chapter 19. organometallic chemistry of bi- and poly-nuclear complexes

19 Organometallic Chemistry of Bi- and Polynuclear Complexes

By G. HOGARTH

Department of Chemistry, University College London, London WCl H OAJ

1 Introduction

The chemistry of organometallic complexes containing two or more metal atoms continues to be an area of intense research activity. During 1991, over 600 papers were published in this area. A thorough review of these contributions appears in the Royal Society of Chemistry’s ‘Specialist Periodical Reports, Organometallic Chemistry, Volume 21’.’ Thus, it is not the aim of this review to cover the literature comprehensively. Rather, publications will be highlighted which in the opinion of the author contain material of special relevance. In this way it is hoped that the chapter will be readable to the non-specialist, whilst also providing research workers in the area with a valuable reference text covering the most significant developments of the year. Rather than listing papers in order of the position of the metal(s) in the Periodic Table, the material is organized under headings which relate to the topic of interest. Thus, for example, under the heading ‘The Nature of the Metal- Metal Bond’ publications considering both theoretical, physical, and structural studies of relevance are grouped together. Under this format there will clearly be an overlap of material between certain sections. Thus their ordering is designed to group relevant material as closely together as possible.

2 The Nature of the Metal-Metal Bond

The debate as to the nature of the metal-metal bond continues. The extent of the metal-metal interaction in the bridged form of C O ~ ( C O ) ~ has been investigated using self-consistent field calculations. The results show an accumulation of density in the region between the two cobalt atoms which, as the authors state, ‘must be due at least in part to a constructive interference between the two atoms’. An analysis of the topology of the charge density, however, shows no interaction line connecting the cobalt atoms. The authors conclude that the results support the existence of a weak, bent cobalt-cobalt bond with a bond order of 1/3.* In contrast, after a study utilizing LCAO calculations on the metal-metal bonding in Fe2(C0)9 the conclusion reached is that, as far as direct metal-metal orbital interactions are concerned, there is net antibonding and one should not consider this compound as possessing a

’ G. Hogarth in ‘Specialist Periodical Reports, Organometallic Chemistry,’The Royal Society of Chemistry,

’ A. A. Low, K. L. Kunze, P. J . MacDougall, and M. B. Hall, Inorg Chem., 1991, 30, 1079. Vol 21, Chapter 9.

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374 G. Hogarth

direct metal-metal bond.3 Extended Huckel calculations on [Tc2( p-O),Cp],, a polymer shown to have a short technetium-technetium bond (1.867 A), show that the net bonding is best described quantitatively as ~ ’ ( v S ) ~ S * . The short bond is consonant with a multiple metal-metal bond superimposed on the already short metal contact enforced by the bridging 0x0 l i g a n d ~ . ~ Two papers document structural changes in trinuclear metal complexes upon varying the total cluster electron count. Thus, the synthesis and structural analysis of a series of electrochemically interconvertible bis(su1fido)capped clusters [Cp~Co3(p3-S),]“+ (Cp’ = C5H4Me, n = 0-2) is reported. The neutral 50-electron cluster contains one long (3.19 A) and two short (2.48 A) metal-metal interactions. Successive oxidations essentially leave the short vectors unchanged but lead to a dramatic shortening ( n = 1,2.87 A; n = 2,2.52 A) of the long vector, such that the dication has approximate C3 symmetry. This correlation of metal-metal distance with the number of ‘excess’ trimetal antibonding electrons in a localized bond scheme provides persuasive support for the validity of the electron counting f~rmula t ion .~ In contrast, a delocalized bonding picture is inferred from a comparison of the X-ray crystal structures of a series of 44, 46, and 48-electron phosphido-bridged stabilized trirhenium clusters. Thus, the addition of electrons to the metal framework results in a gradual opening up of the triangle as a consequence of the occupation of metal-metal T* orbitals, which leads to a loss of metal-metal double bond character.6 Oxidation of non metal-metal bonded [Cp’Ru(p,-S)], affords the dication [Cp’Ru(p3-S)];+ which formally differs from the neutral cluster by addition of a ruthenium-ruthenium bond. An X-ray structural analysis reveals that the complex contains two types of ruthenium atoms joined by three long (3.564-3.464 A) and three short (2.783-2.792 A) vectors. In solution, however, variable temperature NMR spectroscopy shows the equivalence of all ruthenium centres, seemingly indicating that the ruthenium-ruthenium bonds are mobile within the cluster.’ Reaction of the bis(pyrazo1ate)-bridged, para-cymene complex [( T6-cymene)RuC1( p-pz),Ir(COD)] with carbon monoxide affords the isomers [( q6-cymene)Ru(p-pz)21r(C0)2Cl] and [ ( T6-cymene)RuC1(p- pz),Ir(CO),], both of which have been structurally characterized. The former contains a short ruthenium-iridium interaction (2.696 A), while the separation in the latter (3.663 A) is almost 1 A greater, indicating the lack of a direct interaction. Interestingly, the two interconvert in solution by chloride transfer.8 Even more remarkable is the cluster core isomerism observed for tetranuclear [Cp2Pt2M2(CO),(PR3),] (M = Mo, W). Two interconvertible isomers are seen, one with a planar triangulated rhombohedra1 ( 1 ) core geometry, the second being tetrahedral (2). Both experimental and theoretical aspects of the isomerism have been studied, the latter with extended Huckel calculations. The solution ratio is found to be dependent upon the temperature, solvent, and the electronic and steric

A. Rosa and E. J . Baerends, New J. Chem., 1991, 15, 815. A. W. E. Chan, R. Hoffmann, and S. Alvarez, Inorg. Chem., 1991, 30, 1086. C. R. Pulliam, J. €3. Thoden, A. M. Stacy, B. Spencer, M. H. Englert, and L. F. Dahl, J A m . Chem. Soc., 1991, 113, 7398. H-J. Haupt, U. Florke, and H . Schneider, Acta Crystallogr., Sect. C. 1991, 47, 2531. ’ E. J . Houser, J . Amarasekera, T. B. Rauchfuss, and S. R. Wilson, J. Am. Chem. Soc., 1991, 113, 7440.

* D. Carmona, J. Ferrer, A. Mendoza, F. J. Lahoz, J . Reyes, and L. A. Oro, Angew. Chem., Znt. Ed. Engl., 1991, 30, 1171.

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Organometallic Chemistry of Bi- and Polynuclear Complexes 375

Ka I

OC-Mo-CO I

PR3

demands of the platinum-coordinated phosphine, with the authors concluding that the latter is the dominant feature.'

3 Physical Studies

The equilibrium between R U ~ ( C O ) ~ ~ and Ru(CO)~ has been investigated by infra-red spectroscopy in a high pressure cell. The rate at which equilibrium is attained is slow even at high temperatures. The temperature dependence of the equilibrium constant allows the mean ruthenium-ruthenium bond dissociation enthalpy to be calculated at 159 f 3 kJmol-'.lo The fluxionality of organometallic complexes in solution has received much attention in recent years while, in contrast, their behaviour in the solid-state has not. Solid-state 13C CP/MAS NMR spectroscopy has now been utilized to probe the latter. A kinetic analysis of the stereochemical non-rigidity of [Os,(CO),( q2-CH2CH2)( p3-C,H,)] reveals that four of the five fluxional processes observed in solution persist in the solid-state." A similar spectral analysis of the propargyl cation [Cp,Mo,(CO),( p-HCCMe,)]+ reveals that while the fluxional process observed in solution, which interconverts molybdenum atoms, is absent in the solid-state, rotation about the C-CMe, bond persists.12 In contrast to these findings, a study on a variety of ruthenium clusters containing phosphine- stabilized group 1 1 metals concludes that the dynamic processes seen in solution are not observed in the solid-state.13 In the solution state, high pressure 'H NMR spectroscopy has been utilized to determine the volumes of activation for hydride fluxionality in [ R u ~ ( C O ) ~ ( ~ - H ) ~ ( ~ ~ - C H ~ O M ~ ) ] and [H0s3(CO),,(PPh3)(p-H)]. The former is proposed to occur via the conversion of both hydrides from bridging to terminal positions, while in the latter a di-bridging transition state is f0rmu1ated.l~

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P. Braunstein, C. de MCric de Bellefon, S-E. Bouaoud, D. Grandjean, J-F. Halet, and J-Y. Saillard, J. Am. Chem. SOC., 1991, 113, 5282. R. Koelliker and G . Bor, J. Organomer. Chem., 1991,417, 439. S. J. Heyes, M. A. Gallop, B. F. G. Johnson, J. Lewis, and C. M. Dobson, Inorg. Chem., 1991,30,3850. M. V. Galakhov, V. I . Bakhmutov, I. V. Barinov, and 0. A. Reutov, J. Organornet. Chem., 1991,421, 65 . S. S. D. Brown, I. D. Salter, D. J . Smith, N. J. Clayden, and C. M. Dobson, J. Organornet. Chem., 1991, 408, 439. J . B. Keister, U. Frey, D. Zbinden, and A. E. Merbach, Organometallics, 1991. 10, 1497.

