Synthesis, structure and reactions of seven-coordinate, phosphine-supported, halide-activated carbonyl- and alkyne-complexes of niobium(I)

The complexes (Hal)Nb(CO)₃(PR₃)₃ (PR₃ = PEt₃, Hal = I; PR₃ = PMe₂Ph, Hal = Cl, Br, I) and (Hal)Nb(CO)_4/2(dppe)_1/2 (Hal = Br, I) have been prepared by oxidative halogenation of carbonylniobate with pyridinium halides (Hal = Cl, Br) or iodine (Hal = I). In the tricarbonyls, one CO and one PR₃ are labile and can be displaced by a four-electron donating alkyne to give all-trans-[(Hal)Nb(CO)₂(RCCR′)(PR₃)₂] (PR₃ = PMe₂Ph; Hal = Cl, Br, I: R, R′ = H, Et, Ph; R = H, R′ = Ph. PR₃ = PEt₃, Hal = I: R, R′ = Pr; R = H, R′ = Bu, Ph; R = Me, R′ = Et). In the case of acetylene, INb(CO)(HCCH)₂(PEt₃)₂ is also formed. PR₃ can be displaced by P(OMe) ₃. In the tetracarbonyls, two CO ligands are replaced by two isonitriles to form INb(CO)₂(CNR)₂dppe (R = ^tBu, Cy), or by one alkyne to form (Hal)Nb(CO)₂(PhCCPh)dppe (Hal = Br, I). In these complexes, the remaining CO ligands occupy cis positions. The structure of BrNb(CO)₂(dppe)₂·THF, INb(CO)₂(dppe)₂·hexane and INb(CO)₂(PEt₃)₂(MeCCEt) have been determined by a single crystal X-ray diffraction study. The alkyne complexes are best regarded as octahedral with the centre of the alkyne ligand occupying the positions trans to the halide and the CC axis aligned with the OC---Nb---CO axis. The complexes (Hal)Nb(CO)₂(dppe)₂ adopt a trigonal prismatic structure with the halide capping the tetragonal face spanned by the four phosphorus functions. The crystal structure of a by-product, Br₂Nb(CO)(H₂CPhPCH₂CH₂PPh₂)₂·1/2THF has also been determined. The geometry is pentagonal bipyramidal, with one of the bromine atoms and the CO on the axis. Some ⁹³ Nb NMR data for the Nb^I complexes are presented, and preliminary observations on the reactions between the π-alkyne complexes and H₂ or H⁻ are reported.     

P and C NMR investigation of the system tetracarbonyldi-μ-chlorodirhodium(I) — tertiary phosphine

¹³C and ³¹P{¹H} NMR data at low temperature prompted us to characterize cis-[Rh(CO)₂(PR₃)Cl] (3) (3a, PR₃ = PPh₃; 3b, PR₃ = PMe₂Ph), as surprisingly stable products of the reaction between [{Rh(CO)₂(μ-Cl)}₂] (1) and tertiary phosphines in toluene (P : Rh = 1). Every attempt to isolate solid 3a led to the cis- and trans- halide-bridged dimers [{Rh(CO)₂(μ-Cl)}₂] (5a) and 6a which are formed from 3a by slow decarbonylation, a process which is greatly accelerated by the evaporation of the solvent under vacuum.

The analogous reaction of 1 with dimethylphenylphosphine follows a similar pathway; in this case, however, low temperature NMR spectra allowed us to characterize the pentacoordinated dinuclear species [{Rh(CO)₂(μ-Cl)}₂] (2b) as the unstable intermediate of the bridge-splitting process.

The reaction of 3 with a second equivalent of phosphine (P : Rh = 2) leads, at room temperature, to the well known product trans-[Rh(CO)(PR₃)₂Cl] (8) accompanied by evolution of CO; however our data show that when the reaction is performed at 200 K, decarbonylation is prevented and spectroscopic evidence of trigonal bipyramidal pentacoordinate [Rh(CO)₂(PR₃)₂Cl] (7), stable only at low temperature, can be obtained.     

