Multiple Bonds Between Metal Atoms / 06-X3M _ MX3 Compounds of Molybdenum and Tungsten

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With less sterically demanding alkoxide ligands, closely related compounds M2(OR)6(µ- CO)(py)2 have been isolated.207,210 Here the pyridine ligands bind in a trans position to the M–C bond. However, the py ligands are labile, and in solution the tungsten complex W2(OPri)6(µ- CO)(py)2 reacts by py dissociation to give the tetranuclear complex W4(µ-CO)2(OPri)12(py)2.211 The addition of PriOH to W2(OBut)6(µ-CO) leads to W4(µ-CO)2(OPri)12, 6.15.207,210

6.15

The reaction that takes a W2(µ-CO) compound to a W4(µ-CO)2 containing compound comes about with an increase in W–W distance, from 2.50 to 2.67 Å and an increase in CO distance from 1.25 to 1.35 Å. The W–C distance decreases from 2.00 to 1.95 Å. These changes are consistent with a further reduction of the CO ligand and an oxidation of the ditungsten center. A good case can be made that in 6.15 the C–O bond and W–W bond distances represent single bonds and the chemical shift of the bridging carbonyl carbon at 310 ppm is in the range often seen for µ-alkylidyne carbon atoms. The W–O distances associated with the W4(µ-CO)2 moiety are 1.97 Å which is comparable to an alkoxide O to W distance. This is indicative of Op/ to Wd/ donation. The reduction of the CO ligand in this sequence of reactions arises from the combination of W2 d/ to CO /* back-bonding and Op/ to Wd/ donation. The former reduces the CO / bond by adding electron density to the CO /* molecular orbital and the latter by removing electron density from the filled CO / bonds by Op/ to Wd/ donation. Recognition of this fact led to investigations of the reactivity between W4(OR)12 compounds and CO and the alcoholysis reaction between W2(OBut)6(µ-CO) and PriOH in the presence of W2(OBut)6. In both cases, reductive cleavage of CO was observed with the formation of tetranuclear W4(µ-C) containing clusters.212

In the presence of more than one equiv of CO, higher carbonylated complexes have been obtained such as Mo(OBut)2(py)2(CO)2, Mo2(OPri)8(CO)2,213 and W2(OPri)6(CO)4 (6.16).214 In 6.16 a WVI(OR)6 acts as a bidentate ligand to W0(CO)4.214 These compounds reveal how redox disproportionation occurs leading to M(CO)6 and higher oxidation state metal alkoxides M(OBut)4 or W(OPri)6.

6.16

Changing from alkoxides to siloxides or fluoroalkoxides changes the nature of CO uptake at the (M>M)6+ center. W2(OCMe2CF3)6(CO)2 is a compound of the type M2(OR)6L2 with two terminal δ1-CO ligands that are disposed so as to maximize MM and M to CO /-bonding.215 The same formation of W2(OR)6(CO)2 is seen for R = Me2ButSi and 2,6-Me2C6H3.

The ethane-like dimer W2Cl2(silox)4 also reacts with CO to give a carbonyl adduct of type shown in 6.17 when ArNC is replaced by CO. Upon heating to 120 ºC over 4 h this compound looses CO and reacts to give the oxo-carbide shown in 6.18.216

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6.17

6.18

Carbon monoxide has also been found to react with the compounds W2Cl2(NMe2)4 and W2(NMe2)2(OCMe2CF3)4 to give terminal carbonyl adducts and products of CO insertion into the amide bonds.215,217

6.9.2 Isocyanide complexes

As noted earlier, M2(OR)6 reacts in the presence of excess arylisocyanide to give M(CNAr)6 by disproportionation. However, monoisocyanide adducts of ditungsten hexaalkoxides have been isolated and fully characterized.218 The compounds W2(OBut)6(CNAr) and W2(OPri)6(CNAr)(py), where Ar = 2,6-C6H3Me2, were similar to their carbonyl adducts in having WW distances of c. 2.52 Å, which are comparable to M=M bonds. These compounds also have bridging isocyanide ligands. However, the bridging isocyanides were asymmetrically bonded and had C–N–C angles of c. 130 º. The µ-CNC plane was aligned along the WW axis and a theoretical investigation into the bonding revealed that this was favored by W2 to CNC backbonding. In solution, these compounds are fluxional and it was not possible to freeze out the inversion at nitrogen of the bridging isonitrile ligand on the NMR time scale.