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376 G. Hogarth

Electrochemical studies include the reduction of [CpM(CO)], (M = Co, Rh) in non-aqueous solvents. The radical anion of the cobalt cluster decomposes rapidly, affording [CpC0(p-C0)1~ via cobalt-cobalt bond cleavage, while the rhodium analogue is far more stable and may be reduced further to the dianion without metal-metal bond rupture. The enhanced stability of the trirhodium core is attributed to the greater metal-metal bond strength of second versus first row transition metals.” The redox properties of the heterodinuclear fulvalene complexes [ WM(CO),( p- CloHs)] (M = Fe, Ru) have been studied in some detail. The two metals are either reduced or re-oxidized at well separated potentials, thus allowing the specific redox properties of each metal centre to be distinguished. Reduction of both complexes leads (via different pathways) to dimeric dianions [ (CO)3W(pCloH~)M(CO)2]~- in which a new M-M bond has been formed at the expense of the heterobimetallic interaction.16 A series of papers by Atwood and co-workers is concerned with electron transfer between neutral and anionic clusters and between neutral clusters and mononuclear anions. Half-reaction potentials can be used to predict the electron transfer process. Electron transfer between M3(CO)12 (M = Fe, Ru) and [Mi(C0)ll]2- (M’ = Ru, 0 s ) occurs with carbonyl transfer, and has been shown by labelling studies to proceed via the single-electron reduced species [ M,(CO) 12]-.’7 Finally, a new mechanism is proposed to account for the substitution reactions of metal carbonyl dimers, the key initial step being heterolytic metal-metal bond fission. It fits well with experimental data for [MM’(CO),,] (M, M’ = Mn, Re) and c0,(c0)~.18

4 Early Transition Metal Complexes

While there were no publications on the synthesis of complexes containing direct metal-metal bonds between elements of group 4, extended Huckel and ab initio self-consistent field calculations on the model zirconium complex [Cp,Zr( p-PH2)], indicate that despite the long metal-metal distance of >3.5 8, a substantial amount of through space coupling between metal atoms is seen. Thus, bimetallic complexes of this type may be considered as possessing ‘superlong’ metal-metal bonds. l9 Three publications reported on bimetallic complexes of group 5. Irradiation of monomeric [CpV(CO),] with organodisulfides or thiols affords V” dimers [CpV( CO),( p-SR)I2 which are substitutionally labile,20 while oxidation of related monomers [Cp*V(CO),L] (Cp* = q-C,Me,; L = SMe,, MeCN) with elemental tellurium or sodium polytelluride ( -Na2TeS) affords [CpTV2(p-O)(p-Te)(p-Te2)], in which V’” centres are linked. The latter undergoes reactions with elemental sulfur or selenium to afford a variety of mixed tetrachalcogenide complexes.21 The niobium( 11) dimers [Cp’Nb(CO),(p-Cl)], and [Cp’Nb( v2-ArC,Ar)( p-Cl)12 have been synthesized by the sodium amalgam reduction of niobium( 111) complexes [Cp’NbCl(CO),( p-C1)I2

I s J. M. Mevs, T. Gennett, and W. E. Geiger, Organornetallics, 1991, 10, 1229. M-H. Delville-Desbois, D. S. Brown, K. P. C. Volhardt, and D. Astruc, J. Chern. SOC., Chern. Cornrnun., 1991, 1355. M. S. Corraine and J . D. Atwood, Organornetallics, 1991, 10, 2647; M. S. Corraine and J. D. Atwood, Organornetallics, 1991, 10, 2985; Y. Zhen and J. D. Atwood, Organometallics, 1991, 10, 2778.

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*’ B. F. G . Johnson, J. Organornet. Chern., 1991,415, 109. l9 M-M. Rohmer and M. BCnard, Organornetallics, 1991, 10, 157. *’ F. Y. Pktillon, P. Schollhammer, and J. Talarmin, J. Organornet. Chern., 1991, 411, 159. 2 1 M. Herberhold and M. Schrepfermann. J. Organornet. Chern., 1991, 419. 85.

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Organometallic Chemistry of Bi-

oc co oc co \ / \ /

m' and Polynuclear Complexes 377

(3) (4)

(3) and [Cp'NbC12( q2-ArC2Ar)] (4) respectively. Both have been characterized crystallographically. Interestingly, while the metal-metal bond lengths are similar in both complexes (3.056 and 3.073 A), the carbonyl complex contains a puckered Nb2C12 core and a cis disposition of cyclopentadienyl ligands, while the alkyne derivative has a planar inner core geometry with trans cyclopentadienyls. The origin of this difference is not clear but may arise from differences in symmetry in the lowest unoccupied molecular orbitals of each complex. The same paper also contains details of the synthesis and structure of trans-[ Cp*Ta( CO),( P - C ~ ) ] ~ . ~ *

5 Multiple Bonds

The synthesis, structures, and reactivity of a number of multiply bonded complexes were reported in 1991. The unsupported triply bonded complexes [Cp'WX2I2 (X = C1, Br) react with a variety of reagents HY (Y = PPh2, SR, C1, H), resulting in oxidative-addition across the triple bond giving doubly bonded [Cp', W2X4( p-H)- (p-Y)]. Most significant is the reversible reaction with hydrogen; reductive-elimina- tion occurring readily at 50 "C under reduced pressure.23 The chloro complex also reacts with nitriles, which after addition of HCl gas gives the perpendicular bridging alkylidyne-amine complexes [Cp;W,Cl,( p-C1)( p-HNCR)] (5).24 Triply bonded

R I

diruthenium complexes also feature prominently. Reduction of [Cp*Ru(p-OMe)], with HN(SiMe,), affords [ C ~ T R U ~ ( ~ - H ) ~ ( ~ - C O ) ] which has been crystallographically characterized, while with LiBHEt, a mixture of the latter and [Cp*Ru( p.-H),I2 is formed.25 This tetrahydride reacts with a variety of carboxylic acids via hydrogen

'' D. Kwon, J . Real, M. D. Curtis, A. L. Rheingold, and B. S. Haggerty, Organomerallics, 1991, 10, 143. 2 3 Q. Feng, M. Ferrer, M. L. H. Green, P. Mountford, V. S. B. Mtetwa, and K. Prout, J. Chem. SOC., Dalton

Q. Feng, M. Ferrer, M. L. H. Green, P. C. McGowan, P. Mountford, and V. S. B. Mtetwa, J. Chem. SOC., Chem. Commun., 1991, 5 5 2 .

Trans., 1991, 1397. 24

'' B-S. Kang, U. Koelle, and U . Thewalt, Orgonornernllics, 1991, 10, 2569.

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G. Hogarth 378

loss to give singly bonded [Cp*Ru(p-H)( p-O,CR)], which shows some remarkable chemistry. Thus, reaction with dichloromethane results in carbon-chlorine bond cleavages affording [C~;RU~(~-CH,)(~-C~)(~-O~CR)].~~ The unsupported doubly bonded complex [Cp*Co=CoCp*] is formed at low temperature upon co-condensation of cobalt atoms with C5Me5H. The cobalt-cobalt distance is short (2.253 A ) but remarkably this unsaturated 28-electron dimer does not react with carbon monoxide or ethylene.,' A somewhat related rhenium complex [Cp*Re(CO)2]2 is formed from [Cp*Re(CO),(thf)] upon standing in the solid-state. It contains two semi-bridging carbonyls and reacts readily with carbon monoxide and hydrogen affording [CpfRe,(CO),( p-CO)] and [Cp*Re(CO),(p-H)], respectively.28 The doubly bonded complex [Cp2Fe,(p-C0)2(p-CHMe)] ( 6 ) is the only photolysis

0 C 0

product of the corresponding tricarbonyl p-alkylidene. It reacts thermally with a variety of reagents at rates which have been elucidated by laser flash photolysis experiment^.^^ The double bond in the diphosphine-stabilized dimanganese complexes [Mn(Co)6(p-H)2(p-L)] (L = diphosphine) oxidatively adds primary and secondary phosphine~,~' while cumulenes (e.g. C 0 2 ,Me3SiN3 ,CyNCO,CyNCNCy) insert into the metal hydride bonds. With carbon dioxide a mixture of hydroformate [Mn2(Co),(p-H)(p-02cH)(p-L)] and hydroxide [Mn(CO),(p-H)( p-OH)(p-L)] complexes is formed. The latter appears not to be a decomposition product of the former.31 Finally, addition of the doubly bonded dirhenium dihydride [ Re(CO),( p- H)], to labile platinum(0) monomers has been used in cluster synthesis. Thus, addition of one equivalent to [ Pt(COD),] affords triangular [ Re,Pt(CO),( p- H),(COD)] while reaction with a second equivalent under hydrogen gives the bow-tie cluster [Re,Pt(Co),,(p-H),], the platinum centre being bound to all four rhenium atoms.32

6 Organic Ligands

A fantastic variety of both small and large organic ligands has been reported bound to bi- and polynuclear metal centres. Section 7 deals specifically with the coordination of alkynes, while this section highlights other organic ligand systems. Thus, reaction

H. Suzuki, T. Katigano, M. Igarashi, M. Tanaka, and Y. Moro-oka, J. Chem. SOC., Chem. Cornmun., 1991, 283. J . J . Schneider, R. Goddard, S. Werner, and C. Kriiger, Angew. Chem., Int. Ed. Engl., 1991, 30, 1124. C. P. Casey, H . Sakaba, P. N. Hazin, and D. R. Powell, J. Am. Chem. SOC., 1991, 113, 8165. S. D. McKee and B. E. Bursten, J. Am. Chem. SOC., 1991, 113, 1210. R. Carreho, V. Riera, and M. Ruiz, J. Organomet. Chem., 1991, 419, 163.

G. Ciani, M. Moret, A. Sironi, P. Antognazza, T. Beringhelli, G. D.'Alfonso, R. D. Pergola, and A. Minoja, J. Chem. SOC., Chem. Commun., 1991, 1255.