Electrochemical studies on Mo2Cl4(PR3)4 compounds containing phosphines of varied basicity

The electrochemical behaviour of a series of Mo₂Cl₄(PR₃)₄ complexes (PR₃ = PMe₃, PEt₃, PPrⁿ₃,PBuⁿ₃, PH₂Ph, PMe₂Ph, PEt₂Ph, PHPh₂, PMePh₂, PEtPh₂, P(OMe)₃, P(OMe)Ph₂) has been examined by cyclic voltammetry in dichloromethane solution. The phosphines were chosen to provide a wide range of Lewis basicity/π acidity as reflected by Tolman's _co IR and Bodner's Δδ_co ¹³C NMR spectral parameters for Ni(CO)₃(PR₃). The Mo₂ compounds undergo either quasi-reversible or irreversible one-electron oxidations except for P(OMe)₃ and P(OMe)Ph₂ for which no clectroactivity was observed before the solvent limit. The anodic peak potentials, E_p,a, span a range of nearly 700 mV. The half-wave potentials, E_1/2,for the quasi-reversible couples and E_p,a for all were plotted against the IR and NMR values and against the δ → δ* transition energies for the Mo₂ species in dichloromethane and in the solid state. For the organometallic spectral parameters excellent linear correlations were obtained while with the electronic spectral data fair correlations resulted. These results indicate that the Mo₂Cl₄(PR₃)₄ complexes become more difficult to oxidize as the electron-withdrawing nature of the PR₃ substituents increases and the δ → δ* band energy decreases.     

Reactivity of ruthenium complexes with uninegative (X,S)-donor ligands (X = S,P)—II. Phosphine substitution in [Ru(S,S)2(PR3)2] derivatives. Crystal structure of trans-bis(O-ethyldithiocarbonate)bis(dimethylphenylphosphine)-ruthenium(II)

The complexes [Ru(S,S)₂(PPh₃)₂] [S,S = EtCOCS₂⁻, (CH₂)₄NCS₂⁻] react with a variety of tertiary phosphines with the substitution of triphenylphosphine and the formation of [Ru(S,S)₂(PR₃)₂]. The reaction occurs with the formation ofthe cis isomer, except for the complex with PMe₂Ph that gives rise to the trans isomer as the crystal structure shows. The effect of the different phosphines on the ruthenium complex is analysed in terms of the spectroscopic and electrochemical properties of the isolated compounds. The cyclic voltammetric studies of the cis complexes show that isomerization to the trans isomer occurs on oxidation. This isomerization is not observed in the trans-[Ru(S,S)₂(PMe₂Ph)₂] complexes that give rise to stable trans-ruthenium(II)/ruthenium(III) couples. In a similar way the diphosphine complexes afford a quasi-reversible cis-ruthenium(II)/ruthenium(III) process.     

Substituted cyclopentadienyl ligands—10. Intramolecular steric interactions in [(η-C5H3(SiMe3)2)Mo(CO)2(L)I]

Reaction of C₅H₄(SiMe₃)₂ with Mo(CO)₆ yielded [(η⁵-C₅H₃(SiMe₃)₂)Mo(CO)₃]₂, which on addition of iodine gave [(η⁵-C₅H₃(SiMe₃)₂Mo(CO)₃I]. Carbonyl displacement by a range of ligands: [L = P(OMe)₃, P(OPrⁱ)₃,P(O-o-tol)₃, PMe₃, PMe₂Ph, PMePh₂, PPh₃, P(m-tol)₃] gave the new complexes [(η⁵-C₅H₃(SiMe₃)₂ MO(CO)₂(L)I]. For all the trans isomer was the dominant, if not exclusive, isomer formed in the reaction. An NOE spectral analysis of [(η⁵-C₅H₃(SiMe₃)₂)Mo(CO)₂(L)I] L = PMe₂Ph, P(OMe)₃] revealed that the L group resided on the sterically uncongested side of the cyclopentadienyl ligand and that the ligand did not access the congested side of the molecule. Quantification of this phenomenon [L = P(OMe)₃] was achieved by means of the vertex angle of overlap methodology. This methodology revealed a steric preference with the trans isomer (less congestion of CO than I with an SiMe₃ group) being the more stable isomer for L = P(OMe)₃.     

