J. CHEM. soc. DALTON TRANS. 1994 243Reactions of the Hexanuclear Mixed-metal Cluster[ Ru,RhC(CO),,(q5-C,Me,)] and the Synthesis,Characterisation and Structural Analysis of thePentanuclear Derivative [ Ru,RhC( CO),,(q5-C,Me,)] tBrian F. G. Johnson,8 Jack Lewis,**a Hilary Curtka Trushar Adatia,*abMary McPartlin and Jill Morrisba University Chemical Laboratory, Lensfield Road, Cambridge CB2 IEW, UKSchool of Chemistry, University of North London, London N7 8D8, UKTreatment of the hexanuclear cluster [Ru5RhC(C0),,(q5-C,Me5)] 1 with 80 atmospheres of CO under80 "C resulted in a series of decapping reactions yielding the complexes, [Ru,(CO),,], [Ru,C(CO),,] and anew green compound [Ru,RhC(CO),,(q5-C,Me,)] 2. Cluster 2 crystallises in the monoclinic space groupP2Jn with a = 19.1 80(3), b = 13.940(3) and c = 10.728(2) A, p = 97.1 57(2)". The five metal atomsadopt a square-based pyramidal metal framework [Ru-Ru 2.776(1)-2.873(1), Ru-Rh 2.798(1)-2.881 (1 )A].The octahedral cluster [Ru,R~C(CO),,(T~-C,M~~)] 1 reacted at room temperature with a methanolicbase to yield the methoxide adduct [Ru,RhC(CO),,(CO,Me) (q5-C,Me5)] - 3 which on acidificationimmediately regenerated the parent compound. On prolonged stirring, cluster 1 reacted irreversibly withthe methanolic base to form t h e salt [N(PP~,),],[RU,R~C(CO)~~(~~-C~M~,)] 4.In recent years, the chemistry of homonuclear carbido clustersof the iron triad has been well documented, l4 however, reportsof the chemistry of their mixed-metal analogues still remainsrelatively rare.Several strategies have now been employed byvarious workers to synthesise these novel compounds andextend the investigations of their reactions in Asynthetic route which has generated success in cluster build-upof such heteronuclear compounds involves 'redox-condens-ation', a term first applied by Chini and co-workers' toreactions between metal carbonyls and carbonylate ions.Reactions of this sort often involve condensation of a metal in aformally negative oxidation state with one in oxidation statezero, but may also include reactions between metal halides orcations and carbonylate anions. For example, the octahedralhexanuclear cluster [Ru, RhC(C0) , ,(q '-C5 Me,)] has recentlybeen synthesised by the reaction of the salt [N(PPh,),],[Ru,C-(CO),,] with an equivalent amount of the complex[R h(C , M e ,)(NCMe) 3] [BF,] , .The pentamethylcyclopenta-dienylrhodium unit, Rh(C,Me,), is extremely robust and hasbeen widely used as a metal half-sandwich in mono- and bi-nuclear complex ~hemistry,~ where it can remain intact throughmu1 tiple transformations involving acidic, basic, oxidising andreducing conditions. The fifteen equivalent protons of the q5-C,Me, ligand provide a convenient nuclear magnetic resonancemarker, assisting in the characterisation of products. For thesereasons the cluster [Ru,RhC(CO) i4(q 5-C,Me,)] which incor-porates the Rh(C,Me,) unit offers considerable scope forexamination of the cluster reactivity.Results and DiscussionUnder ambient conditions, the hexanuclear