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31 F. J . Garcia Alonso, M. Garcia Sanz, and V. Riera, J. Organornet. Chem., 1991, 421, C12. 32

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of [Pt3( p-dppm)3(p3-CO)]2+ with methyl isocyanide results, in a stepwise fashion, in coordination of the isocyanide with a concomitant rearrangement of the carbonyl from p3 + p2 +. 73 + free CO. This may be envisaged as a model system for the chemisorption of isocyanides on platinum surfaces.33 A series of p-alkylidene complexes [Co,(CO),( p-CO){p-CR(OSiR~)}] has been ~ynthesized,~, while reaction of the square palladium cluster [ Pd4( p-CO),( p-OAc),] with four equivalents of diphenyldiazomethane results in full carbonyl substitution yielding [ Pd4( p- CPh2)4( p-OAc),], the first Schrock-type carbene palladium complex.35 Addition of alkyl- and arylmercuric halides to [ Fe(CO),(p-CO)( p-SR)]- results in formal alkyla- tion of the bridging carbonyl, providing a convenient synthesis of p-acyl complexes [ Fe2(Co)6(p-R'C=o)(p-sR)].36

Olefins have been coordinated to a number of metal centres. The complex [Ir2(C0)( 72-C2H4)(p-CO)(p-I)(p-dppm)2]+ represents a rare example of an A- frame complex containing coordinated ethylene.37 Reaction of both cis- and trans- ( Me0,C)CH=CH(C02Me) with [ R u ~ ( C O ) ~ ( p-CO)(p-dmpm),] yields the trans substituted diruthenacyclobutane [ Ru2(C0),{p-( MeO,C)CHCH(CO,Me)}( p- dmpm),] (7 ) , which has been characterized structurally. The pentacarbonyl complex is reformed upon exposure of (7) to carbon m~nox ide .~ ' A single crystal neutron

PMe2 A

Me2P I I 1 1

(C 0 ) 2 R u Ru(C0)z T:C=C~ 1 Me2P vPMe2

diffraction study of the related osmium complex [ O S ~ ( C O ) ~ ( ~ - H ~ C C H , ) ] shows that the ethylene carbons are rehybridized to sp3 and the carbon-carbon bond length is comparable with that in ethane.39 Addition of di-iodomethane to [CpW(C0)2{P(OMe)3}]- forms the p-ketene complex [Cp2W2(CO),{P(OMe)3}2(p- CO){p-CH,C(O)}] which loses water at -5°C to give the non metal-metal bonded [{CpW(CO),( P -C~) ] .~ ' Examples of unsaturated two-carbon ligands include the p-alkynyl complexes [Cp2M02(CO),( p-C,R)]-. Addition of acids results in selective attack at the P-carbon, affording side-bound p-alkenyl complexes which rearrange

R. J. Puddephatt, M. Rashidi, and J . J . Vittal, J. Chern. Soc., Dalron Trans., 1991, 2835. A. Sisak, A. Sironi, M. Moret, C . Zucchi, F. Ghelfi, and G. Palyi, J. Chern. Soc., Chern. Cornrnun., 1991, 176. T. A. Stromnova, 1 . N. Busygina, D. I. Kochubey, and I . I. Moiseev, J. Organornet. Chern., 1991, 417, 193; I. N. Busygina, T. A. Stromnova, D. I. Kochubey, M. Ya. Botnikov, and 1. I . Moiseev, Organornet. Chern. USSR., 1991, 4, 35. D. Seyfreth, C . M. Archer, D. P. Ruschke, M. Cowie, and R. W. Hilts, Organornetallics, 1991, 10, 3363. B. A. Vaartstra, J . Xiao, J . A. Jenkins, R. Verhagen, and M. Cowie, Organornetallics, 1991, 10, 2708. K. A. Johnson and W. L. Gladfelter, Organornetallics, 1991, 10, 376. 0. P. Anderson, B. R. Bender, J. R. Norton, A. C. Larson, and P. J . Vergamini, Organornetallics, 1991, 10, 3145. M-C. Chen, Y-J. Tsai, C-T. Chen, Y-C. Lin, T-W. Tseng, H-H. Lee, and Y. Wang, Organornetallics, 1991, 10, 378.

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380 G. Hogarth

uia a 1,2-hydrogen shift to give perpendicular alkyne c o m ~ l e x e s . ~ ~ The bis-alkynyl complex [Cu3(p-dppm),(p3-q '-C2Ph),]+ is formed quantitatively from the reaction of [Cu,( MeCN),(p-dpprn),l2+ with phenylethyne, and contains alkynyl ligands in the rare p3-q'-capping mode:, while reduction of [CpIrBr,], in the presence of substituted olefins affords bis-alkenyl complexes [CpIr( p-CH,CHR)], via carbon- hydrogen bond a~tivation.~, The reaction of allene with the multiply-bonded ditungsten complex [ W,(OBu'),] has been investigated in some detail. Simple addition of one equivalent of the unsaturated organic ligand affords the adduct [ W,(OBu'),(p- C3H4)] (8) in which allene symmetrically bridges the metal-metal bond. With excess allene, however, [W2(0Bu'),(p-OBu'),(p-C3H4)( q2-H2CC=CH2)] (9) is formed.

Here one allene moiety bridges the tungsten atoms in an asymmetric-ally1 fashion, while the second is bound to a single metal atom as a coordinated olefin.44 Larger organic fragments are also found bound to two or more metal centres. Trimethyl- enemethane caps three rhodium atoms in the 44-electron cluster [Rh,(COD),(pu,- H){p3-q4-(CH2)2C=CH2}]. Two methylene groups are cr-bound to single rhodium atoms, while the C=CH, unit is .rr-bound to the third. The nature of this binding has been investigated with extended Hiickel calculation^.^^ Arenes, and arene derived fragments, are found bound to a variety of metal centres. Irradiation of [ Pd( PBu:),] with phenol affords [Pd,( PBu~H)~(~-PBu')(~-~~-~~-C,H~OH)] as a result of the facile cleavage of three phosphorus-carbon bonds.46 The mwe common binding of arenes to a cluster has been investigated by structural studies on [ R ~ , ( c O ) ~ ( p , - q ~ - q2-q2-C6H6)] at room temperature and at 193 K. The benzene ring shows Kekulk- type distortions of alternating long and short bonds, a feature which has been studied further by Fenske-Hall calculation^.^^ Reaction of [Cp*IrCl( p-H)]* with triphenylphosphine under phase transfer conditions affords the ortho-phenylene complex [Cp:Ir(p-H)( p-C6H4)( p-PPh,)] uia phosphorus-aryl and carbon- hydrogen bond a ~ t i v a t i o n , ~ ~ while a number of heterobimetallic metallabenzene

S. F. T. Froom, M. Green, R. J. Mercer, K. R. Nagle, A. G. Orpen, and R. Rodrigues, J. Chem. SOC., Dalton Trans., 1991. 3171. J. Diiz, M. P. Gamasa, J. Gimeno, A. Aguirre, and S . Garcia-Granada, Organornetallics, 1991, 10, 380. A. Nessel, 0. Niirnberg, J. Wolf, and H. Werner, Angew. Chem., Int. Ed. Engl., 1991, 30, 1006. S. T. Chacon, M. H. Chisholm, K. Folting, J . C. Huffman, and M. J. Hampden-Smith, Organometallics, 1991, 10, 3722. G. E. Herberich, U. Englert, L. Wesemann, and P. Hofmann, Angew. Chem., Int. Ed. Engl., 1991,30,313. M. Sornmovigo, M. Pasquali, P. Leoni, D. Braga, and Sabatino, Chem. Ber., 1991, 124, 97. D. Braga, F. Grepioni, B. F. G. Johnson, J. Lewis, C. E. Housecroft, and M. Martinelli, Orgnnornetallics, 1991, 10, 1260. V. V. Grushin, A. B. Vymenits, A. I . Yanovsky, Yu. T. Struchkov, and M. E. Vol'pin, Organometallics, 1991, 10, 48.

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Organometallic Chemistry of Bi- and Polynuclear Complexes

I / - ,

H M O , oc co 0

381

complexes [( R3P)31r(p-q2-qS-CsHs)M~(C0)3] (10) have been ~yn thes i zed .~~ Zinc reduction of the diacetylene C1CH,C4CH2Cl in the presence of Fe3(C0)12 affords both exo and endo (1 1) isomers of the tetranuclear iron-hexapentaene complexes [{Fe2(C0)6}2(p-p-q3-q3-q3-q3-c6H8)], believed to be the first examples of poly-.rr- ally1 comple~es.’~

7 Alkyne Complexes

Alkynes have been found bound to metal centres in a variety of ways. Two papers report on parallel binding to diruthenium centres. Thermolysis of [ R u , ( C O ) ~ ( ~ - CO)(p-L),] (L = diphosphine) affords unsaturated tetracarbonyls which do not show a short metal-metal bond indicative of multiple bonding but rather contain two semi-bridging carbonyls. They react with alkynes to give mixtures of p-alkenylidene and the parallel alkyne complexes [ R u ~ ( C O ) ~ ( p-RC=CR)( p-L),].” The corresponding complex (R = C0,Me; L = dmpm) is obtained by an unprecedented series of transformations. Thus, direct reaction of [ Ru,(CO)~( p-CO)( p-dmpm),] with excess alkyne results in addition of two equivalents of the latter affording the non-metal-metal bonded complex [ Ru,(CO)~{ q’-C(O)CR=CRC(O))(p- RC=CR)( p-dmpm),], in which one alkyne has inserted into two carbonyl moieties. Thermolysis results, initially, in loss of one alkyne to give a metallacyclopentenone complex, and later one carbon monoxide to give the metal-metal bonded parallel alkyne complex [R~,(Co),(p-RC=CR)(p-dmprn)~] (R = CO,Me)’, Reaction of [ W,Cl,(thf),]- with a range of alkynes affords perpendicular derivatives [ W2Cl4(thf),(p-Cl),(p-RC2R)] but with but-2-yne the major product, [ W2C14( q2- MeC,Me)(p-Cl),]-, contains the alkyne bound only to a single metal ~en t r e . ’~ Reaction of disubstituted alkynes with the fulvalene complexes [ (CO),Ru( p- CI,H8)Mo(CO),] results in selective substitution at molybdenum to give [ ( C O ) , R U ( ~ - C ~ ~ H ~ ) M O ( C O ) ~ ( q2-RC2R)], in which the alkyne acts as a two-electron donor. Thermolysis, however, results in carbonyl loss affording the 4-electron alkyne complex [(CO)2Ru(p-C,,H8)Mo(CO)( q2-RC2R)], a process which is reversed under a pressure of carbon monoxide. In contrast, with primary alkynes the ruthenium centre is not a spectator. Reaction affords tetracarbonyl semi-bridging alkenylidene complexes, uia a 1,2-hydrogen shift, which lose carbon monoxide upon

