1973 1439Crystal and Molecular Structure of Di-p-carbonyl-cis-p-(l-5-q : 1'-5'-q-dicyclopentad ienyldi met hylsilane) bis( carbonyl iron) (Fe-Fe)By John Weaver and Peter Woodward," Department of Inorganic Chemistry, The University, Bristol BS8 1 TSCrystals of the title compound are monoclinic. space group P2,ln. with Z = 4 in a unit cell of dimensions: a =11.66, b = 13.27, c = 10.82 A, p = 105" 50'. The structure was determined from diffractometer data by thesymbolic addition method and refined to R 0.072 for 1309 independent reflections. The silicon atom i s tetra-hedrally 0 bonded to two methyl groups and to two cyclopentadienyl rings; the latter are in turn each x-bonded toone iron atom. The two iron atoms are bonded to one another (2.51 A) and are also linked by two symmetricalcarbonyl bridges; the four-membered ring so defined is non-planar, being folded along the Fe-Fe axis away fromthe C5 rings a t an angle of ca.160". Each iron atom also carries one terminal carbonyl group, which makes theoverall co-ordination approximately octahedral.The Si atom does not lie in the Fe,(CO),(terminal) plane, nor are the cyclopentadienyl rings symmetrically relatedto it (though their centroids lie in it), and the overall symmetry of the molecule i s only C,(rn). Reasons for thisconfiguration are discussed. The Fe-C and C-0 distances are as expected ; the four Si-C distances relativelyequal, mean 1.88 8.CRYSTALS of the title compound Fe,[x-C,H,*Si(Me,)*-C5H4](CO), were prepared by heating under reflux amixture of (C,H,),SiMe, and Fe(CO), in octane for 20 hunder nitr0gen.l The structural interest of the com-pound lies in its relation to the known structures ofcompounds such as [Fe(CO)2(x-C5H,)],,2,3 in which thecyclopentadienyl rings are not connected to one another,[ (x-C,H5),Fe,(CO),],~Ph2PCCPPh2,4 which contains asimilar iron carbonyl cyclopentadienyl moiety, and[Fe(x-C,H4*[CH(NMe2)]2*C5H4)(CO),],S in which thecyclopentadienyl rings are linked by two-carbon bridges.EXPERIMENTALDark red crystals were obtained as thin plates developedin the (010) plane.The dimensions of the crystal chosenfor X-ray diffraction (0.36 x 0.06 x 0.41 mm) weremeasured on a binocular microscope and the crystalmounted on b as rotation axis. Lattice parameters weremeasured from Weissenberg and precession photographs,and Mo-K, radiation was used throughout.Intensities1 S. Moorhouse, Ph.D. Thesis, University of Bristol, 1971.0. S. Mills, Acta Cryst., 1958, 11, 620.R. F. Bryan and P. T. Greene, J . Chem. Soc. ( A ) , 1970,3068.were measured for reciprocal layers 120-91 on a Buerger-Supper-Pace 0-01 "-incrementing two-circle autodiffracto-meter, with equi-inclination geometry and a stationary-background-w-scan-stationary-background sequence. Thediffractometer angular settings T and # were input frompunched paper tape prepared on an Atlas computer.6For each reflection hkl a scan interval of (2-0 + 0.4/L)Owas used, where L is the Lorentz correction, with a scanspeed of 1" min-l; during this scan a total count, N,,was recorded.The stability of the apparatus and ofthe crystal were monitored by remeasuring the intensityof one particular reflection