Reaction of heterocumulenes R---N=C=N---R and R---P=C=P---R with a di-iron aminocarbene complex

Reaction of R---N=C=N---R (R=p-Me-C₆H₄) and R---P==C=P---R (R=2,4,6-^tBu₃C₆H₂) with the di-iron aminocarbene complex [Fe₂(CO)₇{1μ-C(Ph)C(NEt₂)}] (1c) gave corresponding complexes [Fe₂(CO)₆{C(Ph)C(NEt₂)C(NC₆H₄Me)N (C₆H₄Me)}] (2) and [Fe₂(CO)₆{C(Ph)C(NEt₂)C(PC₆H₂^tBu₃)P(C₆H₂^tBu₃)}] (4), resulting from a coupling reaction with carbon-carbon bond formation. [Fe₂(CO)₅(CNC₆H₄Me){C(Ph)C(NEt₂)N(C₆H₄Me)}], complex 3, obtained in the reaction with R---N=C=N---R, resulted from C=N bond rupture insertion of a nitrene fragment into the Fe=C bond. Complexes 2–4 were characterized by X-ray diffraction. The different geornetries of complexes 2 and 4 are discussed. The formation of these complexes may be explained by cycloaddition on the Fe =C metal-carbene bond.     

Equilibrium constants and kinetics of carbon monoxide insertion in alkyl complexes of ruthenium(II)

The equilibrium constants of the reaction of cis, trans-[Ru(CO)₂(PMe₃)₂(CH₃)I] (Mc) with carbon monoxide to give cis, trans[Ru(CO)₂(PMe₃)₂ (COMe)i] (Ac) and trans, trans[Ru(CO)₂(PMe₃)₂(COMe)I] (At) were measured at various temperatures in toluene. The thermodynamic parameters are compared with those obtained for the isoelectronic complexes of iron, and the trend is discussed. The kinetics of the carbonylation reaction of Mc, as well as those of the inverse decarbonylation reaction of At were measured. The kinetics of the carbonylation of the new complex trans, trans-[Ru(CO)₂(PMe₃)₂(CH₃)I] (Mt) were also investigated. All the results afford further support to the previously proposed CO insertion mechanism occurring via methyl migration. The comparison of these kinetic results with those of isoelectronic complexes of iron indicates that ruthenium is more reactive than iron, which is reflected by its greater aptitude to act as catalyst in many processes.     

Carbene derivatives of areneruthenium(II) complexes in one step from terminal alkynes

The complexes [(η⁶-arene)Ru=C(OMe)CH₂R′)Cl(PR₃)]PF₆ (R′ = Ph; ARENE = Me₄C₆H₂, ⁱPr₃C₆H₃, Et₃C₆H₃; PR₃ = PMe₃, PPh₃, P(OMe)₃) have been made from RuCl₂(PR₃)(arene) precursors by activation at room temperature of phenylacetylene in methanol containing NaPF₆. The complex with R′ = ⁿBu, ARENE = Me₄C₆H₂, and PR₃ = PMe₃ is similarly formed from hex-1-yne but much more slowly, and a complex of the type [(p-cymene)Ru=C(OMe)CH₂R′)Cl(PR₃)]⁺PF₆⁻ could be obtained only when the phosphine was the bulky PPh₃ (10b). It has been shown that the steric hindrance by both arene and phosphine ligands contributes to the stabilization of the carbeneruthenium complexes.     