carbido cluster[Ru,RhC(CO),,(q 5-CSMe5)] 1 forms immediately on reactionof [N( PPh,),] , [Ru, C( CO) dl with [Rh( C5 Me,)(NCMe),]-[Sbf;,],.The very dark brown product is stable indefinitelyt Supplementary data available: see Instructions for Authors, J. Chem.SOC., Dulton Trans., 1994, Issue 1, pp. xxiii-xxviii.Non-SI unit employed: atmosphere = 101 325 Pa.in air in the solid state, and is virtually unaffected by heat-ing in n-octane even up to a temperature of 125 "C for aperiod of twenty hours under nitrogen. This stability reflects thestrength of the rhodium-pentamethylcyclopentadienyl bondtogether with the compact octahedral structure l o of the cluster,which presents no easy pathway for attack. The cluster[Ru,RhC(C0)14(qs-C,Me,)] 1 reacts with methanolic sodiumcarbonate or potassium hydroxide to yield the anion tentativelyformulated as [Ru,RhC(CO),,(CO2Me)(q5-C5Me,)]- 3.Onprolonged stirring with KOH-MeOH a dianion 4 isformed, formulated as [Ru,RhC(CO), 3(qs-C5Mes)]~~ on thebasis of fast atom bombardment (FAB) mass spectroscopyresults (Scheme 1). This behaviour directly parallels thatobserved for the cluster [Ru6C(CO), which is rapidljattacked by the methoxide ion to form the adduct [Ru,C-(CO),,(CO,Me)] -, and on prolonged stirring with methanolic.base, or rapidly with aqueous base, reacts irreversibly to formthe dianion [Ru,C(C0),,]2-." At 80°C and under eight4atmospheres of carbon monoxide cluster 1 again behave5similarly to its homonuclear analogue, undergoing a series oldecapping reactions to form pentanuclear carbides.As with[Ru,C(CO), ,I, the major product is [Ru,C(CO), ,I, but a newgreen compound [Ru4RhC(CO),.,(q5-CSMe5)] 2 is also formedas established by X-ray analysis. The decapping process isbelieved to involve nucleophilic attack of carbon monoxide onthe cluster, (Scheme 2), followed by displacement of either aruthenium or a rhodium atom. Initial characterisation of thepentanuclear cluster [Ru,R~C(CO),~(~~-C,M~~)] 2 wascarried out by infrared, 'H and I3C NMR spectroscopictechniques (Tables 1 and 2). The carbonyl stretch frequencies incluster 2 are consistent with the presence of only terminal COgroups. Variable-temperature ' 3C NMR spectra of 2 suggestcarbonyl fluxionality.At + 20 "C the carbonyl signals arecollapsed but at - 80 "C these signals differentiate as six peaksin approximate ratio 2 : 2 : 2 : 3 : 1 : 2, all in the terminal region. Atthis temperature the carbide resonance is also detected at 6409.1. The coupling constant, J(Rh-C) 51 Hz, is significantlyhigher than previously reported values for rhodium mixed-metal carbide clusters ' , (Table 3). In contrast with the carbide,no splitting of the cyclopentadienyl carbon signal was observe244 J. CHEM. soc. DALTON TRANS. 