on each layer between every20 measured intensities. No deviation > 3 . N34 was ob-served. Peak profiles were checked on a chart recorder;this also served t o confirm the accuracy of centring of 9.Background counts were taken for 0.5 min both beforeand after each scan ( N , and N , respectively). The in-tensity of the reflection I(hkZ) is then proportional to[N3 - t,(N1 + N,)] and the structure factor F(hkZ) =A. J. Carty, T. W. Ng, W. Carter, G. J. Palenik, andT. Birchall, Chem. Comm., 1969, 1101.F. S. Stephens, J . Chem. SOC. ( A ) , 1970, 1722.J. C. Cuppola, TAPSET, A Fortran programme for calcul-ation of diffractometer setting anglesJ.C.S.Dalton[l(hkZ)]*(s/Lp)*, where s is a scale factor arbitrarilyassessed at 10, p is the polarisation correction and t, isthe scan time in min; the standard deviation a~(hkZ) =(s/L+)[N, + tS2(N1 + N2)]1/2F(hRZ). Structure factors andtheir standard deviations were computed from the punchedpaper tape output of the diffractometer by means of aspecially written data reduction programme on an Atlascomputer.' Reflections for which [N3 - t,(N1 + N 2 ) ] /[N3 + t,(N1 + N,)]* <2-0 were regarded as ' unobserved 'and were not included in the final refinement; the totalnumber of ' observed ' reflections was 1403.RESULTSCrystal Data.-Cl,Hl,Fe2Si0,, M = 410.1, Monoclinic,= 11*66(1), b = 13*27(1), c = 10*82(1) A, p = 105" 50'TABLE 1Atomic positional and thermal parameters, withAtom %laFe( 1) 0.5129(2)Fe(2) 0*5806(1)s1 0.4655(3)Carbonyl groupsC(l) 0-5120(11)0(1) 0.5090(9)0*6014(11) g:) 0.6157(10)C(3) 0-6678(11)O(3) 0.7692(8)0.43 14( 10) gp) 0.3319(9)Methyl groupsC(5) 0.3079(12)C(6) 0*5417(11)Cyclopentadienyl ringsC(11) 0.4686(9)C(12) 0*5724( 10)C(13) 0-5413(11)0-421 1( 12) :[:$ 0-3733(10)0.549 1 ( 1 0)0.6756( 10)C(23) 0*6967(12)C(24) 0*5863(11)C(25) 0*4962(10)Y lb0*1657( 1)0.3462( 1)0.2 7 7 5 (3)0.1 35 7 (9)0.1 179(7)0.3 7 3 4 ( 1 0)0.3955 (9)0.2 2 62 (9)0*1961(7)0.2873(9)0.3 138 (7)0*3193( 11)0*2552(9)0.1 585 (8)0.0978 (9)0.03 6 3 ( 1 0)0.1 183(9)0.3743(8)0*3754( 9)0.4507(10)0-4980(9)0.451 O( 9)0.021 9(10)Hydrogen atoms (not refined)H(12) 0.665 0.111H(14) 0-374 - 0.01 1H(13) 0.595 - 0.039H(16) 0.279 0.143H(22) 0.744 0-320H(23) 0.784 0.472H(24) 0.574 0.564H(25) 0.395 0.467standard deviations in parentheses* B = 8x2U.ZIC0.22 19(2)0.2419(2)0-4914(3)0*0652(13)0*0957(14-0*0063(11]0.2532 (1 1)0.2734(9)0*1640( 11)0*1094(9)-0.0405(10)0.4584( 13)0*6664( 12)0*3984( 10)0-4058(11)0.3 103 (1 2)0.2472 ( 12)0-2995( 11)0*4237(11)0.437 7 ( 1 2)0.353 9 ( 1 3)0.29 1 7 (1 2)0.3347(11)0.4750.2890.1660.2690.5000.3370.2280.302u x 102(A2) *3.35(5)3.1 4 ( 6)3.1 6 (9)4*1(3)6.7 (3)4.9 (4)7.9(3)5.4(3)5*9(3)3*8(3)3.2 (3)5.1(4)4.3(5)2*6(2)3*6(3)4.5(3)4-3(3)3.6(3)3.0(3)4.0(3)4*6(3)3.8(3)3.3(3)4.44.44.44.44.44.44.44-4(lo'), D, (by flotation) = 1-60,Z = 4, D, = 1-65, F(000) =832.Space group P2,/n. Mo-K, X-radiation, A = 0.7107 A; y(Mo-K,) = 11-44 crn-l.The structure was solved by the symbolic additionmethod and by successive electron-density differencesyntheses. Attempts to refine the structure with isotropicS. F. Watkins, SPADD, a Fortran data reduction programmefor use on an Atlas computer.8 M. M. Woolfson, ' Direct