J. R. Bleeke, Y-F. Xie, L. Bass, and M. Y. Chaing, J. Am. Chem.. SOC., 1991, 113, 4703. M. Iyoda, Y. Kuwatani, M. Oda, K. Tatsumi, and A. Nakamura, Angew. Chem., In?. Ed. Engl. 1991, 30, 1670.

5 1 J . S. Field, R. J. Haines, J . Sundermeyer, and S. F. Woollams, J. Chem. SOC., Chem. Commwn., 1991, 1382. K. A. Johnson and W. L. Gladfelter, J. Am. Chem. Soc., 1991, 113, 5097. S. G. Bott, D. L. Clark, M. L. H . Green, and P. Mountford, J. Chem. SOC., Dulron Trans., 1991, 471.

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382 G. Hoga rt h

thermolysis giving rare examples of side-on bound alkenylidene species [ (CO)Ru( p- ClOH8)( ~ - C = C R H ) M O ( C ~ ) ~ ] . ~ ~ Room temperature addition of ethyne to [W,(OR),] (R = SiBu'Me,) results in the formation of polyacetylene. However, at -1O"C, in the presence of pyridine, a perpendicular alkyne complex [W2(OR),(py)(p-Hc2H)] can be isolated. Warming to room temperature results in silanol loss to give the palkynyl compound [ W2(OR),( p-C2H)]." Related cycloalk- yne complexes [W2(oR) , (p -oR) (p -R'c2R' ) ] [R' = (CH2)4,(CH2)5] have been pre- pared as a result of the carbonyl induced coupling of alkylidyne moieties in [(R0),WC(CH2),CW(OR),1 ( n = 2,4). When n = 3 or 6, alkyne complexes were not formed due to unfavourable steric effects.56 In the dipalladium complexes [Cp#Pd2L3( p-PhC,Ph)]+ (Cp# = C,Ph,) the alkyne is bound asymmetrically across the metal-metal bond,57 while reaction of the acetonitrile derivative [ Mn2(CO),( MeCN)] with substituted alkynes MeC,X (X = OEt, NMe2) affords acyl substituted p-alkenylidene complexes [ Mn,(CO),{p-C=C( Me)C(O)X}] in which the acyl oxygen is bound to rnangane~e.,~

A number of examples in which alkynes are coordinated to three or more metal centres are reported. Reaction of 1,4-dimethoxy-2-butyne with [ Fe,(CO), ,( MeCN)] affords [Fe,(CO),(p3-RC2R)] (R = CH20Me; n = 9 or 10) containing perpendicular 6-electron and parallel 4-electron alkyne moieties re~pec t ive ly .~~ The related triruthenium complexes [ Ru,(CO),(p-dppm)( p,-PhC2Ph)] ( n = 7 or 8 ) have also

Ph Ph \ / Ph Ph

oc co 0 co c 0

A, -CO

+co - -

$ % oc co

been reported. Here parallel (12) and perpendicular (13) complexes are interconver- ted upon CO loss/addition. The perpendicular complex also reacts with a variety of other reagents, for example hydrogen, to afford parallel derivatives.,' A report on the synthesis and structural characterization of [Cp,Rh3( p3-CO)( p3-RC,R)] (R = CF3) provides the first example of a 46-electron cluster with a parallel attach- ment of the alkyne;61 while the rearrangement of ethyne at a triplatinum centre has been studied as a model for the reactions on platinum (111) surfaces.62 Dipheny-

R. Boese, M. A. Huffman, and K. P. C. Vollhardt, Angew. Chem., Int. Ed. Engl., 1991, 30, 1463. M. H. Chisholm, C. M. Cook, J . C. Huffman, and W. E. Streib, J. Chem. SOC., Dalton Trans., 1991, 929. M. H. Chisholm, K. Folting, J . C. Huffman, and E. A. Lucas, Organometallics, 1991, 10, 535. N. G. Connelly, W. E. Geiger, A. G. Orpen, J . J . Orsin'jun, and K. E. Richardson, J. Chem. Soc., Dalton Trans., 1991, 2961. R. D. Adams, G. Chen, L. Chen, M. P. Pompeo, and J . Yin, Organometallics, 1991, 10, 2541. D. Lentz and M. Reuter, Chem. Ber., 1991, 124, 773. S. Rivomanana, J. Lavigne, N. Lugan, and J-J. Bonnet, Inorg. Chem., 1991, 30, 411.

L. Manojlovic-Muir, K. W. Muir, M. Rashidi, G . Schoettel, and R. J . Puddephatt, Organometallics, 1991, 10, 1719.

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lethyne displaces the coordinated benzene in [ H20s4(CO),o( q6-C6H6)] to give [ H,OS~(CO)~( q2-PhC2Ph)(p3-PhC,Ph)] containing terminally bound and capping parallel alkyne units, both of which act as four-electron donor ligands to the cluster.63

8 Organic Transformations

A vast array of organic transformations has been reported ranging from the simple to the sublime. Carbonylation of [Cp$Rh,Me( MeCN)(p-CH,)]+ affords methylethyl ketone, a result of the coupling of both methyl and methylene moieties with carbon monoxide;64 carbonylation of [ Rh,Me,( p-CO)( p-dppm),] affords acetone and CH3C(0)C(O)CH3 uia competitive pathways.65 Reaction of the ferrole complex [Fez( CO),( p-C4Et4)] with dppm and trimethylamine- N-oxide results in carbonyl insertion into the central carbon-carbon bond to give the flyover bridge compound [ Fe,(CO),( r)1-dppm){p-C,Et,C(0)C2Et2}] containing a pendant dppm ligand.66 Reaction of excess diazomethane with [ Fe,( CO),( p-C,Bu')( p-PPh,)] yields the butadienyl complex [Fe(CO),(p-PPh2){p-(H2C=CBu')C=CH2}] as a result of methylene addition to both carbons of the p-alkynyl ligand. These complexes are also accessible uia the reaction of vinylalkynes with [HFe2(C0)7(pPPh2)].67 Re- action of dimethyldiazomethane with [Cp2Rh2(p-CO)( p-RC=CR)] (R = CF3) affords the doubly bonded p-alkylidene complex [Cp,Rh,(p.-CO){p- RC(CR=CMe,)}], which rearranges via a 1,4-hydrogen shift to the p-dienyl isomer [Cp,Rh,( p-CO)(p-q4-HRC=CR-CMe=CH2)].68 Facile carbon-carbon bond formation also occurs upon addition of unsaturated organics to [Cp*Ni( p-CO)( p- CH,)W(CO),Cp]. Thus, with allene the trimethylenemethane complex [Cp*Ni(p- CO)2{p-CH2C(CH2)}2WCp] (14) results, the new organic ligand being a-bound to nickel while binding to tungsten in a n-ally1 fashion. This complex displays remarkable fluxional behaviour which is attributed to a 'helicopter' like rotation of the hydrocarbon ligand about an axis perpendicular to its plane. With phenylethyne, the tungstenacyclopentenone complex [Cp*Ni{p-PhC=CH-C(0)- CH,}W(CO),Cp] (15) results from the regioselective coupling of methylene, carbon monoxide, and alkyne units.69 An extremely novel trimerization of a primary alkyne has been reported. Thus, while addition of tolylethyne to [ CpT Ru2( p-SR)J (R = Prl) gives the bis-alkynyl complex [ Cp*Ru( q1-C2tol)( p-SR)],, reaction with trimethyl- silyethyne gives the novel trimerization product [Cpf Ru2( p-H)( p-SR)- {p-R'C,C(=CHR')C,R'}] (16) (R' = SiMe3).70 A number of novel organic transformations have been achieved at the tri-osmium centre. Thus, the conversion of an ynamine to an aminocarbene occurs upon asymmetric hydrogenation of the tri-

H. Chen, B. F. G. Johnson, J. Lewis, D. Braga, F. Grepioni, and P. Sabatino, J. Organomet. Chem., 1991,405, C22. G. J . Sunley, I . M. Saez, D. J. Gulliver, P. S. Williams, and P. M. Maitlis, J. Chem. SOC., Chem. Commun., 1991, 193. K. W. Kramarz, T. C. Eisenschmid, D. A. Deutsch, and R. Eisenberg, J. Am. Chem. SOC., 1991,113,5090. R. Giordano, E. Sappa, D. Cauzzi, G. Predieri, A. Tiripicchio, and M. Tiripicchio Camellini, J. Organomet. Chem., 1991,412, C14. S. M. Breckenridge, S. A. MacLaughlin, N . J. Taylor, and A. J. Carty, J. Chem. SOC., Chem. Commun., 1991, 1718. R. S. Dickson and D. C. Greaves, J. Chem. SOC., Chem. Commun., 1991, 1300. M. J . Chetcuti, P. E. Fanwick, and B. E. Grant, Organometallics, 1991, 10, 3003. H. Matsuzaka, Y. Mizobe, M. Nishio. and M. Hidai, J. Chem. Soc., Chem. Commun., 1991, 1011.