1994t 3Table 1 Analytical and physical data for compounds 1-4Analysis' (%)Compound v,,,(CO) */cm-' C H N1 2075s, 2025vs, (br), 26.4 1.32005(sh), 198Om (br), (26.2) (1.3)1815w (br)2 2075m, 2070(sh), 28.1 1.52047s, 2032m, (27.9) (1.5)2010s, 1992w, 1975(sh)3 2041m, 1997s 43.6 2.9 0.84 2022w (br), 1999m, 52.6 3.4 1.3(43.4) (2.8) (0.8)1954s, 1941s, 1889w (br) (52.5) (3.4) (1.3)Calculated values in parentheses. Measured in dichloromethanesolution.at a data-point separation of 4 Hz, suggesting a couplingconstant J(Rh-C) of this order or less.This is consistentwith other Rh(C,MeS) compounds, which show J(Rh-C) upto about 8 Hz, and with the idea that the carbon-metal bondessentially involves only the carbon 2p-71 orbitals of the cyclo-pent adieny 1 ligand. 6 ,The molecular structure of the pentanuclear mixed-metalcluster [RU,R~C(CO)~ 2(q5-C,Me,)] 2 has been establishedby single-crystal X-ray diffraction analysis and is displayed inFig. 1. Selected interatomic distances and angles aresummarised in Table 4.In the solid state the pentanuclearcluster 2 adopts a square-based pyramidal or nido-octahedralmetal-core geometry. The square base of the metal framework isnx WO(33)Fig. 1 The molecular structure of the pentanuclear cluster [Ru,RhC-(CO), 2(r)s-C,Me,)] 2 showing the crystallographic numberingscheme. The carbon atom of each carbonyl group has the samenumbering as the oxygen atomdefined by the metal atoms Ru(2), Ru(3), Ru(4) and Rh, withRu(1) forming the apex of the pyramid, and the carbido atomco-ordinates to all five metal atoms.The range of metal-metal distances in cluster 2 [Ru-Ru2.776(1)-2.873(1), Ru-Rh 2.798(1)-2.881(1) A] is lower thanthat reported for the parent hexanuclear compound [Ru,RhC-(CO),4(q5-C5Me5)] 1 [Ru-Ru 2.813(1)-2.959(1), Ru-Rh2.825(1)-2.895(1) A].'' The mean Ru-Ru bond in 2 [2.810(1)A] is notably shorter than that observed in 1 [2.890(1) A],the homonuclear analogue [Ru,C(CO), ,] [2.840(2) A]'"and the organopentaruthenium cluster, [Ru,C(CO) , 3(q6-C6H,)] [2.831(2) However, the mean Ru-Ru bond in 2closely resembles the mean distance calculated for the arenederivative, [Ru,C(CO), 3(q6-c6H6)] C2.8 13(2) A].The rangeof Ru-Rh distances in the pentanuclear cluster 2 aresignificantly longer than that in the tetranuclear clusters[Ru3R hH 3(C0) 2] [2.757( 3)-2.762( 3) A] and [Ru2Rh2H2-(CO),,] [2.756(3)-2.768(3) A].'5 This could be a result of theincreased nuclearity in 2 coupled with the expansion of themetal framework due to the presence of a carbido atom. Thefour metal-metal bonds associated with the square base in thestructure of 2 show a marked variation with one metal-metalbond [Ru(3)-Ru(4) 2.873(1) A] being significantly longer thanthe other three [Ru 2)-Ru(4) 2.830(1), Ru(2)-Rh 2.798(1) andwith the Ru(1tRh bond markedly longer than the other threeapical Ru-Ru bonds (Table 4).This may be attributed to thepresence of a different coinage metal atom in the Ru4Rhframework which does not carry any carbonyl groups, and tothe steric influence enforced by the bulky C,Me, group co-ordinating to the rhodium atom. This steric influence could alsoaccount for the observation that the isomer with the Rh-(C,Me,) unit at the basal site appears to form in preferenceRu(3)-Rh 2.810(1) a 3.The apical bonds from Ru(1) also varyJ . CHEM. SOC. DALTON TRANS. 