The compound W2Cl2(silox)4 was noted to form a bis-isocyanide complex and a carbonylisocyanide complex W2Cl2(silox)4(CO)(CNAr) which, based on NMR studies, was assigned the structure shown in 6.17.216

6.9.3 Reactions with alkynes

Alkynes and Mo2(OR)6 compounds were first noted to react via adduct formation and CC coupling reactions.219,220 This led to characterization of the compounds Mo2(OPri)6(µ-C2H2)(py)2 and Mo2(OCH2But)6(µ-C4H4)(py). W2(OBut)6 and the alkynes RC>CR where R = Me, Et and Pr were shortly thereafter reported to enter into the metathesis reaction, the Schrock “Chop Chop” reaction:221

W2(OBut)6 + RC>CR Α 2[W(OBut)3(CR)]

Schrock, et al. extended this to a general route to (ButO)3W>CR compounds by employing terminal alkynes.221 They also reported (ButO)3Mo>CPh could be prepared similarly.222 The reactions between W2(OR)6 compounds and alkynes were subsequently shown to be very sensitive to the nature of the steric and electronic properties of the R groups.

Compounds such as W2(OPri)6(µ-C2H2)(py)2, W2(OCH2But)6(µ-C2Me2)(py)2, and W2(OPri)6(µ-C4R4)(δ2-C2R2), where R = Me and H were also structurally characterized.223 The alkyne adducts were shown to enter into C–C coupling reactions with alkynes and nitriles.58, 224-225 The ethyne adduct W2(OBut)6(µ-C2H2)(py) was shown to exist in equilibrium with the methylidyne complexes (ButO)3W>CH on the basis of the following double labeling experiment:226

W2(OBut)6(µ-C2D2)(py) + W2(OBut)6(µ-13C2H2)(py) 2W2(OBut)6(µ-H13CCD)(py)

Further evidence for the generality of the equilibrium between alkyne adducts and (ButO)3W>CR compounds was presented based on trapping experiments. The addition of CO to (ButO)3W>CMe in hydrocarbon solutions gave the butyne adduct W2(OBut)6(µ-C2Me2)(CO).227

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Addition of CO to (ButO)3W>C–(CH2)n–C>W(OBut)3 gave W2(OBut)6(µ-C2(CH2)n)(CO) where n = 4 and 5.228

The compounds (ButO)3W>CR are alkyne metathesis catalysts229,230 and react via the reversible formation of metallacyclobutadienes. In reactions between alkynes and W2[OCH(CF3)2]6, W2(OC6H3-2,6-Me2)6 or W2[OCMe2(CF3)]6 tungstacyclobutadiene complexes (RO)3WC3Et3 were isolated and characterized.231,232

In reactions between certain W2(OR)6 compounds and alkynes where the alkyne to W2(OR)6 ratio is 1:3, alkylidyne capped tritungsten clusters such as W3(µ-CMe)(OPri)9 are formed.128, 233 These products form as a result of the alkyne metathesis reaction being followed by a comproportionation between the reactive (RO)3W>CR' species and the W2(OR)6 starting material. In the reaction between W2(OPri)6 and 3-hexyne, the M>M/C>C metathesis reaction, alkynealkyne coupling and formation of the alkylidyne clusters are all competitive reactions.234

The general scheme of reactions for alkynes and M2(OR)6 is shown in Scheme 6.4. The alkyne adducts have µ-perpendicular alkyne ligands that span MM bonds of distance c. 2.65 Å. A notable feature of these compounds is the presence of long CC (alkyne) distances that fall in the range 1.38 to 1.44 Å. These are longer than those typically seen in alkyne adducts such as Co2(CO)6(µ-C2H2) that have distances in the range 1.30-1.35 Å. Also, it was noted from the spectroscopic studies of µ-13C2H2 compounds that the carbon-carbon coupling constants were very small, in the range of 12-24 Hz. These are notably smaller than the 56 Hz coupling in Co2(CO)6(µ-C2H2) which in turn can be compared to 256 Hz in free acetylene. This, together with the observed long CC distances testifies to the rehybridization of the alkyne upon binding to the (M>M)6+ center. The compounds can be viewed as dimetallatetrahedranes and the following reversible reaction as an internal redox reaction.235