Multiple ligand transfer reaction between [CpRu(L)(AN)2][PF6] (L=AN, CO, P(OMe)3; AN=acetonitrile) and CpFe(CO)L′X (L′=CO, PMe3, PMe2Ph, PMePh2, PPh3, P(OPh)3; X=Cl, Br, I)

Treatment of ruthenium complexes [CpRu(AN)₃][PF₆] (1a) (AN=acetonitrile) with iron complexes CpFe(CO)₂X (2a–2c) (X=Cl, Br, I) and CpFe(CO)L′X (6a–6g) (L′=PMe₃, PMe₂Ph, PMePh₂, PPh₃, P(OPh)₃; X=Cl, Br, I) in refluxing CH₂Cl₂ for 3 h results in a triple ligand transfer reaction from iron to ruthenium to give stable ruthenium complexes CpRu(CO)₂X (3a–3c) (X=Cl, Br, I) and CpRu(CO)L′X (7a–7g) (L′=PMe₃, PMe₂Ph, PMePh₂, PPh₃, P(OPh)₃; X=Br, I), respectively. Similar reaction of [CpRu(L)(AN)₂][PF₆] (1b: L=CO, 1c: P(OMe)₃) causes double ligand transfer to yield complexes 3a–3c and 7a–7h. Halide on iron, CO on iron or ruthenium, and two acetonitrile ligands on ruthenium are essential for the present ligand transfer reaction. The dinuclear ruthenium complex 11a [CpRu(CO)(μ-I)]₂ was isolated from the reaction of 1a with 6a at 0°C. Complex 11a slowly decomposes in CH₂Cl₂ at room temperature to give 3a, and transforms into 7a by the reaction with PMe₃.     

Nouveaux composes heterotrinucleaires fer—ruthenium a ponts hydroxo et hydruro

Two new types of heteropolymetallic complexes FeRu₂(μ-OH)₂(CO)₈(PR₃)₂ and FeRu₂(μ-H)(μ-OH)(CO)₈(PR₃)₂ have been obtained in acetone and isopropanol, respectively, from the reaction of FeRu₂(μ-Cl)₂ complexes with bases. They contained a Ru---Fe---Ru bent-chain with the bridging groups holding the ruthenium atoms.     

9-BBN activation. Synthesis, crystal structure and theoretical characterization of the ruthenium complex Ru[(μ-H)2BC8H14]2(PCy3)

Reaction of the bis(dihydrogen) ruthenium complex RuH₂(H₂)₂(PCy₃)₂ (1) with an excess of 9-borabicyclononane yields Ru[(μ-H)₂BC₈H₁₄]₂(PCy₃) (6) and the phosphine adduct PCy₃·HBC₈H₁₄. The new complex is characterized by NMR spectroscopy and X-ray diffraction. New X-ray data on 9-BBN dimer, from a measurement at 180 K, are also reported. DFT calculations (B3LYP) on Ru[(μ-H)₂BC₈H₁₄]₂(PMe₃) (7), the PMe₃ analogue of 6, confirm the ruthenium (II) formulation with two dihydroborate ligands. The data obtained using PH₃ or PMe₃ as models for PCy₃ in PR₃·HBC₈H₁₄ are also discussed.     

Bis(di-t-butylphosphino)methane complexes of rhodium: homogeneous alkyne hydrosilylation by catalyst-dependent alkyne insertion into Rh---Si or Rh---H bonds. Molecular structures of the dimer [(dtbpm)RhCl]2 and of the silyl complex (dtbpm) Rh[Si(OEt)3](PMe3)