1994 245Table 2 Proton and I3C NMR data' for compounds 1-4Compound T/OC 'H b(6) 13CC(6)21 Ambient 2.05 (s, C,Me,) 12.2 (s, CsMe,)104.3 (s, C,Me,)201 .o (s, CO)- 60 12.1 (s, C,Me5)104.1 (s, CsMe5)202.0 (br s, CO)422.4 [d, J(Rh-C) (carbide)] 44Ambient 2.18 (s, C,Me,) 11.1 (s, C,Me,)102.7 (s, CsMe5)- 80 11.1 (s, CsMes)102.7 (s, CsMe5)189.7 (s, 2CO)192.6 (s, 1CO)197.0 (s, 3CO)198.1 (s, 2CO)198.6 (s, 2CO)200.5 (s, 2CO)409.1 [d, J (Rh-C) (carbide)] 513 Ambient 2.08 (s, C,Me,)4.44(s, C02Me)7.5-8.1 (m, p(PPh,),] ')4 Ambient 2.08 (s, C,Me,)7.3-7.9 (m, p(PPh,),]+)a Chemical shifts (6) in ppm, coupling constants (J) in Hz.Measured in CD,C12. ' Measured in CDCl,.Table 3diumCarbon-13 NMR data for cluster carbides coupled to rho-6 (carbide) J(Rh--C)/Hz Reference264.7 13.7 12(4291.2 10, 15 1 2(b)281.8 13.2 12(4300.3 12.7 12(4300.3 12.7 12(4461.1 19.5 12(4422.4 44409.1 51285.3 10.7, 12.7 12(c)Table 4 Selected bond lengths (A) and angles (") for [Ru4RhC-(CO), 2(q5-C5Me,)] 2, with estimated standard deviations (e.s.d.s) inparenthesesRu( 1 bRu(2) 2.792( 1 ) Ru(l)-R~(3) 2.776( 1)Ru( l)-Ru(4) 2.795( 1) Ru( 1)-Rh 2.881( 1)Ru(2)-Ru( 4) 2.830(1) Ru( 2)-R h 2.798(1)Ru(3)-Ru(4) 2.873(1) Ru( 3)-R h 2.8 1 O( 1 )Ru( 1 )-C 2.146(7) Ru(2)-C 2.037(7)R U( 3)-C 2.01 l(7) Ru(4)-C 2.033(8)Rh-C 1.943(8)Ru-CO(termina1) 1.855( 1 1)-1.892( 1 1)Rh-C (CsMe5 ring) 2.179(8)-2.260(8)Ru( ~)-Ru( 1 )-Ru(~)Ru(~)-Ru( l)-Ru(3)Rh-Ru( l)-Ru(3)Ru(~)-Ru(~)-Ru( 1 )R h-Ru(2)-R u(4)Ru(2)-Ru(4)-Ru( 1)Ru( 3)-Ru( 4)-Ru( 2)R~-Ru(~)-Ru( 1)Ru(3)-Rh- Ru( 1)9 2 3 1)62.1(1)59.5( 1)59.6( 1)89.7( 1)62.1 ( 1)5 9 3 1)89.7( 1)58.4( 1)Ru-C-O(termina1) 173(8)-179(9)Ru(~)-Ru( ltRu(2)Rh-Ru( ltRu(2)Rh-Ru(2)-Ru( 1)Rh-Ru( 1 )-Ru(~)Ru(~)-Ru(~)-Ru( 1)Rh-Ru( 3)-Ru(4)Ru(~)-Ru(~)-Ru( 1)Ru(~)-R~-Ru( 1)Ru( 3)-R h-Ru(2)60.9( 1)59.1 (1)88.7(1)62.1(1)59.3( 1)88.6( 1)58.6(1)58.9( 1)91.7(1)to the alternative isomer in which this fragment would have toadopt a more sterically hindered apical position.The basalisomer may also be kinetically and thermodynamically morefavoured due to the probability of basal co-ordination beingfour times as likely as the apical one.In the pentanuclear cluster 2 the metal-carbide bond lengthsshow a marked variation with the Rh-C distance of 1.943(8) Abeing significantly shorter than the Ru-C lengths C2.01 l(7)-2.146(7) A, Table 41.Interestingly, this feature is consistent withwhat has been observed in numerous penta- and hexa-ruthenium carbidoarene clusters where shortening of the Ru-Cbonds occurs systematically when a tricarbonyl unit is replacedby a poorer n-acceptor arene ligand as for example in theclusters [R~,C(CO),~(q~-arene)] [arene = C6H3Me,-1,3,5,C,H@e,-1,3 or C,H,Me] and [Ru,C(CO),,(~-C~H,)].~In cluster 2 the carbido atom is found to lie 0.17(2) below thesquare plane, and results in the apical Ru(1tC bond being0.1 19(7) A longer than the mean of the other three basal Ru-Cdistances.The cyclopentadienyl ligand in 2 is found to bondin a terminal fashion to the rhodium atom with the range ofRh-C (C,Me, ring) distances [2.179(8)-2.260(8) A] beingsimilar to that observed in the related hexanuclear cluster,[Ru,RhC(CO) i4(q ,-C,Me,)] C2.2 17(9)-2.254(9) A],' andto those reported for numerous mononuclear rhodium(r)complexes.' 