Methods in Crystallography,'Clarendon Press, Oxford, 1961, pp. 88-92.thermal parameters for all atoms gave R 0.119 (R' 0.0'74);the quantity minimised was C{zo]F,} - lFc])2), and in thefirst instance w was taken as l / o ~ ( h k Z ) ~ .Further im-provement in R was obtained by ' smoothing ' the back-ground count so as to reduce the effect of random variations ;reflections for which N , and N , differed by >[30(Nl) +3o(N,)] were also deleted at this stage. The number ofmeasured intensities used in the final refinement wasthereby reduced to 1309. Individual weights were thenrecalculated according to the scheme: l/w = o2 = oP2 +IFo] + 0.002 IF0I2, devised so as to keep w(AF), as nearlyconstant as possible for ranges of F, and of (sin O/h).Convergence was obtained at R 0.072 (R' 0,082), and thevalue of the ' error-of-fit ' (= [ Z W ( A F ) ~ / ~ ~ - nv]), whereno = number (1309) of observations and n, = number ofvariables) was 1.09. As there was no evidence on the elec-tron-density maps for anisotropic thermal motion of eitherthe iron or silicon atoms, no correction was applied. Finallythe hydrogen atoms of the cyclopentadienyl rings wereincorporated a t calculated positions, assuming a CTH1-10 and an isotropic thermal parameter of 3.5 A2.Neither the positional nor thermal parameters of the hydro-TABLE 2Molecular geometry *(a) Distances (A)Fe(1)-Fe(2) 2-512(3)Fe(l)--C(l) 1.74(1)Fe(l)-C(3) 1.92(1)Fe(1)-C(4) 1.89(1)C(1)-0(1) 1*16(1)C(3)-0(3) 1-21(1)Fe(1)-C( 11) 2.1 1( 1)Fe (1)-C( 12) 2.12 (1)Fe ( 1 )-C ( 1 3) 2 * 1 1 ( 1)Fe(1)-C( 14) 2-08(2)Fe(1)-C(15) 2-11(1)C(11)-C(l2) 1.43(2)C(12)-C(13) 1.41(2)C( 13)-C(14) 1*38(2)C(14)-C(15) 1-40(2)C( 15)-C(11) 1-41 (2)Si-C(11) 1.88(1)Si-C(5) 1.86(1)Fe (2)-C (2)Fe (2)-C( 3)Fe (2)-C (4)C(2)-0 (2)C(4)-0 (4)Fe (2)-C (2 1)Fe (2)-C(22)Fe (2)-C (2 3)Fe (2)-C(24)Fe(2)-C(25)C( 22)-C (23)C (2 3)-C (2 4)C (24)-C (25)C (25)-C (21)Si-C(21)Si-C(6)C(2 1)-c (22)1*70( 1)1-87(1)1.88(1)1*20(2)1.2 1 (1)2-13(1)2.13(1)2-08(1)2*08(1)2.1 O ( 1)1-43(2)1*40(2)1*42(2)1 * 40( 2)1*41(2)1*88(1)1-58(1)(b) Some intramolecular non-bonding distances (A)Fe(1) - - - Si 3.448(5) Fe(2) - - * Si 3.449(5)C(11) - - - C(21)C(5) - * * C(6)C(3) - * * C(4)Si.. . C(3) 4*00(1) Si . . . C(4) 3.45(1)3*00(2)3.14(2)2-77(2)* Estimated standard deviations (in parentheses) includecell parameter errors.(c) Angles (")Fe(1)-C( 1)-0(1)Fe (1 )-C (3) -0 (3)Fe ( 1 )-C (4) -0 (4)Fe(2)-C( 2)-0 (2)Fe (2)-C( 3)-0 (3)Fe (2)-C (4)-0 (4)C( 1)-Fe( 1)-C( 3)C ( 1 )-Fe ( 1 )-C( 4)C(1)-Fe( 1)-Fe(2)C (2)-Fe (2)-C( 3)C (2)-Fe (2)-C (4)C(2)-Fe (2)-Fe ( 1)C( 3)-Fe (1)-C(4)C (3)-Fe( 2)-C( 4)Fe (1)-C( 3)-Fe (2)Fe (1 )-C( 4)-Fe (2)C(15)<(11)-C(I2)C( 1 1 )-c (1 2)-C( 13)178-1(11)136.1 (1 1)138.0( 10)178.5 (1 3)140.8 (10)138.q 10)9 1 -4 (5)90.3 (6)102.8 (4)9 1 * 9 (6)91-4(6)104-3(6)93-5(5)95.3(5)83-0(6)83*7(5)106*8(11)108*5( 11)C( 12)-C( 13)-c ( 1 4)C( 13)-C( 14) -C ( 1 5)C(14)-C(16)-C(ll)C (2q-C (2 l ) - C (22) c (2 1 )-c (22)-c (23) c (22)-C (23)-c(24) c (23)-C( 24)-C( 25)C(24)-C(25)-C(21)Si-C(ll)-C(l5)Si-C ( 1 1 1 2 ) )-C(Si-C(2 1)-C(25)Si-C(2 1)-C(22)C( 1 1)-Si-C( 21)C (5)-Si-C (6)C(5)-Si-C(l1)C (5)-Si-C(2 1)C(6)-Si-C( 1 I )C( 