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3 84 G. Hogarth

Me& \ C

SiMe3 *c /

osmium complex [Os3(CO),(p-OMe){p3-q4-CHMeCNMe2)] affording [0s3(C0),{ =CEt( NMe2)}( p H ) ( p-OMe)171 Tri-osmium aminocarbene complexes themselves undergo a variety of novel transformations. Reaction of [ O S ~ ( C O ) ~ ~ { =CEt( NMe2)}] (17) with terminal alkynes initially results in simple carbonyl substitution, the alkyne coordinating in a parallel fashion to the tri-osmium centre (18). Warming to 68”C, however, leads to carbonyl loss and oxidative-addition of the carbon-hydrogen bond of the alkyne to the cluster, giving [Os3(CO),{=CEt(NMe2)}(p-H)(p3-C,R)] (18). Coupling of carbene and alkynyl ligands occurs under high pressures of carbon monoxide to give zwitterionic parallel alkyne complexes (19) (Scheme l).72 The reactivity of an hydrido-tri-osmium cluster containing a bridging aminocarbene has also been reported. Here addition of alkynes results in initial insertion into a metal-hydride bond affording u-T vinyl complexes, while later thermolysis results in carbon-carbon bond formation.73 The reactivity

Et\ ,NMe2 Me2 +; ( 3 3

Et, /NMe2 C II /C,A \

-, HC<Os(CO), - A,CO Et //s\H M e , N O /22&.o s( o)

C II

(co)4oS- OS(CO)4

RC,H

/C\\ 3 ( c o ) 3 0 s - ~ - o s ( c o ) , (CO)30~-

/0Y0)3 ‘H’ R

( 1 7 ) (18) (19)

Scheme 1

” R. D. Adams, M. P. Pompeo, and J . T. Tanner, Organometallics, 1991, 10, 1068. R. D. Adams and G. Chen, Organometallics, 1991, 10, 3020. R. D. Adams and G. Chen, Organometallics. 1991, 10, 3028.

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of benzyne coordinated to three or four ruthenium centres is the subject of one publication. At the trinuclear centre in [ Ru3(C0),( p-PPh2)( p3-C6H4)], carbonylation results in carbon-carbon bond formation and the opening up of one metal-metal bond to give [ Ru,(CO),( p-PPh2)2(p3-C6H4C=0)]. Carbonylation of tetranuclear [ Ru4( CO),,( p34-PPh)(p4-C6H4)] (20), however, proceeds uia a novel series of transformations to afford binuclear [RU~(C~),(~-~~-P~P~(O)C~H~}] (21) and [ R ~ ~ ( C o ) , ~ ( p ~ - P P h ) { p - q ~ - ~ ( ~ ) ~ ~ ~ ~ ~ = ~ } ] (22). Even more remarkably these transformations are reversed upon thermolysis (Scheme 2).74 Two publications

0

A

co d e-

document the coupling of alkynyl ligands at cluster centres. Thus, thermolysis of [Ru3(CO),(p-PR,)(p3-C2But)] (R = C2But) affords [RU~(CO)~~(~~-~~-BU~C~BU~)- (p4-PR)] in which the diacetylene acts as an eight-electron donor to the square ruthenium cluster;,' while [ FeIr,(CO),( PPh3),( p3-v2-PhC4Ph)] is one of the products of the reaction of Fe(CO), with [Ir(CO),(PPh,),(C,Ph)]. In the latter the dialkyne acts only as a six-electron ligand, being simply an alkynyl substituted perpendicular alkyne ~omplex . '~

9 Heterocyclic Ring Opening Reactions

The 1991 literature contains reports of the coordination and subsequent opening up of a variety of sulfur, selenium, tellurium, and phosphorus containing organic rings. Reaction of the saturated four-membered ring 3,3-dimethylthietane (DMT), cyclo-SCH2CMe2CH2, with [0s3(C0), (MeCN)] initially affords the simple derivatives [Os3(C0),,( q-DMT)] and [Os3(CO),o(p-DMT)].77 U.V. irradiation of the latter results in ring opening to give the metallacyclic complex [Os3(C0),,(p-q3-

J. P. H. Charmant, H. A. A. Dickson, N. J. Grist, J. B. Keister, S. A. R. Knox, D. A. V. Morton, A. G. Orpen, and J . M. Vitias, J. Chem. SOC., Chem. Commun., 1991, 1393. B. J. Bobbie, N. J. Taylor, and A. J. Carty, J. Chem. SOC., Chem. Commun., 1991, 1511. M. I . Bruce, G. A. Koutsantonis, and E. R. T. Tiekink, J. Organomet. Chem., 1991, 407, 391. R. D. Adams and M. P. Pompeo, J. Am. Chern. Soc., 1991, 113, 1619.

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386 G. Hogarth

SCH2CMe2CH2)] containing a new metal-carbon bond, while addition of tetraethyl- ammonium chloride followed by aci'd gives another product of ring opening, namely thiolato-bridged [OS~(CO),~(~-H)(~-SCH~CM~~CH~C~)].~~ In the presence of excess DMT, the bridged complex [Os,(CO),,(p-DMT)] undergoes a novel ring opening trimeri~ation,~' while a somewhat similar ring opening dimerization occurs upon photolysis of the rhenium complex [ Re2(CO),( DMT)2].79 Reaction of 2,5- dihydrothiophene with Ru3(CO) 12 affords the carbon-hydrogen oxidative-addition product [RU,(CO),(~-H)(~~-~~-SCH~CH=CHCH)], no ring opening being observed.80 The interaction of thiophenes with metal centres continues to attract attention with a view to understanding homogeneous hydrodesulfurization catalysis. Reaction of the iridium 2,5-dimethyl thiophene (DMT') complex [Cp*Ir( q4-DMT')] (23) with Fe3(C0)12 initially affords the ring opened sulfido-capped ferrole cluster [Cp*IrFe2(CO),( p-CMeCHCHCMe)( p3-S)] (24). Complete removal of sulfur is attained upon reaction with carbon monoxide affording [Cp*Ir(p-CO)( p- CMeCHCHCMe)Fe(CO),] ( 2 5 ) (Scheme 3) .81 The related rhodium complex

Ir

I

(23)

I + q ; F e ( C 0 l 2

[Cp*Rh(DMT*)Fe(CO),] (DMT* = tetramethylthiophene) ring opens at 110°C to afford the triangular sulfido-capped cluster [ Cpq Rh2Fe( CO),( p-CO)( P ~ - S ) ] . ~ , One potential hydrodesulfurization catalyst is the mixed metal cluster [Cp~Mo,Co,(CO),( ~ ~ - s ) ~ ( p ~ - S ) ] . It reacts with thiophene at temperatures between 110 and 150°C and under an hydrogen atmosphere to eliminate alkanes and give the new tetrasulfido cluster [Cp;Mo,Co,(CO),( P ~ - S ) ~ ] . Under lOOOpsi of carbon monoxide at 120"C, the latter is converted to the starting cluster in low yield thus providing the basis of a catalytic Selenophene and tellurophene (cyclo- C4H4X) are readily ring opened by trinuclear complexes of group 8. Thermolysis of selenophene with [Os,(CO),,( MeCN),] initially affords the ring opened product [OS~(CO),~(~-~~-S~CHCHCHCH)], in which cleavage of only one selenium- carbon bond has occurred. Thermolysis of this complex with a second equivalent of [ O S ~ ( C O ) , ~ ( M ~ C N ) ~ ] , however, results in cleavage of the second Se-C bond affording [Os3(CO),o(p3-Se)(p-CHCHCHC)Os3(CO)lo(p-H)], in which two tri- osmium units are linked via the cleaved organic moiety. Reaction of selenophene

R. D. Adams, J . A. Belinski, and M. P. Pompeo, Organometallics, 1991, 10, 2539. R. D. Adams, J . A. Belinski, and J. Schierlmann, J. Am. Chern. Sol . 1991, 113, 9004. M-G. Choi, L. M. Daniels, and R. J . Angelici, Inorg. Chern., 1991, 30, 3647.

'' J . Chen. L. M. Daniels, and R. J . Angelici, J . Am. Chern. Soc., 1991, 113, 2544. S. LUO, A. E. Oglivy, T. B. Rauchfuss, A. L. Rheingold, and S. R. Wilson, Organomelallics, 1991, 10, 1002. U. Riaz, 0. Curnow, and M. D. Curtis, X Am. Chern. Soc., 1991, 113, 1416.

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Organometallic Chemistry of Bi- and Polynuclear Complexes 3 87

with both Fe3(C0),, and R u ~ ( C O ) ~ ~ directly affords (together with other products) the ferrole complexes [ M2(C0)6( p-C4H4)] via facile extrusion of selenium. Tel- lurophene appears to undergo similar reactions due to the relative instability of the p3-Te moiety; fewer products could be isolated and ~haracterized.'~ Two papers describe the coordination and ring opening of phenylphospholes at the triosmium centre. Initially, simple substitution products are formed in which the phosphole is bound through phosphorus to a single metal centre. Thermolysis of these complexes, however, results in competitive phosphorus-carbon bond cleavage between the phosphorus-phenyl and phosphorus-ring carbon bonds. The products of the latter always contain the p-PhPCHCRCRCH unit and no evidence is found for the occurrence of a second carbon-phosphorus bond cleavage which would lead to extrusion of the phosphorus moiety." These results clearly reflect the increased stability of (group 15)-carbon versus (group 16)-carbon bonds.