(RO)6M2(µ-C2R2') 2[(RO)3M>CR']

Notable in this context is the observation that addition of donor ligands such as quinuclidine drive the equilibrium to the right while acceptors such as CO capture the alkyne adduct. Also, whereas this equilbrium is often seen for W, it has not been observed for Mo which is easier to reduce and harder to oxidize.

A theoretical investigation into the reaction pathway leading to the cleavage of the CC bond as shown in the equation above, implicated the asymmetrical transition state shown in 6.19.236 The WW distance is 2.63 Å and the CC bond is clearly broken as one CH group becomes a terminal alkylidyne and the other is bridging.

Scheme. 6.4. Reactions of alkynes with (W>W)6+.

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6.19

As noted earlier, the nature of the alkoxide group also influences the reaction pathway. Whereas W2(OBut)6 and ethyne establish the equilibrium shown in the equation above, W2(OSiMe2But)6 reacts to form a kinetically labile µ-ethyne adduct that yields

W2(OSiMe2But)5(µ-δ1,δ2-C2H) with elimination of ButMe2SiOH. The eliminated silanol then enters into reaction with the ethyne adduct leading to µ-vinyl (µ-CHCH2) and the µ-ethylidyne complex W2(OSiMe2But)7(µ-CCH3).33

Whereas the dichloride W2Cl2(silox)4 failed to react with alkynes, the bis-hydrido complex W2H2(silox)4 reacts at low temperatures with RC>CR' to give kinetically labile alkyne adducts W2(µ-H)2(silox)4(µ-RCCR') where R = R' = H,Me; R = H, R' = Ph. Based on spectroscopic data, these compounds were proposed to have C2 molecular symmetry with a µ-perpendicular alkyne and asymmetric hydride bridges.237 Upon warming, these compounds eliminate H2 and give alkylidyne bridged compounds W2(silox)4(µ-CR)2, with a planar central W2C2 ring. For R = Me, W>W = 2.72 Å and W>C = 1.95 Å.

The introduction of alkyl or benzyl groups in compounds of the type 1,2-W2R2(OPri)4 leads to some fascinating reactions with alkynes. Alkyne adducts such as W2(CH2Ph)2(OPri)4(µ-C2Me2), are formed along with products derived from _-hydrogen activation, such as W2(H)(OPri)4(µ- CPh)(µ-C4Me4)2. Other compounds such as W2(Pr)2(OPri)4(µ-C2Me2)2, W2(µ-C2Me2)2(OPri)4, and W4(µ-CEt)2(µ-C2Me2)2(δ2-C2Me2)2(OPri)6, are formed from alkyne metathesis and from C–C couplings and _- or `-hydrogen activation.238-241

Addition of alkynes to mixed chloride-dimethylamide compounds also led to µ-alkyne adducts and in a study of the reaction between ethyne and W2Cl3(NMe2)3 in the presence of PR3, the formation of the µ-vinyl ligand in (PR3)Cl2W(µ-NMe2)(µ-δ1,δ2-CHCH2)(µ-δ2- CH2NMe)WCl(NMe2)(PR3) was observed by hydrogen atom transfer from a dimethylamide ligand.242 This formation of a µ-δ2-CH2NMe ligand provides a clue to the likely first step in the reaction between W2(NMe2)6 and PriOH that leads to the carbido-imido cluster compound W4(µ4-C)(µ-NMe)(OPri)12.59

The replacement of alkoxide by thiolate groups shuts down reactions with alkynes as evidenced by the lack of reactivity of M2(OBut)2(SBut)4.69 Calculations on model compounds indicate that the alkoxide ligands are much stronger /-donor ligands than thiolates and thus labilize the MM /-bonding MO’s.69 Furthermore, replacement of t-butoxide by o-tolyl thiolate243 converts an alkylidyne to a µ-alkyne complex:

2[(ButO)3W>CPh] + 6C7H8SH Α W2(SC7H8)6(µ-C2Ph2) + 6ButOH

6.9.4 Reactions with C>N bonds

W2(OBut)6 and organic nitriles enter into the Schrock “Chop Chop Reaction” to give an equivalent of the alkylidyne complex (ButO)3W>CR and the nitride (ButO)3W>N. However, this reaction is unique to tungsten as Mo2(OBut)6 and related molybdenum alkoxides are inert to reaction with acetonitrile at ambient temperature. The reaction is also very sensitive to the nature of the alkoxide and replacement of t-butoxide by fluorinated alkoxides or siloxides greatly reduces the propensity of the reductive cleavage reaction. Schrock noted that acetonitrile binds reversibly to W2[OCMe2CF3]6 to give an adduct of the form W2(ORF)6L2.231 Sub-

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sequently, the binding of acetonitrile to M2(OCMe2CF3)6 was studied in some detail. Adduct formation was enthalpically favored for tungsten (where ¨Hº = 26(1) kcal mol-1) relative to molybdenum (where ¨Hº = 22(1) kcal mol-1).244 Arylnitriles bind less strongly and undergo the following metathesis reactions.

W2(OCMe2CF3)6 +2ArC>N Α 2[(CF3Me2CO)3WN] + ArC>CAr

W2(OSiMe2But)6 + 2ArC>N Α [(ButMe2SiO)3W>N]2 + ArC>CAr

A similar reaction was observed for W2(OSiMe2But)6 and the nitridotungsten compounds were shown to adopt the trimeric and dimeric structures shown below in 6.20 and 6.21 for OCMe2CF3 and OSiMe2But, respectively.244,245

Studies of the kinetics of this reductive cleavage reaction indicated that the reaction was suppressed by excess benzonitrile and the active species leading to cleavage was proposed to be a mononitrile adduct W2(OR)6(NCPh).244,246

6.20

6.21

Although Mo2(OR)6 compounds do not react with alkyl or aryl nitriles, beyond showing reversible adduct formation, Me2NCN was found to form a 1:1 adduct with a structure wherein the C>N bond bridges the two metal atoms.247 Based on the C–N and MoMo distances, this complex was formulated as having double bonds and as such provides a model for a reactive intermediate on the pathway to reductive cleavage of the CN bonds. The analogous reaction with W2(OBut)6 led to C>N cleavage,247 although in a reaction involving the less bulky neopentoxide, a compound W2(OCH2But)6(NCNMe2)3 was obtained and structurally characterized.248 This compound contained three NCNMe22- ligands, each bound to the ditungsten center in a different manner. This reaction proceeds with complete cleavage and loss of the WW bond.

6.9.5 Reactions with C=C bonds

Allene adds to W2(OBut)6 to give a 1:1 adduct and a 2:1 adduct. The 1:1 adduct contains a V-shaped bridging allene as depicted by 6.22 and an essentially eclipsed W2O6 skeleton.249,250 The WW distance in the green 1:1 allene adduct is 2.58 Å which indicates extensive backbonding to the µ-allene ligand. By NMR spectroscopy, the two methylene carbons and their protons are equivalent. However, the methylene protons appear to be coupled to both tungsten nuclei in an equivalent manner, which led to the suggestion that the µ-allene ligand was fluxional on the NMR time scale. Due to backbonding, the allene in this compound can be construed as an (allene)2- ligand.

Addition of allene to this 1:1 allene adduct yields the 2:1 allene adduct which, in the solid state, has the structure depicted by 6.23.249,250 The bridging allene can now be considered as a metallated δ3-allyl group while the terminal δ2-allene is typical of allenes bonded to mononuclear metal centers.