6)-Si-C( 2 1)106-8( 13)1 10-7 ( 13)107.1 (12)106*6( 11)108.2 ( 1 2)108.1 (1 3)107*9( 12)127-1 (9)125.8 (8)126*0(9)126-8(9)106-3(6)11 4*2( 6)l08-0(6)1 0 8- 2 { 6)109.4( 5)110*2(6)109-2( 111973 1441gen atoms were refined.The final R 0.072 (R' 0.082)resulted from a refinement in which the average shift-to-error was 0.01, and a final electron-density differencesynthesis showed no peaks >0.7 or < -0.6 eA-3. Bondlengths and angles with their respective standard deviationsTABLE 3(a) Some least-squares planes and lines : distances (A) of relevantatoms from these planes are given in square bracketsPlane (1) : Cyclopentadienyl ring 1[C(ll) -0.007, C(12) -0*001, C(13) 0.010, C(14) -0.014,C(15) 0.0131-5.1676~ - 8.0150~ + 8.20362 = -0.4236Plane (2) : Cyclopentadienyl ring 2[C(21) 0.017, C(22) -0.006, C(23) -0-008, C(24) 0.018.C(25) -0.0221-0*7140~ + 8.7537~ + 7.97822 = 6.2816Plane (3) : Fez and (CO),(terminal)[Fe(l) 0.008, Fe(2) 0.015, C(l) 0*008, C(2) 0.002, 0(1)-0.017, O(2) 0.003, Centroid ring (1) -0.002, Centroid - .,ring (2) -0.017110.4846~ - 3.9552~ + 0.68002 = 4.8663Plane (4) : Fez and p ( C 0 ) group (1)[Fe(l) 0.006, Fe(2) 0.005, C(3) -0-022, O(3) 0.011]-2.0678~ - 0.4170~ + 10.76282 = 1.2553Plane ( 5 ) : Fez and p(C0) group (2)[Fe(l) 0.002, Fe(2) 0.003, C(4) -0.012, O(4) 0.0061-5.7431~ + 0.9945~ + 10.4796~ = -0.4559Plane (6): Fe(l), Fe(2), Si9.2711~ - 3.8166~ + 3.21052 = 4.8371Plane (7): Si, C(11), C(21)8.1368~ - 3.8018~ + 4.77342 = 5.0812Plane (8): Si, C,(Me), V-(CO)~[Si 0.022, C(3) -0.044, O(3) 0.015, C(4) 0.045, O(4)C(5) -0.020, C(6) -0.OOOlJ3.4199% + 12.6305~ + 0.06762 = 5.1271Line (9) : Si, C(11)0.01 7,x = 0.4662, y = 0.2186 + 0*0632T,* z = 0.4439 + 0.0504T* T is a parameter which can assume all real values.Line (10): Si, C(21)x = 0-5086 + 0.0448T, y = 0.3278 + 0.0518T1 z = 0.4586- 0.0352TLine (11): Fe(l), Fe(2)x = 0.5467 + 0*0268T, y = 0.2558 + 0*0719T, z = 0.2321 + 0.0079T(b) Angles (") between least-squares planes and lines[;;I[;\) g:: [;;:;;\) 4;:;(3)-(7) 22.8 (3)-(8) 89.5(1)-(3) 89.6 (2)-(3) 89.3were computed from the full variance-covariance matrixobtained in the last refinement cycle.Positional andthermal parameters are in Table 1, interatomic distancesin Table 2, and some least-squares planes in Table 3.No absorption correction was applied, and the atomict For details of Supplementary Publications see Notice t oAuthors No. 7 in J.C.S. Dalton, 1972, Index issue. Items lessthan 10 pp. are sent as full-sized copies.9 H. P. Hanson, F. Herman, J. D. Lea, and S. Skillman,Acta Cryst., 1964, 17, 1040.scattering factors used were those of ref. 9. All com-putational work was carried out on an Atlas computerusing the ' X-Ray System of programs.1° Observedand calculated structure factors are listed in SupplementaryPublication No. SUP 20703 (5 pp.).?DISCUSSIONOur crystallographic study confirms that the overallgross structure of the molecule is that expected fromspectroscopic considerations.1 The silicon atom istetrahedrally a-bonded to two methyl groups and totwo cyclopentadienyl rings, while the latter are inturn each n-bonded to one iron atom.The two ironatoms are bonded to one another (Fe-Fe 2-51 A) andare