10 0 x 0 and Imido Ligation

The chemistry of organometallic complexes containing the strong T-donating 0x0 (02-) and imido (NR2-)ligands is one of extreme current interest. This is reflected in the increasing number of publications concerned with the stabilization of these ligands at bi- and polynuclear metal centres. The synthesis and reactivity of alkyl-oxo complexes [ R,ReO( p-O)], is the subject of three papers. The complexes are formed upon addition of alkylzinc reagents to CpReO, (R = Me) or Re207 (R = Et)," and can be reduced to the radical anion by c o b a l t o ~ e n e , ~ ~ while their reaction with hydrogen sulfide leads to substitution of one or both bridging 0x0 moieties by sulfido ligands (R = CH2CMe3),88 Re-oxidation of the radical anion [ Me,ReO( p-O)], by molecular oxygen affordx7 the novel trinuclear anion [ Me2ReO( p-O),ReMe,( p- O),ReOMe,]-. The related dirhenium complexes [ ReO( T ~ - RC2R),I2 are formed upon reaction of [ ReOI( v2-RC2R),] with dimethylzinc. Despite the fact that all the ligands can potentially bridge the metal-metal bond it is somewhat surprising that an unsupported rhenium-rhenium interaction is f a ~ o u r e d . ' ~ Four papers report on the synthesis and reactivity of cyclopentadienyl-stabilized dimolybdenum complexes [Cp2MoXX*(p-Y)(p-Y*)] (X,X*,Y,Y* = O,S,Se,NR). They are synthesized from the reactions of [CpMo(CO),], with As4S4 90 or PhN02,91 or from Na[ M o ( C O ) ~ C ~ ] and SeOC12.90 The nitrobenzene route has also been used to synthesize benzo-15- crown-5 crown ether imido complexes. The electrochemistry of these has been studied, addition of Na+ causing significant anodic shifts in the potentials of the oxidation processes.92 Aspects of their reactivity are also reported. Thus in contrast

A. J. Arce, R. Machado, C. Rivas, Y. DeSanctis, and A. J. Deeming, J. Organornet. Chem., 1991,419,63. A. J. Acre, Y . DeSanctis, J . Manzur, A. J . Deeming, and N. I. Powell, J. Organornet. Chern., 1991, 408, C18; A. J. Deeming, N. 1. Powell, A. J. Arce, Y. DeSanctis, and J. Manzur, J. Chern. SOC., Dalton Trans., 1991, 3381. W. A. Herrmann, C. C. Rornao, P. Kiprof, J. Behrn, M. R. Cook, and M. Taillefer, J. Organornet. Chern., 1991, 413, 11. W. A. Herrmann, R. W. Albach, and J . Behrn, J. Chern. SOC., Chern. Cornmun., 1991, 367. S. Cai, D. M. Hoffman, and I>. A. Wierda, Inorg. Chem., 1991, 30, 827. E. Spaltenstein and J. M. Mayer, J. Am. Chem. SOC., 1991, 113, 7744. M. Gorzellik, H. Bock, L. Gang, B. Nuber, and M. L. Zeigler, J. Organomet. Chern., 1991, 412, 95.

M. L. H. Green, G. Hogarth, and G. C . Saunders, J. Organomet. Chem., 1991, 421, 233.

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388 G. Hogarth

to the dirhenium complex [R2ReO(p-0)I2, the tri(oxo)complex [Cp$Mo202(p-O)- (p-NPh)] reacts with hydrogen sulfide to substitute selectively one terminal 0x0 moiety yielding [Cp;Mo,OS(p-O)( P - N P ~ ) ] . ~ , Reports on ditungsten oxides include the synthesis of [Cp*WO(p-Te)], ,90 and the formation of p-alkylidene complexes [W20(OR)4(py)(p-OR)2(p-CHAr)] (R = CH2Bu') (26 ) from the reactions of steri- cally unencumbered aldehydes with [ W2(OR)6(py)2].94 Chromocene and permethyl chromocene have been used in the synthesis of bi-, tri-, and tetranuclear 0x0 and imido complexes. Thus, reaction of CpTCr with oxygen affords [Cp*CrO(p-O)], , while with N 2 0 the cubane cluster [Cp*Cr( p3-O)l4 is produced. Interestingly, further reaction of the latter with N 2 0 affords the binuclear oxide.95 Chromocene itself reacts with azobenzene to give the 49-electron paramagnetic cluster [ Cp,Cr,( p- NPh),(p,-NPh)] which undergoes smooth oxidation on alumina to give the corresponding 48-electron cation.96 Capping imido ligands are also produced upon reduction of nitrosyl ligands, reaction of [Cp'Fe(p-NO)], with [Cp'Co( q2-C2H4),] affording [Cp~CoFe2(p3-NH),] as a minor p r ~ d u c t . ~ ' A new synthetic route to bridging imido complexes involves the addition of mononuclear imido moieties to other labile metal fragments in a fashion akin to that widely exploited in p-alkylidene and p-alkylidyne chemistry. Thus, addition of [ Pt( q2-dppe)( q2-C2H4)] to [Cp*IrNBu'] affords binuclear [ ( q2-dppe)Pt( ~ - N B U ' ) I ~ C ~ * ] , ~ ~ while reaction of C O ~ ( C O ) ~ with [Re( NBu'),CI] gives tetranuclear [Co,Re(CO),( p-CO),( p3-NBu'),] (27).99 The organic chemistry of complexes containing these ligands has received

0 RO o c o

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R R

very little attention. However, thermolysis of the imido-carbene cluster [Fe3(C0),{=CPh(0Et>}(p3-NPh),] affords the imine EtN=CPh(OEt), resulting from carbon-nitrogen bond formation."'

11 Large Clusters

The synthesis of a number of large clusters of group 8 and 9 metals has been reported. Thermolysis of R U , ( C O ) ~ ~ with the alkylurea ( Me2N)2CS affords

J. Fletcher, G . Hogarth, and D. A. Tocher, J. Organomet. Chem., 1991, 405, 207. M. H. Chisholm, E. A. Lucas, A. C . Sousa, J . C . Huffman, K. Folting, E. B. Lobkovsky, and W. E. Streib, J. Chem. SOC., Chem. Commun., 1991, 847. F. Bottomley, J. Chen, S. M. MacIntosh, and R. C. Thompson, Organomefallics, 1991, 10, 906. I. L. Eremenko, A. A. Pasynskii, E. A. Vas'utinskay, A. S. Katugin, S. E. Nefedov, 0. G. Ellert, V. M. Novotortsev, A. F., Shestakov, A. I . Yanovsky, and Yu. T. Struchkov, J. Organornet. Chem., 1991, 411, 193. J. Miiller, I . Sonn, and T. Akhnoukh, J. Organomet. Chem., 1991, 414, 381.

V. Saboonchian, A. Gutierrez, G . Wilkinson, B. Hussain-Bates, and M. B. Hursthouse, Polyhedron, 1991, 10, 1423. M. Tasi, A. K. Powell, and H. Vahrenkamp, Chem. Ber., 1991, 124, 1549.

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[ HRU,(CO),~(~~-S),]- , a planar ruthenium cluster with almost perfect c3" symmetry;'" while vacuum pyrolysis of [OS, (CO)~~( MeCN)2] affords [ HOs,(CO),,]- shown to contain a tricapped octahedral core.'02 Decanuclear clusters of group 8 are particularly prevalent. Reaction of R u ~ ( C O ) ~ ~ with [ Ru,N(CO),,]- affords [ RU,~N(CO)~,]-, the first decametal nitrido cluster,103 while heating [ O S ~ ( C O ) ~ ~ ( M ~ C N ) ~ ] at 190°C affords [os~O(co)26]2- which loses carbon dioxide upon vacuum pyrolysis to give [OS~,,C(CO)~~]~- in 85% yield.lM Reports on the synthesis of large clusters of group 9 metals include that of the electron-rich carbonyl [ co6(co)6(p~-s)8], formed directly from the reaction of Co2(C0)* with sulfur,1o5 and [ Ir,(CO) 13(p-CO)7]3-, a cofacial bioctahedral cluster.lo6 Three publications document the synthesis of clusters with greater than ten metal atoms. Thermolysis of [ Rh6N( CO) 15]- affords [ Rh14( N)2( CO)25]2- which contains two interstitial nitrogens, each bound to five rhodium atoms.'07 From the reaction of [Pd4(CO)SL4] and [Pdlo(C0)12L,] (L = PEt,), [Pd16(C0)13L9] has been isolated and crystallographically characterized,'08 while thermolysis of [Os,( CO),,( MeCN),] at 300°C for 70h affords [ O S ~ ~ ( C O ) ~ ~ ] ~ - in 20% yield. This latter cluster is shown to have a tetrahedral cubic close packed metal core, and preliminary cyclic volta- metric studies indicate a rich electrochemistry reflecting the increased density of accessible oxidation states for large metal particle^.'^^

Reactivity studies have also been reported for large metal clusters. Thus, for pentanuclear [ Ru,(CO),,( p-PPh2)( ps-C2PPh2)] basal carbonyl substitution by triethylphosphite is noted upon thermolysis, while activation with trimethylamine- N - oxide results in wing-tip substitution. No interconversion of isomers was noted.' lo