Addition of CO also leads to the formation of a dimetallaallyl, W2(OBut)6[(µ-δ1,δ3- C(CH2)2](CO)2 having the structure depicted in 6.24.250 An allene adduct of W2(OCMe2CF3)6 of structural type seen in 6.22, was also characterized.150 Carbodiimides ArN=C–NAr, which are isoelectronic with allenes were also found to give structurally related 1:1 adducts.150,251

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6.22

6.23

6.24

Ethylene adds to W2(OCH2But)6 to give a 2:1 adduct.252,253 The structure, shown in Fig. 6.14, bonding and dynamic behavior of this molecule proved particularly interesting.253 The reversible uptake of ethylene occurs in a cooperative manner and in the 2:1 ethene adduct, the C–C axes are perpendicular to the WW axis and the two C2 units may be viewed as metallacycylopropanes where C–C = 1.45 Å and W–C = 2.14 Å. There are four bridging alkoxide ligands that span the WW bond of distance 2.53 Å in an asymmetric manner forming four short W–O distances, 2.00 Å and four long W–O distances, 2.31 Å. The two C2 units are orthogonal to each other so as to maximize Wd/ to ethylene /* back-bonding. Tungsten-olefin bond rotation is restricted on the NMR time scale and the olefinic protons appear as an ABCD spin system. The two carbon atoms are chemically inequivalent and in the 13C labeled compound derived from reaction with 13C2H4, 1JCC is 67 Hz.

Fig. 6.14. Structure of W2(OCH2But)6(C2H4)2.

This compound reacts further with ethylene to give an alkylidyne bridged metallacyclic compound W2(µ-CCH2CH2CH2)(OR)6 with the elimination of ethane. This reaction254 proved to be general for W2(OR)6 compounds where R = Pri, c-C5H9 and c-C6H11.

W2(OR)6 + 3C2H4 Α W2(µ-CCH2CH2CH2)(OR)6 + C2H6

In the case of R = Pri, the reaction pathway was found254 to proceed by the reversible formation of a metallacyclopentane ethylene complex:

W2(OPri)6 + 3C2H4 W2(OPri)6(CH2)4(δ2-C2H4) ΑW2(µ-CCH2CH2CH2)(OPri)6 + C2H6

In W2(OPri)6(CH2)4(δ2-C2H4) the δ2-ethene ligand can again be viewed as a metallacyclopropane and the WW distance of 2.65 Å is consistent with a (M–M)10+ center.

W2(OCH2But)6(py)2 reacts with 1,3-butadiene and isoprene to form 1:1 adducts in which all four carbon atoms of the conjugated diene are coordinated to the dinuclear center in a µ-δ1,δ4-manner255, 256 as in Fig. 6.15. This addition was reversible and in the presence of H2, the 1,3-dienes were selectively hydrogenated to the 3-enes.257 _-Olefins were also found to be hydrogenated by W2(OCH2But)6(py)2 in the presence of H2.257

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Ene-yne couplings have been observed in reactions involving W2(OSiMe2But)6(µ-C2H2)(py) and ethene and allene. The hydrido alkylidyne bridged compound W2(H)(µ-CCH=CHMe)- (OSiMe2But)6 and the analogous bridged compound W2(H)(µ-CC(=CH2)(CH=CH2)- (OSiMe2But)6 were formed, respectively.258,259

Fig. 6.15. Structure of W2(OCH2But)6(µ-δ1,δ4-C4H6)(py).

_,`-Unsaturated aldehydes and ketones were found to add to W2(OR)6 compounds to form 1,2- and 1,4-adducts.260 Aldehydes and ketones undergo reductive cleavage of the C=O bond to give oxo-alkylidene complexes which are themselves capable of undergoing CC coupling with CO bond cleavage in further reactions with aldehydes and ketones.261-264 This forms the basis of a selective two step olefination reaction. The first step, the reduction of the first aldehyde or ketone is quite general but the second step is less efficient and does not proceed in high yield for aryl or bulky alkyl substituted ketones. Rather interestingly, the product in the first step is a (W–W)10+ containing compound having a terminal oxo group and a bridging alkylidene. The reaction involving c-C3H5CHO gave a cyclopropylidene complex and this was taken as evidence that the C=O bond cleavage did not proceed via a radical process or one in which significant positive charge was localized on the ketonic carbon atom.263 However, the reaction involving cyclohexanone gave a product of vinyligous coupling. An overall scheme for the olefination reaction and its competing side reactions is shown in Scheme 6.5.