7,1 *ExperimentalSpectroscopic and analytical data for compounds 1 4 aresummarised in Tables 1 and 2.Infrared spectra were recorded on a Perkin Elmer 983spectrophotometer. Proton NMR spectra were recorded on aBruker WP 80 SY instrument. Carbon-13 NMR spectra wererecorded on a Bruker WM 250 instrument. Mass spectra wererecorded on an MS-12 instrument and the fast atombombardment mass spectra on an AEMS 50 machine.Synthesis of [R h(C, Me,)(NCMe),] [ SbF,] .-Crude [R h,-(C,Me,),CI,] was prepared from a purchased sample ofRhCl,.xH,O (1 g) and was stirred in freshly distilled aceto-nitrile (50 cm3). Excess of silver hexafluoroantimonate wasadded, which resulted in an immediate precipitation of silverchloride and a change in the colour of the solution from red toyellow.The solution was carefully filtered through a bed ofCelite (approximately 2 cm thick), then reduced in volume to 1246 J. CHEM. SOC. DALTON TRANS. 1994cm3. Diethyl ether was added to precipitate the yellow-orangeproduct, which was collected, washed with diethyl ether andused without further purification.IR (Nujol mull): 2320 andTable 5 X-Ray crystal structure determination data for[R%RhC(CO) 1 z(q 5-C5Me5)l 2(a) Crystal dataMolecular formulaMCrystal dimensions/mmCell determined from 25 reflections in28 range/"Crystal systemSpace group4blACIAPI"ul A 3DJg cm-3ZF(OO0)p( Mo-Kol)/mm-'TlK(b) Data collectionDiffractometer20 RangerNo. of reflections measuredhkl RangeScan modeNo. steps in scanStep width/"Minimum counts for reflection to ben, where F > no(F)Total no. of reflections [Z > 3o(Z)]No. of standard reflectionsVariation in intensity (%)Maximum, minimum transmission factorsmeasured (counts s-')(c) Structure refinementNo.of reflections used in refinementNo. of refined parametersRR' = Cwt/CwflF,IMaximum, minimum electron-densitypeak/e A-3c2 3H 1 So 1 2 RhRu4990.670.23 x 0.21 x 0.1315 < 2% c 25Monoclinicp2 1 In19.180(3)1 3.940( 3)10.728(2)97.157(2)2845.992.312418800.79298Philips PW 1 1006.0-50.04317-27 to 27, &18, &150-20300.0510639463c51.0450.92337501910.03970.04410.33, -0.512293 cm-'. 'H NMR(CD,CN): 6 1.68 (s, C,Me,) and 1.8-2.2(various peaks, co-ordinated and free NCMe).Synthesis of [RU,R~C(CO),~(~~-C~M~,)] 1.-The salt~(PPh3)2]2[Ru,C(CO)i,] (0.212 g, 0.107 mmol) was dis-solved in dichloromethane (20 cm3) and mixed with asuspension of [Rh(C5Me,)(NCMe),][SbF,1, (0.2 g, 0.24mmol, excess) in dichloromethane (10 cm3).The solutionimmediately became dark brown. After stirring for 10 min, thesolution was eluted through a short column of Kieselgel60 withdichloromethane-hexane (1 : 1). The dark brown product (0.1 14g, 0.10 mmol, 70%) was crystallised by repeated addition ofhexane and partial evaporation of the eluted solution. [m/z1 150 ( M +) and a sequence of peaks due to the loss of CO fromM '3.Reactions of [Ru,RhC(CO) 4(q '-C,Me,)] 1 .