also linked by two symmetrical carbonyl bridges.The Fe&, ring so defined is non-planar, with an anglebetween the two Fe-C-Fe planes of ca. 160". Eachiron atom carries one terminal carbonyl group on theside away from the cyclopentadienyl ring. The co-ordination state of the iron atoms can be regarded asoctahedral if the cyclopentadienyl rings are assumedto occupy three sites and any iron-iron interaction isignored; the 18-electron rule is obeyed only if thepresence of an iron-iron bond is assumed.A stereo-scopic view of the molecule drawn by computerll isgiven in Figure 1. The position of the molecule in itsmonoclinic unit cell, its relation to the symmetryelements of the space group, and the system of number-ing the atoms, are illustrated by a view along the bdirection of the cell (Figure 2).The tetrahedral co-ordination around the siliconatom, coupled with the equivalence of the two ironatoms, constrains the cyclopentadienyl rings to aneclipsed (cis) configuration as seen along a line parallelto the Fe-Fe bond. Indeed, the whole moleculepossesses idealised mirror symmetry about the planewhich is the perpendicular bisector of the Fe-Fe bond,as shown in Figure 3, which also illustrates the mostsurprising feature of the molecule: that there is nosecond plane of symmetry.In fact the silicon atomlies more than 0.5 A away from the plane defined bythe iron atoms and the terminal carbonyl groups,although it is, of course, equidistant from both ironatoms. Furthermore, although the cyclopent adien ylrings are necessarily placed asymmetrically with respectto the Fe,(CO),(terminal) plane, their centroids lie in thisplane and the ring planes are perpendicular to it. Wemust ask why the silicon atom also does not lie in thisplane to give the whole molecule C,, (2mm) symmetryinstead of its actual C, (m) symmetry?If it can be assumed that in the compound [Fe(CO)2-(x-C5H5)'J2 3 the cyclopentadienyl rings take up theoptimum configuration with respect to the iron atomsbecause they are not otherwise bonded to extraneousatoms, then the ' natural ' position is one in which the10 J.M. Stewart, F. A. Kundell, and J. C. Baldwin, ' TheX-ray System of Crystallographic Programmes for any Computer, 'Computer Science Centre, University of Maryland, 1970.11 C. K. Johnson, ORTEP, a Fortran programme for crystalstructure illustrations, Oak Ridge National Laboratory Report,ORNL 37941442 J.C.S. Daltondihedral angle between the two ring planes is very plane in which the Si atom would lie if the overallclose to 90". In the compound Fe,(x-C,H,*CH(NMe,)*- symmetry were C2v. By swinging out of this plane, theCH(NMe,)-C,H,)(CO),, where there is a relatively Si atom forces the cyclopentadienyl rings even closerflexible two-carbon bridge between the two rings, the together (thus making the dihedral angle nearer to thedihedral angle again remains close to 90" (SS-SO).In tetrahedral value), and in conjunction with the deviationour silicon-bridged structure, however, the angle between of the Si-C bond from the plane of the C, rings towardsFIGURE 1 A stereoscopic view of the moleculethe ring planes has increased to 97.2", doubtless pri-marily