Diarylthiones, Ar2C=S, also undergo reductive cleavage of the C=S bond yielding sulfido bridged complexes of the structural type depicted in 6.25. The PMe3 adduct, W2(OCH2But)6-

(S)(CPh2)(PMe3) was structurally characterized.265 In this study, the kinetics of the reductive cleavage of (p-XC6H4)2C=S compounds was studied by NMR spectroscopy as a function of X, where X = NMe2, OMe, Me, H, F, Cl and CF3. Both electron donating and electron withdrawing groups accelerated the rates of reaction. From Eyring plots, the activation parameters ¨H& = 10.2(2) kcal mol-1, ¨S& = -29(1) eu were obtained for Ph2C=S cleavage.

6.25

A general reaction scheme was proposed involving the initial reversible formation of a 1:1 adduct followed by an irreversible cleavage.265 The kinetic parameters were compared with those for the reversible uptake of Et2NC>N by Mo2(OCH2But)6 to give the µ-δ1,δ2-CN adduct. A further analogy was made with the µ-δ1,δ2-SCPh2 adduct of Cp2Mo2(CO)4 which has the structure depicted in 6.26.266

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Scheme 6.5. Some reactions of ditungsten oxo/alkylidene compounds with ketones.

6.26

6.9.6 Reactions with H2

Although H2 is not usually observed to add directly to the M>M bond (see Section 6.9.8), it has been noted to react with attendant metal-carbon bonds as in the hydrogenation of 1,3-di- enes and _-olefins.257 Also, in reaction with W2(Bui)2(OPri)4, a complex reaction ensues leading to the unusual octahedral cluster W6H5(CPri)(OPri)12.267 This cluster has the central skeleton shown in Fig. 6.16 and has the unusual property of being sufficiently kinetically slow toward bridge to terminal exchange that the reactivity of bridging and terminal hydrides can be distinguished within the same molecule.

The terminal W–H group participates in the hydrogenation of ethene while the other hydrides do not. The stepwise coupling of W2 units containing hydride ligands formed by hydrogenation of the butyl ligands, together with _-CH activation, presumably leads to formation of this W6 cluster. In the presence of chelating diphosphines, dinuclear W2H2(OPri)4(dmpe)2 and tetranuclear W4(H)4(OPri)8(dmpm)3 complexes were isolated.

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Fig. 6.16. The core in W6H5(CPri)(OPri)12.

6.9.7 Reactions with organometallic compounds

This is a relatively unexplored field of chemistry although the following indicates the potential for this area of research.

The compound Fe2(CO)6(µ-S2) was shown to react with W2(OPri)6(py)2 to give a planar “Fe2W2(µ-S)2” containing cluster, Fe2W2(OPri)6(CO)5(µ-S)2(py) having both WW and FeW bonds.268 The reaction could be viewed as an oxidative addition to the WW triple bond.

Alkynylplatinum(II) compounds enter into a complex series of reactions with W2(OBut)6 and from the reactions involving trans-Pt(C>CH)2(PMe2Ph)2, the dicarbido compounds (ButO)3W>C–C>W(OBut)3 and trans-(PMe2Ph)2Pt[C2W2(OBut)5]2 were isolated and fully characterized.

From the reaction between CpCo(C2H4)2 and W2(OCH2But)6, the compound CpCoW2(OCH2But)6 was isolated and fully characterized.271 As shown in Fig. 6.17, this molecule contains unsupported Co–W bonds of distance 2.28 and 2.34 Å. The WW distance of 2.50 Å is typical of a double bond distance and an interesting analogy can be made with this addition of a CpCo fragment across a W>W bond with that of the addition of CO across W>W.271

Fig. 6.17. Structure of CpCoW2(OCH2But)6.