The homogeneous, Rh-catalysed hydrosilylation of but-2-yne with triethoxysilane has been studied. All rhodium complexes employed as catalyst precursors contain ^tBu₂PCH₂P^tBu₂ (“dtbpm”) as a chelating ligand. The crystal and molecular structure of the dimer [(dtbpm)RhCl]₂ (10) has been determined by X-ray diffraction. Complex 10 is shown to be a sluggish catalyst in hydrosilylation reactions of hex-1-ene, whereas but-2-yne is hydrosilylated more rapidly. A much more efficient and highly selective catalyst is 10 with added PPh₃, equivalent to the use of monomeric (dtbpm)RhCl(PPh₃). (E)-2-Triethoxysilylbut-2-ene is formed exclusively and with high turnover numbers in this case. For both 10 and its PPh₃ derivative, the 14-electron fragment [(dtbpm)RhCl], formed by dissociation processes, is the most likely active intermediate in a Harrod-Chalk-type catalytic cycle. The PPh₃ dissociation equilibrium has been studied in detail for (dtbpm)RhCl(PPh₃) and its thermodynamic parameters have been determined. With rhodium alkyl complexes as catalyst precursors, a different type of alkyne hydrosilylation catalysis, involving direct alkyne insertion into the Rh---Si bond of an intermediate rhodium silyl complex, (dtbpm)Rh[Si(OEt)₃](PMe₃) (14), has been found. Complex 14 was synthesized independently from (dtbpm)RhMe(PMe₃) and characterized by X-ray diffraction. It is an equally active catalyst itself, yielding (E)-2-triethoxysilylbut-2-ene as the major product (90%) from but-2-yne and HSi(OEt)₃ (turnover number 1000 per 30 min). The insertion step of the alkyne into the Rh---Si bond of 14 and the formation of two stereoisomeric rhodium vinyl complexes were established independently for MeO₂CCCCO₂Me as a more reactive alkyne substrate. A catalytic cycle is proposed for this unprecedented hydrosilylation reaction. The synthesis of the ν³-benzyl complex (dtbpm)Rh(η³-CH₂C₆H₅) (23) is described. This compound allows an alternative, more efficient access to the new silyl complex (dtbpm)Rh[Si(OEt)₃](PMe₃).     

钌的二甲基氯硅烷羰基化合物和衍生物的合成及表征

用SiMe₂ClH与Ru₃(CO)₁₂反应,得到顺式-Ru(CO)₄(SiMe₂Cl)₂(I)和[Ru(CO)₄(SiMe₂Cl)]₂(Ⅱ)。它们的SiMe₂Cl配位基呈现较强的反位效应,由此合成得到一系列含膦(氧磷)或含卤素的衍生物。进行了Ⅰ-Ⅵ的元素分析、IR、¹HNMR和MS表征。     

exo-closo-Rhodacarboranes: synthesis and characterisation of [{exo-(R3P)2Rh}(closo-CB11H12)] [R3P=P(OMe)3, PCy3, 1/2dppe]

Addition of H₂ to CH₂Cl₂ solutions of [(diene)Rh(L)₂][closo-CB₁₁H₁₂] (diene=norbornadiene, cyclooctadiene, L=PCy₃, P(OMe)₃, 1/2dppe) results in the formation of the exo-closo complexes [(PR₃)₂Rh(closo-CB₁₁H₁₂)]. These have been characterised in solution by ¹H- and ¹¹B-NMR spectroscopy, and for L=PCy₃ by a single crystal X-ray diffraction study. This suggests that the metal fragment is bound with the cage through the 7,8- and not the 7,12-{BH} vertices. DFT calculations on a model system where L=PMe₃ show that there is only a negligible energy difference between these two isomers (1 kcal mol⁻¹), suggesting that both represent stable structures. The salient spectroscopic markers that indicate an interaction of [closo-CB₁₁H₁₂]⁻ with a metal fragment are discussed and compared across a range of metal complexes. Large upfield shifts in the ¹¹B-NMR spectrum and a small downfield shift of the CH vertex in the ¹H-NMR spectrum are shown to the most reliable indicators of borane interaction in solution.     

