statistical error (uiz.,