because of the added constraint imposed by thelengths of the Si-C bonds which, at 1.88(1) A, are notthe Fe, axis, achieves an environment which is veryclose to ideal tetrahedral (see Table 2 and Figure 4).nFIGURE 3 A view of the molecule along the c direction of themonoclinic cell, showing the displacement of the silicon atomfrom the central Fe,(CO),(terminal) plane.IC (61 bFIGURE 2 A view of the molecule in its monoclinic cell seen inprojection along b looking towards the origin. Only onemolecule is shown, but the symmetry elements of P2Jn showhow the three symmetry-related molecules are placedsignificantly different from other tetrahedral Si-Cbonds.12 In pulling the rings closer to one another,however, the silicon atom becomes constrained to abond angle equal to the dihedral angle between the twocyclopentadienyl ring planes, except in so far as theSi-C bonds deviate (i) from the plane of the ring towhich they are attached and (ii) from the Fe,(CO),l2 Chem.SOC. Special Publ., No. 18, 1965.QP* n ta)FIGURE 4 Diagrammatic representations of the molecule toshow geometrical relationships, (a) in section through theFe,(CO) ,(terminal) plane, and (b) along the vector perpen-dicular to the cyclopentadienyl ring passing through thecentroi1973 1443The Fe-Fe Interaction.-The value of the Fe-Febond distance [2.512(3) A] is close to those found inother structures containing two iron atoms bridged bytwo carbonyl groups: e.g., 2.510( 1) Fe,((n-C,H,*CH-(NMe,)~CH(NMe,)*C,H,)(C0),,5 2.531(2) cis[(n-C,H,)-Fe (CO)2]2,3 2.534(2) trans[ (n-C5H5) Fe( CO),],,13 265( 1)Fe,(C0),,,14 2.54 [(x-C,H,),Fe2(CO),],*Ph,PCCPPh2,4 and2.53 A [(x-C,H,)Fe(CO)],CO*CNPh,.15The Carbonyl Grou@s.-The geometry of the Fe,(CO),portion of the molecule is identical (within experimentalerror) to that of the similar units in the compoundsof refs. 3 and 5. The terminal groups show no significantdeviation from linearity and form a coplanar unit withthe two iron atoms. The bridging carbonyl groupslikewise form individual coplanar units with the twoiron atoms, but the two planes so defined are not them-selves coplanar : the dihedral angle between theseR. F. Bryan and P. T. Greene, J . Chem. SOC. ( A ) , 1970,l4 C. H. Wei and L. F. Dahl, J . Amer. Chem. SOL, 1969, 91,3064.1351.planes is 160". The individual Fe-C and C-0 distancesare as expected. The mean Fe-C-Fe angle for thecarbonyl bridge is 82.5".The Silicon-Carbon Bonds.-These have a mean lengthof 1.88(1) and the silicon atom achieves a closelytetrahedral environment (Table 2). The two linksto the cyclopentadienyl rings deviate from the planes ofthe rings towards the Fe-Fe axis (see Table 3). Theplane defined by the two Si-(C,-ring) bonds is, of course,parallel to the Fe * - Fe vector but does not include thetwo iron atoms; it is at an angle of ca. 23" to theFe,(CO),(terminal) plane.We thank S. Moorhouse for crystals, and the S.R.C. forfinancial support and for facilities a t the Atlas ComputerLaboratory, Chilton, Berkshire, where help was given byMrs. Carol Hirst.[2/2519 Received, 7th November, 1972315 K. 1:. Joshi, 0. S. Mills, P. L. Pauson, B. W. Shaw, andW. H. Stubbs, Chem. Cornm., 1965, 181