Thirty years of organometallic aryldiazenido arylazo derivatives

The aryldiazenido ligands provide the fourth member of the isoelectronic series CO, NO⁺, RNC, RN₂⁺ of ligands for transition metal complexes. The first aryldiazenido metal complex was reported in 1964 when p-CH₃OC₆H₄N₂Mo(CO)₂C₅H₅ was prepared by the reaction of NaMo(CO)₃C₅H₅ with p-CH₃OC₆H₄N₂⁺BF₄⁻. This review surveys the development of organometallic aryldiazenido chemistry since that time. Such organometallic aryldiazenido derivatives, including RN₂M(CO)₂C₅H₅, RN₂M(CO)₂(Pz₃BH) (M = Cr, Mo, W), [(η⁶-Me₆C₆)Cr(CO)₂N₂Ar]⁺, [(MeC₁₅H₄)M′(CO)₂N₂Ar]⁺ M′ = Mn, Re), [trans-PhN₂Fe(CO)₂(PPh₃)₂]⁺, and PhN₂M′(CO)₂(PPh₃)₂(PPh₃)₂ can be obtained by reactions of arenediazonium salts with suitably chosen transition metal nucleophiles. Analogous methods cannot be used to prepare alkyldiazenido transition metal complexes because of the instability of alkyldiazonium salts. However, the alkyldiazenido derivatives RCH₂N₂M(CO)₂C₅H₅ (R = H or Me₃Si) can be obtained from HM(CO)₃C₅H₅ and the corresponding diazoalkanes. Important aspects of the chemical reactivity of RN₂M(CO)₂Q derivatives (Q = C₅H₅, Pz₃BH) include CO substitution reactions, coordination of the second nitrogen in the RN₂ ligand to give heterobimetallic complexes such as C₅H₅Mo(CO)₂(μ-NNC₆H₄Me)(CO)₂C₅H₅, oxidative addition rections with X₂ X = Cl, Br, I), SnX₄, RSSR, and CINO, and reactions with further RN₂⁺ to give bis(aryldiazenido) derivatives (RN₂)₂MQL⁺ (L = CO, X⁻, etc.). Dearylation of an aryldiazenido ligand to a dinitrogen ligand can be effected by reaction of [(MeC₅H₄)M′(CO)₂N₂Ar]⁺ with certain nucleophiles to give (MeC₅H₄)M′(CO)₂N₂.     

Komplexe mit kohlenstoffsulfiden und -seleniden als liganden : X. Synthese und kristallstruktur von C5H5(PMe3)Co(μ-CS)2CoC5H5: Ein zweikernkomplex mit unsymmetrischen Co(CS)Co-brückenbindungen

The complex C₅H₅(PMe₃)Co(μ-CS)₂CoC₅H₅ (I) is formed by the reaction of C₅H₅Co(PMe₃)CS and CH₂I₂. The X-ray structure analysis shows an unsymmetrical non-planar Co₂C₂-skeleton with different Co---C bond lengths. The Co---Co distance is 239.2 pm. Compound I thus represents a new example of binuclear (18 + 16)-electron complexes in which the more electron-rich metal atom forms a donor bond to the more electron-poor counterpart. The reaction of I with ligands such as P(NMe₂)₃ does not lead to bridge cleavage indicating the stability of the Co(CS)₂Co-framework.     

The initiation of ring-opening metathesis polymerisation of norbornadiene by seven-coordinate molybdenum(II) compounds. X-ray crystal structure of [Mo(μ-Cl)(SnCl3)(CO)3(η-NBD)]

The new heterobimetallic complex [Mo(μ-Cl)(SnCl₃)(CO)₃(η⁴-NBD)] (1) has been prepared by reaction of [(CO)₄Mo(μ-Cl)₃Mo(SnCl₃)(CO)₃] with norbornadiene (NBD) at room temperature. The structure of complex 1 was established by X-ray crystallography. The IR, ¹H- and ¹³C-NMR spectra of 1 are also described and can be correlated with the crystallographically observed geometry. In the presence of an excess of NBD compound 1 initiates the ring-opening metathesis polymerisation (ROMP). The initiation mechanism of ROMP by seven-coordinate molybdenum(II) compounds have been discussed. The microstructure of polynorbornadiene formed was determined by ¹H- and ¹³C-NMR spectroscopy.     