6.9.8 (δ5-C5H4R)2W2X4 compounds where R = Me, Pri and X = Cl, Br

Cp2W2X4 compounds exhibit δ5-bound Cp rings, in contrast to aforementioned, slipped δ3-Cp-dimethylamides: W2Cp2(NMe2)4, W2(MeCp)2(NMe2)4, and W2(indenyl)2(NMe2)4. Green and Mountford discovered these halo-compounds formed in the reactions of piano-stool (RCp)WX4 compounds (where R = Me, Pri and X = Cl, Br) with Na(Hg). The solid state structure of (PriCp)2W2Cl4 revealed an unbridged (W>W)6+ bond of distance 2.368(1) Å and

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an anti-conformation for the central 1,2-W2Cp2Cl4 skeleton.272, 273 The presence of the unbridged W>W bond is in contrast to the related compounds [CpMX2]2 which have either four halide bridges (for M = Cr) or two halide bridges (for M = Mo).274

This lack of halide bridging testifies to the increasing importance of MM bonding in descending from Cr to W within the group 6 transition metals. However, as can be seen in Table 6.2, the ditungsten distance in (PriCp)2W2Cl4 is slightly longer than those seen for most compounds of formula 1,2-M2X2Y4.

The addition of chelating Lewis bases such as Me2P(CH2)2PMe2 or the addition of halide ions to these compounds leads to the formation of bridged species with disruption of the M>M bond.275 CO reacts with them to form [Cp'WCl(CO)]2(µ-Cl)2 which contains a rather long WW bond of 2.965(1) Å. Nitriles add to these compounds to form 1:1 adducts in which the nitrile bridges the ditungsten center in a µ-δ1,δ2 fashion. A similar structure was proposed for a 1:1 isocyanide adduct.

Alkynes react with these Cp'2W2X4 species to give both alkyne adducts and products from alkyne coupling.276 The product from simple alkyne addition exhibits a relatively long W–W bond of 2.795(3) Å while the alkyne moiety within exhibits a long µ-(CC) distance of 1.41(4) Å which is indicative of the formation of a dimetallatetrahedrane. The skewed alkyne bridge (25° dihedral between WW and CC) was the subject of an EHMO computational study by Mountford who concluded that steric and not electronic factors were responsible for the unique alkyne coordination geometry.277

The perpendicular nature of the µ-C4Me4 bridging ligand in the alkyne-coupled product, (δ5-MeCp)2W2Cl4(µ2-C4Me4) contrasts with that for analogous M2(OR)6(µ-C4R'4) compounds. Again, a rather long WW distance of 2.930(1) Å is observed for the alkyne-coupled product, possibly as a result of steric crowding around each tungsten atom.

The compounds Cp'2W2X4 are unique among ditungsten compounds in showing reversible reactivity with H2 at room temperature to give the hydrido-bridged species: Cp'2W2X4(µ-H)2.278 The bridging hydride was formulated based upon NMR spectroscopic data including the appearance of hydride resonances at β 1.2 with J183W-1H = 112-116 Hz and T1 ~ 1-2 s at –90 °C. Several other oxidative addition reactions were noted for reactions involving HCl, HSR and HPR2 compounds. Notable among these was the complex (δ5-PriCp)2W2Cl3(µ-H)(µ-Cl)(µ-PPh2)(PMe3) which was structurally characterized.

6.10 Conclusion

The coordination chemistry of the X3M>MX3 “ethane-like dimers” of molybdenum and tungsten is rich and varied. Though the chemistry of the (Mo>Mo)6+ and (W>W)6+ units are very similar, there are significant differences. The ditungsten center is notably more readily oxidized and this leads to a much more extensive organometallic chemistry of small, unsaturated organic molecules. Many of these reactions lead to the reduction and cleavage of C–X multiple bonds. In contrast, reductive eliminations occur more readily from the Mo26+ center to give Mo24+ compounds having MM quadruple bonds. Furthermore, the ditungsten compounds are much more labile towards forming clusters. The organometallic chemistry of the M2(OR)6 compounds bears a superficial resemblance to that of the Cp2M2(CO)4 compounds, though it is evident that despite the difference in formal oxidation states, the M2(OR)6 compounds are more reactive as electron reservoirs.