Heating the parent carbonyl cluster in oxirane results in the isolation of isomeric [ Ru5(CO),,(p-PPh2)(p4-PPh)(p5-C2Ph)] formed as a consequence of two carbon- phosphorus bond cleavage reactions and formation of a new carbon-carbon bond.'' ' While it is generally assumed that high temperature thermolysis is required to convert a cluster bound carbonyl into a carbido ligand, the reaction of [ R U ~ ( C O ) ~ ~ ] ~ - with triflic anhydride has been found to produce [ Ru6C( CO) 17] quantitatively.' l2 Ther- molysis of [ H20~7(C0)20] with diphenylethyne initially results in the formation of [Os,(CO)18( p3-CPh),], formed upon scission of the alkyne, while after longer reaction times the cleavage of a second alkyne gives [ O S ~ ( C O ) ~ ~ ( ~ ~ - C P ~ ) , ] . " ~

U. Bodensieck, H. Stoeckli-Evans, and G. Suss-Fink, Angew. Chem., Int. Ed. Engl., 1991, 30, 1126. A. J . Amoroso, B. F. G. Johnson, J . Lewis, P. R. Raithby, and W. T. Wang, J. Chem. Soc., Chem. Commun., 1991, 814. P. J . Bailey, C . C . Conole, B. F. G. Johnson, J. Lewis, M. McPartlin, A. Moule, and D. A. Wilkinson, Angew. Chem., Int. Ed. Engl., 1991, 30, 1706. A. J . Amoroso, B. F. G. Johnson, J . Lewis, P. R. Raithby, and W. T. Wang, Angew. Chem., Int. Ed. EngL, 1991, 30, 1505. E. Diana, G. Gervasio, R. Rossetti, F. Valdemarin, G. Bor, and P. L. Stanghellini, Inorg. Chern., 1991, 30, 294. R. D. Pergola, F. Demartin, L. Garlaschelli, M. Manassero, S. Martinengo, N . Masciocchi, and D. Strumolo, Inorg. Chem., 1991, 30, 846. S. Martinengo, G. Ciani, and A. Sironi, J. Chem. SOC., Chem. Commun., 1991, 26. E. G. Mednikov, Yu. L. Slovokhotov, and Yu. T. Struchkov, Organomet. Chem. USSR., 1991, 4, 65. A. J. Amoroso, L. H. Gade, B. F. G. Johnson, J . Lewis, P. R. Raithby, and W-T. Wong, Angew. Chem., Inf. Ed. Engl., 1991, 30, 107. M. I. Bruce, M. J. Liddell, E. R. T. Tiekink, and B. K. Nicholson, J. Organomet. Chem., 1991,410, 211. C . J. Adams, M. I. Bruce, B. W. Skelton, and A. H. White, J. Organomet. Chem., 1991, 420, 87. P. J . Bailey, J. Organornet. Chem., 1991, 420, C21. D . Braga, F. Grepioni, B. F. G. Johnson, J . Lewis, and J . Lunnis, J. Chem. SOC., Dalton Trans., 1991,2223.

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390 G. Hogarth

Addition of diphenylethyne to decanuclear [ R U ~ ' C ~ ( C O ) ~ ~ ] ~ - (28) gives a very different result. The alkyne displac\es two carbonyls and bridges the octahedral metal clusters affording [ RU~,C~(CO)~~(~-P~C,P~)]~- (29) and a new ruthenium- ruthenium bond. The process can even be reversed, reaction of (29) with carbon monoxide resulting in alkyne displacement and metal-metal bond cleavage (Scheme 4).", 7 2-

0 0 C C I I

1*- PhC,Ph, 125 "C

2C0.125"C

b 4-

Ph Ph

12 Heterometallic Complexes

A number of early-late heterobimetallic complexes have been reported. Addition of [ MeTi( OBu'),] to [ HCo( CO),] gives [ (Bu'O),Ti-Co( CO),] which contains an unsupported early-late interaction.' l 5 The p-methylene complexes [Cp,Ti(p-CH,)- (p -~~-c&x)Rh(CoD)] (X = OMe, NMe,) also contain bridging aryl ligands,'16 while reaction of the metal carbonyl dimers [CpM(CO),], (M = Mo, W) with group 5 metal hydrides [Cp2M'(H),] (M' = Nb,Ta) affords the new complexes [Cp2M'(p- CO),M(CO)Cp].' l 7 Other reports on heterobimetallics include a study of the structures of the unbridged datively bonded complexes [(CO),-,( Bu'NC),Os +

Cr(CO),] (x = 1,2).'18 and a report of the synthesis of [(CO),Cr(p-q', q2- C6H5=CH2)Pd( q3-ally1)]."9 A number of unusual heterometallic clusters have been synthesized. Reaction of [Cpr Ni2( p-P2)] (Cp# = C,HPrh) with W(CO),(thf) gives the bis-phosphide cluster [CpTNi2W(C0),(p3-P),] in which there is a weak phosphorus-phosphorus interaction. Oxidation with bis(trimethylsily1)peroxide then affords [Cp2#Ni2W(C0),(p3-PO),] (30), the first example of a PO complex.'*' Reaction of excess [WS,]'- with [Cp*RuCl,], affords the novel trinuclear cluster [ C ~ T R U , W S ( ~ - S ) ~ ( p3-S)( p-S,)] (3 1) which contains terminal, doubly, and triply bridging, sulfido moieties.12' Two papers document the synthesis of exotic anti- ferromagnetic chromium-iron cluster^,'^^"^^ including the bow-tie cluster cation 114

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L. Ma, D. P. S. Rodgers, S. R. Wilson, and J. R. Shapley, Inorg. Chem., 1991, 30, 3591. D. Selent, R. Beckhaus, and T. Bartik, J. Organomet. Chem., 1991, 405, C1S. J. W. Park, L. M. Henling, W. P. Schaefer, and R. H. Grubbs, Organometallics, 1991, 10, 171. D. Perry, J. C. Leblanc, C. Moi'se, and J . M a r t i n G I , J. Organomet. Chem., 1991, 412, 363. J. A. Shipley, R. J . Batchelor, F. W. B. Einstein, and R. K. Pomeroy, Organometallics, 1991, 10, 3620. V. M. Kalinin, I. A. Cherepanov, S . K. Moiseev, A. S. Batsanov, and Yu. T. Struchkov, J. Chem. SOC., Mend. Commun., 1991, 17. 0. J. Scherer, J. Braun, P. Walther, G. Heckmann, and G. Wolmerhauser, Angew. Chem., Int. Ed. Engl., 1991, 30, 852. Y. Mizobe, M. Hosomizu, J. Kawabata, and M. Hidai, J. Chem. Soc., Chem. Commun., 1991, 1226. S . E. Nefedov, A. A. Pasynskii, I. L. Eremenko, E. E. Stomakhina, A. I. Yanovsky, and Yu, T. Struchkov, J. Organornet. Chem., 1991, 405, 287. A. A. Pasynskii, I. L. Erernenko, E. E. Stomakhina, S. E. Nefedov, 0. G. Ellert, A. I. Yanovsky, and Yu. T. Strickhov. J. Organornet. Chem., 1991, 406, 383.

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Organometallic Chemistry of Bi- and Polynuclear Complexes

0

391

[C~,Cr~(p-!3R)~(p,-S),Fel+ (32) in which iron is linked to all four chromium atoms and each Cr,Fe triangle is capped top and bottom by sulfido ligands.12, A new approach to cluster synthesis has been reported using [(CO)SReC2Re(CO),] as a building block. Thus, reaction with C O ~ ( C O ) ~ results in coordination of the alkyne to the dicobalt unit giving the tetranuclear alkynyl complex [Co,Re(CO),(p- co>2{P,-c2Re(co>,}] (33).12,

Examples of complexes in which other transition metals are bonded to copper or gold abound. Heterobimetallic copper complexes include the p-alkenylidene compounds [( q6-C6H6)( PR3)0s(p-C=CHR')CuCp]125 and the diphosphine bridged [( Ph,P)C~(p-dppm)Fe(CO),{Si(OMe)~)1;~~~ while gold compounds include [CpRu( PCy3)(p-H)2Au(PPh3)]127 and datively bonded [ (CNMe),Ir( p- d ~ p m ) ~ A u ] ~ + . ' ~ ~ . Reaction of the carborane-stabilized gold monomer [(CB)Au(PPh,)]- (CB = 7,s-nido-C,B,H,Me,) with trans-[IrCl(PPh,),] in the presence of TlBF, affords heterobimetallic [ (CB)Ir( PPh3)( CO)Au( PPh,)] in essentially quantitative yield, a result of the transfer of the carborane cage from gold to iridium. Similar transfer is noted to r h 0 d i ~ r n . I ~ ~ Copper-containing clusters are formed from the reaction of metal hydrides with copper( I ) chloride. Thus, addition of [Cr2(CO)lo(p-H)]- in liquid ammonia affords trinuclear [Cr2Cu(CO)10( NH3)2(p3- H)] (34) in which copper bridges the dichromium centre,130 while reaction with [Cp*Ru( PCy,)H,] gives [{Cp*RuH( PCy3)(p-H)2Cu}2(p-C1)2] in which

T. Weidmann, V. Weinrich, B. Wagner, C. Robl, and W. Beck, Chem. Ber., 1991, 124, 1363. H. Werner, R. Weinand, W. Knaup, K. Peters, and H. G. von Schnering, Organometallics, 1991, 10,3967. P. Braunstein, M. Knorr, U. Schubert, M. Lanfranchi, and A. Tiripicchio, J. Chem. Soc., Dalton Trans., 1991, 1507. A. Antitiolo, F. A. Jalon, A. Otero, M.Fajardo, B. Chaudret, F. Lahoz, and J. A. Lbpez, J. Chem. Soc., Dalfon Trans., 1991, 1861. A. L. Balch and V. J . Catalano, Inorg. Chem., 1991, 30, 1302. J . A. K. Howard, J. C. Jeffery, P. A. Jelliss, T. Sommerfeld, and F. G. A. Stone, J. Chem. SOC., Chem. Commun., 1991, 1664. P. Klufers and U. Wilhelrn, J. Organomet. Chem., 1991, 421, 39.