--I Thermalstability. Compound 1 (0.005 g, 0.004 mmol) was heated at125°C in n-octane (10 cm3) for 24 h. The solvent wasevaporated and the residue redissolved in dichloromethane.Spot thin-layer chromatography and solution infraredspectroscopy showed it to be at least 90% [Ru,RhC(C0),?(q5-C,Me,)], with a small amount of baseline material immobile innon-polar solvents.2 Base reactions.(a) Compound 1 (0.02 g, 0.017 mmol) wasstirred overnight at - 37 "C in a methanol solution (15 cm3)containing potassium hydroxide (0.1 g, 0.18 mmol) and[N(PPh,),]Cl (0.1 g, 0.17 mmol). The solution was filteredthrough a prechilled Schlenk sinter, which left a brown residuewhich was extracted with dichloromethane. From micro-analysis, IR and NMR data the compound was tentativelyfo&ulated as the salt (IN(PP~,),][Ru~R~C(CO),~(CO,M~~-(rl 5-CsMe~)I 3-(b) Compound 1 (0.02 g, 0.017 mmol) was stirred at roomtemperature in methanol (10 cm3) with sodium carbonate (0.1 g,0.943 mmol). When all the dark brown material had dissolved(after half an hour) the solution was filtered to remove Na2C03.The infrared spectrum of the methanol solution showedabsorbances at 2035m and 1998s cm-' indicating the sameproduct as in (a).One drop of HBF, in Et20 was added, givinga brown precipitate immediately. This was redissolved indichloromethane and identified by its infrared spectrum to bethe starting material.(c) Compound 1 (0.03 g, 0.026 mmol) was stirred for 5 hat room temperature in a methanol solution (15 cm3) con-taining potassium hydroxide (0.1 g, 1.80 mmol). Excess of[N(PPh,)],Cl in methanol was added to precipitate the brownTable 6 Fractional atomic coordinates for [Ru,RhC(CO), z(qs-C,Mes)] 2 with e.s.d.s in parenthesesAtom X Y z Atom X0.168 79(3)0.228 39(3)0.035 54(3)0.152 17(3)0.110 32(3)0.127 O(4)0.150 8(5)0.140 l(3)0.266 4(5)0.326 O(4)0.131 4(5)0.307 O(4)0.355 8(4)0.214 4(4)0.201 8(3)0.286 O(4)0.322 6(3)0.144 4(5)- 0.006 O(5)-0.032 5(4)0.161 89(4)0.331 78(4)0.242 43(5)0.330 37(5)0.259 92(4)0.301 2(5)0.064 3(7)0.145 O(6)0.127 l(6)0.093 2(7)0.050 8(6)0.363 8(6)0.387 4(5)0.457 2(6)0.532 6(5)0.285 9(6)0.256 6(5)0.192 4(8)0.159 O(6)- 0.000 4(5)0.000 86(6)0.102 49(6)-0.041 04(6)-0.141 04(6)0.200 33(6)0.033 6(7)0.1 14 5(9)0.175 6(7)0.011 2(9)0.008 4(7)-0.148 O(10)-0.241 2(9)0.022 4(8)0.158 6(8)0.192 O(6)0.247 4(9)0.331 5(7)-0.023 5(7)-0.196 7(11)- 0.289 9(9)- 0.029 O ( 5 )- 0.065 2(4)-0.011 l(5)- 0.039 5(4)0.088 8(5)0.050 O(4)0.230 9(5)0.278 5(4)0.160 2(5)0.170 l(4)0.067 3(4)0.022 8(4)0.062 O(4)0.131 3(4)0.135 7(4)0.043 6(5)0.032 5(5)0.187 l(4)0.192 8(4)- 0.056 7(5)Y0.343 6(7)0.409 7(6)0.148 2(7)0.086 l(6)0.322 8(7)0.327 3(5)0.298 2(7)0.277 l(6)0.464 7(7)0.546 4(6)0.352 9(6)0.271 9(6)0.188 O(6)0.219 l(6)0.319 6(6)0.456 7(7)0.275 9(7)0.090 l(7)0.154 O(7)0.386 l(7)z-0.037 4(9)-0.031 3(8)0.038 4(10)0.083 l(8)- 0.288 9(9)-0.380 l(7)-0.221 O(9)-0.270 l(7)- 0.152 3(9)-0.161 5(8)0.335 l(8)0.315 4(8)0.357 8(8)0.403 l(8)0.391 6(8)0.317 2(10)0.281 3(10)0.372 O(9)0.470 l(9)0.453 l(9J.CHEM. SOC. DALTON TRANS. 1994 247salt [N(PPh,)2]2[Ru5RhC(CO),3(q5-C5Me5)] 4 (0.043 g,0.019 mmol, 75%).