Synthesis, structure and characterization of M---Sn (M=Mo, W) bonded heterobimetallic complexes containing bis(pyrazol-1-yl)methane ligands

The reaction of bis(pyrazol-1-yl)methane tetracarbonylmolybdenum(0) or tungsten(0) complexes with RSnCl₃ (R=Ph, Cl) at room temperature yielded heterobimetallic complexes CH₂(Pz)₂M(CO)₃(Cl)(SnCl₂R) (Pz represents substituted pyrazole; M=Mo or W; R=Ph or Cl) in good yields, which have been characterized by elemental analysis, ¹H NMR and IR spectroscopy. The reaction of bis(3,5-dimethyl-4-halopyrazol-1-yl)methane tetracarbonyl tungsten with PhSnCl₃ did not take place even in refluxing CH₂Cl₂. The electronic and steric characteristics of substituents on the pyrazole ring remarkably influence the structures of the products. The structures of CH₂(3,5-Me₂-4-BrPz)₂W(CO)₃(Cl)(SnCl₃) (8) and CH₂(4-BrPz)₂Mo(CO)₃(μ-Cl)(SnCl₂Ph) (17) (Pz: pyrazole) determined by X-ray crystallography show that no chlorine-bridged W---Sn bond is observed in complex 8, while one chlorine-bridged Mo---Sn bond exists in complex 17. The Sn---M bond length is 2.7438(5) Å in complex 8 (W---Sn) and 2.7559(4) Å in complex 17 (Mo---Sn).     

Mixed-metal cluster chemistry VI: Phosphine substitution at CpMoIr3(μ-CO)3(CO)8; X-ray crystal structure of CpMoIr3(μ-CO)3(CO)7(PPh3)

Reactions of CpMoIr₃(μ-CO)₃(CO)₈ (1) with stoichiometric amounts of phosphines afford the substitution products CpMoIr₃(μ-CO)₃(CO)_8−x (L)_x (L = PPh₃, x = 1 (2), 2 (3); L = PMe₃, x = 1 (4), 2 (5), 3 (6)) in fair to good yields (23–54%); the yields of both 3 and 6 are increased on reacting 1 with excess phosphine. Products 2–5 are fluxional in solution, with the interconverting isomers resolvable at low temperatures. A structural study of one isomer of 2 reveals that the three edges of an MoIr₂ face of the tetrahedral core are spanned by bridging carbonyls, and that the iridium-bound triphenyiphosphine ligates radially and the molybdenum-bound cyclopentadienyl coordinates axially with respect to this Molr₂ face. Information from this crystal structure, ³¹P NMR data (both solution and solid-state), and results with analogous tungsten—triiridium and tetrairidium clusters have been employed to suggest coordination geometries for the isomeric derivatives.     

Molybdenum-molybdenum quadruple bonds: An asymmetrically substituted dimer containing a single acetate bridge. Crystal and molecular structure of [Mo2Cl3(μ-OAc)(PMe3)3]·PhMe

[Mo₂(OAc)₄] reacts with three or more equivalents of lithium chloride and PMe₃ in thf to give [Mo₂Cl₃(μ-OAc)(PMe₃)₃]0.75thf (1). The IR spectrum of the complex shows Mo---O and Mo---Cl stretches at 350 and 300 cm⁻¹ respectively and the ¹H and ¹³C NMR spectra suggest several species are present in solution. [Mo₂Cl₃(μ-OAc)(PMe₃)₃] converts slowly in thf to [Mo₂Cl₄(PMe₃)₄] and [Mo₂(OAc)₄]. The structure of [Mo₂Cl₃(μ-OAc) (PMe₃)₃]0.5C₆H₅Me (2) has been determined by single-crystal X-ray diffraction methods. Crystals of the toluene solvate are tetragonal with a = 20.726(2), c = 11.776(2) Å, space GROUP = I4cm. The structure was solved by Patterson and Fourier methods and refined to R of 0.035 for the 539 observed data. The molecule contains two metal centres each of which shows 5-fold coordination. The two molybdenum atoms are linked by an acetate bridge and a short Mo---Mo bond of 2.121(3) Å. Remaining coordination sites are occupied on Mo(1) by two Cl and one PMe₃ and on Mo(2) by one Cl and two PMe₃ groups.