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392 G. Hoga rt h

Scheme 5

bis( hydrido)-bridged copper-ruthenium centres are linked together by copper- chloride bridges.',' Trinuclear (34) reacts with pyridine to afford tetranuclear [{(py)C~(p,-H)Cr(C0)~},] (35) , a planar butterfly cluster containing a short copper- copper contact (2.360 A) (Scheme 5 ) . I 3 O Clusters containing gold include trigonal bipyramidal [ ( PPh3),Au,Co( CO),]', in which the cobalt atom lies in the trigonal plane,132 the square tetranuclear complex [ (CO)8Fe2Au2( p-dppm)] which contains a short diphosphine-bridged gold-gold distance,133 and [ Ni,,AU6(C0)24]2-, an 18- vertex cubic tetrahedral complex composed of five fused octahedra, the central octahedron being formed from six gold atoms.134 A number of research groups,135 but most notably Adams and c o - ~ o r k e r s , ' ~ ~ - ' ~ ~ have reported on the synthesis, structures, and reactivity of mixed platinum-osmium clusters. The general method of synthesis of the smaller clusters is from the reaction of tri-osmium clusters with platinum(o) complexes, most commonly [ Pt(COD),]. Thus, with [Os(CO),,( MeCN)2]136 and [ ~ ~ ~ ( C ~ > I O ( C L - H ) ~ I ' ~ ~ pentanuclear [ Pt20~3(C0)10(COD)] and [ Pt,0s3(CO),(COD),(p-H)2] are formed respectively. Heating the smaller clusters provides a convenient entry into the larger mixed metal complexes. Thus, thermolysis of [ P~,OS~(CO), (COD), (~-H)~] gives [Pt40S6(Co),l(CoD)(p-H)2] (36),'37 and from [Pt20~3(C0)10(COD)Z] both [Pt40s6(CO),,(COD)] (37) and [Pt50~6(C0)21(COD)2] (38) can be i s~ la t ed . '~~These large clusters are interesting since they appear to show a segregation of platinum and osmium into alternating layers of the pure element. The reactivity of hexanuclear [ Pt20s4(CO),,] has been investigated towards a number of reagents. Under 100 atmospheres of hydrogen four new clusters are produced including nonanuclear [ Pt20~7(C0)23( P - H ) ~ ] , ~ ~ ~ while the thermal reaction with cycloocta- 1,5-diene results in displacement of four carbonyls affording [ Pt20~4(C0)12(COD)2] (39).I4O Sub- sequent photolysis of (39) results in further carbonyl loss to give the unsaturated cluster [P t20~4(CO)l l (COD),] (40) which contains several short metal-metal interac-

T. Artiluie, B. Chaudret, F. A. Jalon, A. Otero, J . A. Lopez, and F. J . Lahoz, Organometallics, 1991, 10, 1888. G. Beuter, J . Mielcke, and J . Strahle, Z. Anorg. Allg. Chem., 1991, 593, 35. S. Alvarez, 0. Rossell, M. Seco, J . Valls, M. A. Pellinghelli, and A. Tiripicchio, Organometallics, 1991, 10, 2309. A. J. Whoolery and L. F. Dahl, J . Am. Chem. SOC., 1991, 113, 6683. L. J . Farrugia, Acta Crystallogr. Sect. C., 1991, 47, 1310; D. H. Farrar, R. R. Gukathasan, and J . A. Lunniss, Inorg. Chem. Acta, 1991, 179,271; L. J . Farrugia, and S. E. Rae, Organomerallics, 1991, 10,3919. R. D. Adams, G. Chen, J-C. Lii, and W. Wu, Inorg. Chem., 1991, 30, 1007. R. D. Adams, J-C. Lii, and W. Wu, Inorg. Chem., 1991, 30, 3613. R. D. Adams, J-C. Lii, and W. Wu, Inorg, Chem. 1991, 30, 2257. R. D. Adams, M. P. Pompeo, and W. Wu, Inorg. Chem., 1991, 30, 2425. R. D . Adams and W. Wu, Organometallics, 1991, 10, 35; R. D . Adams and W. Wu, Inorg. Chem., 1991, 30, 3605.

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tions indicative of some multiple bond character. It is highly reactive, addition of carbon monoxide regenerating (39) while [ Pt(COD),] gives the expanded cluster [ Pt3Os4(C0), ,(COD),] (41) (Scheme 6).14*

13 Assorted Complexes

Other important developments in bi- and polynuclear organometallic chemistry include the use of metal vapours in cluster synthesis, the linking together of cluster units by mercury atoms, the inclusion of metal-metal bonds into organic polymers, and the synthesis and characterization of clusters impregnated into zeolites. Metal vapour synthesis has long been used in the preparation of novel organometallic compounds. Thus, co-condensation of nickel atoms with CSHSBut gives the clusters [Cp4#Ni4) and [ c ~ , # N i ~ ( p - H ) ~ ] . ' ~ ~ In an important development, metal atoms have been condensed with Fe(CO), in the presence of unsaturated organics to afford

R. D. Adarns, M. P. Pompeo, and W. Wu, Inorg. Chem., 1991, 30, 2899. R. D. Adarns, M. S. Alexander, I. Arafa, and W. Wu, Inorg. Chem., 1991, 30, 4717. J . J. Schneider, R. Goddard, C . Kruger, S. Werner, and B. Metz, Chem. Ber., 1991, 124, 301

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394 G. Hogarth

COD

0 (40)

R(COD), -co

Pt COD

I (COh

COD Pt 0 s

Scheme 6

Pt COD (41 1

new mixed-metal clusters. With nickel atoms and C,Me,H trinuclear [Cp~Ni,Fe(C0)3(p3-CO)2] is produced, while co-condensation with cobalt and mesitylene affords [(me~)~CoFe(CO)~(p~-C0)~].~~

Transition metal centres linked by a single mercury atom are long established. In a more recent development, however, cluster units have been joined by one or more mercury atoms. Thus, reaction of two equivalents of [ R u ~ C ( C ~ ) , , ] ~ - with Hg(CF3C02), affords [ { R U ~ C ( C O ) , ~ } , ( ~ ~ - H ~ ) ] ~ - in which the mercury links octahedral clusters by bridging edges.145 More remarkably, reaction of [ O S , ~ C ( C O ) ~ ~ ] ~ - with Hg(CF3C02), gives [{Os,C(CO),,},(p-Hg3)]2-, two nonanuclear metal fragments being linked via a triangle of mercury atoms. Irradiation results in extrusion of one atom of mercury to give [{Os,C(CO)2,}2(p-Hg2)]2-, a process which is reversed when the latter is exposed to metallic mercury.146

Photochemically reactive polymers are a topic of immense current interest. Tyler and co-workers have succeeded in synthesizing polyurethanes containing [CpMo(CO),], or [CpFe(CO),], dimers in the backbone, molecular weights of up to

144

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147

16000 ( n = 20) being a~hieved . ’~’ Photolysis in the presence of carbon tetra-

J. J. Schneider, R. Goddard, and C. Kriiger, Organomelallics, 1991, 10, 665. B. F. G. Johnson, W-L. Kwik, J. Lewis, P. R. Raithby, and V. P. Saharan, J . Chem. SOC., Dalton Trans., 1991, 1037. L. H. Gade, B. F. G. Johnson, J . Lewis, M. McPartlin, T. Kotch, and A. J. Lees, J. Am. Chem. SOC., 1991, 113, 8698. S . C. Tenhaeff and D. R. Tyler, Organomerallics, 1991, 10, 473.

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chloride results in facile bond homolysis, while heating the molybdenum polymer leads to carbon monoxide loss to give a new polymer incorporating molybdenum- molybdenum triple bonds.’48

The catalytic properties of metals incorporated into zeolites is an area of great potential. A number of anionic platinum clusters [Pt3(cO)6]:- ( n = 3,4) have been synthesized within NaY zeolite uia a ‘ship-in-bottle’ procedure. Thus, cationic [Pt(NH3)4]2- is entrapped into the zeolite by cation exchange. Later reduction of the trapped species by carbon monoxide then affords the included clusters which show high catalytic activity for the reduction of NO by carbon monoxide to give N20 and nitrogen.’49 Both iridium d i m e r ~ ” ~ and hexanuclear cluster^'^' have also been synthesized within NaY zeolite. In the latter, different isomers of Ir6(CO)16 are formed depending upon the conditions of carbonylation. Either isomer can, however, be reversibly decarbonylated, opening up the prospect of a rich iridium chemistry within zeolite cavities.

S. C. Tenhaeff and D. R. Tyler, Organometallics, 1991, 10, 1116. G-J. Li, T. Fujimoto, A. Fukuoka, and M. Ichikawa, J. Chem. SOC., Chem. Commun., 1991, 1337. L. Crowfoot, G. A. Ozin, and S. Ozkar, J. Am. Chem. Soc., 1991, 113, 2033. S. Kani and B. C. Gates, J. Chem. SOC., Chem. Commun., 1991, 994.

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chapter 19. organometallic chemistry of bi- and poly-nuclear complexes

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