[ m / z (FAB) 1121 ( M + of anion) and 1065( M + -2CO)l.Synthesis of [ R u , R ~ C ( C O ) , ~ ( ~ ~ - C ~ M ~ ~ ) ] 2.-The com-pound [RU~R~C(CO),~(~~-C~M~,)] 1 (0.39 g, 0.34 mmol)was sealed with n-heptane (30 cm3) in a glass liner inside a 100cm3 Roth autoclave. The autoclave was filled with carbonmonoxide at ca. 20 atmospheres and the pressure was thenreleased. This cycle was repeated to flush out air. The autoclavewas then pressurised to 80 atmospheres of carbon monoxide,and heated at 80 "C for 4 h with stirring. After cooling, thepressure was released, the n-heptane evaporated and the purpleresidue dissolved in dichloromethane. The products wereseparated by thin-layer chromatography on silica plates, elutingwith dichloromethane (30%)-hexane (70%), to give yellow[RU~(CO),~], pink [Ru5C(CO),,] (0.163 g, 0.17 mmol, 51%)and green [RU,R~C(CO),~(~~-C~M~,)] (0.123 g, 0.12 mmol,36%) together with some unrecovered brown baseline material.Crystal Structure Determination of Cluster [Ru,RhC-(CO), 2(q 5-C5Mes)] 2.-Suitable crystals of cluster 2 weregrown from the slow diffusion of n-pentane-dichloromethane atroom temperature.Details of the crystal parameters, datacollection parameters, and refinement data are summarised inTable 5. The method of data collection and processing havebeen described previo~sly.'~ The positions of the metal atomswere deduced from a Patterson synthesis and the remainingnon-hydrogen atoms were located from subsequent Fourierdifference syntheses.Empirical absorption corrections wereapplied to the data after initial refinement of the isotropicparameters of all the non-hydrogen atoms.20 During the finalcycles of refinement, anisotropic thermal parameters wereassigned to the metal atoms.'l The hydrogens associated witheach methyl substituent on the pentamethylcyclopentadienylligand were geometrically calculated to ride at the respectivecarbon atom at distances of 1.08 A with fixed thermalparameters of 0.08 A'. Full-matrix refinement of the atomicpositional and thermal parameters of all the non-hydrogenatoms converged at final R and R' values of 0.0397 and 0.0441with weights of w = l/02(F,,) assigned to individual reflections.The final atomic coordinates for cluster 2 are listed in Table 6 .Additional material available from the Cambridge Crystallo-graphic Data Centre comprises H-atom coordinates, thermalparameters and remaining bond lengths and angles.AcknowledgementsWe would like to thank the SERC for financial support and foraccess to the Chemical Database Service at Daresbury.References1 J.Evans, P. M. Stroud and M. Webster, J. Chem. Soc., Dalton Trans.,1991, 1017; 2027.2 D. Braga, F. Grepioni, B. F. G. Johnson, J. Lewis, M. Martinelliand A. Gallop, J. Chem. SOC., Chem. Commun., 1990, 53; H. Chen,B. F. G. Johnson, J. Lewis, D. Braga, F. Grepioni and E. Parisini,J. Chem. SOC., Dalton Trans., 1991, 215; D. Braga, F. Grepioni,B. F. G. Johnson, E. Parisini, M. Martinelli, M. A. Gallop andJ. 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