Alkyl Hydride Research Articles

Unifying the mechanisms for alkane dehydrogenation and alkene H/D exchange with [IrH2(O2CCF3)(PAr3)2]: the key role of CF3CO2 in the “sticky’' alkane route

To understand photochemical and thermal alkane activation with IrH2(O2CCF3)(PAr3)2 (Ar = p-FC6H4), H/D isotope scrambling between alkenes and IrD2(O2CCF3)(PAr3)2 was studied. No unique interpretation of the experimental data was possible, so DFT(B3PW91) calculations on the exchange process in Ir(H)2(O2CCF3)(PH3)2(C2H4) were carried out to distinguish between the possibilities allowed by experiment. Of several possible mechanisms for H/D scrambling, one was strongly preferred and is therefore proposed here. It involves the insertion of the olefin to give an alkyl hydride that reductively eliminates to lead to a transition state that contains an η3-bound alkane. This transition state, which achieves a 1,1′ geminal H/D exchange, is significantly lower in energy than a dihydrido carbene, located as a secondary minimum, eliminating the alternative carbene mechanism. The unexpectedly large binding energy (BDE) of the alkane (“sticky alkane”) to the Ir(O2CCF3)(PH3)2 fragment (BDE = 11.9 kcal mol−1) in this transition state is ascribed in part to the presence of a weakly σ- and π-donating (CF3CO2) group trans to the alkane binding site. The H/D exchange selectivity observed requires that 1,1′-shifts (i.e., M moving to a geminal C–H bond), but not 1,3-shifts, be allowed in the alkane complex. In a key finding, a 1,3-shift in which the metal moves down the alkane chain is indeed found to have a much higher activation energy than the 1,1′-process and is therefore slow in our system. A 1,2-shift has not been considered since it would involve a strong steric hindrance at a tertiary carbon in this system. The mechanism ia an alkane path provides an insight into the closely related photochemical and catalytic thermal alkane dehydrogenation processes mediated by IrH2(O2CCF3)(PAr3)2; the thermal route requires tBuCHCH2 as the hydrogen acceptor. These two alkane reactions are intimately related mechanistically to the isotope exchange because they are proposed to have the same intermediates, in particular the sticky alkane complex. Remarkably, the rate determining step of the thermal (150 °C) alkane dehydrogenation process is predicted to be substitution of the hydrogen acceptor-derived alkane by the alkane substrate.

Insertion of Dioxygen into a Platinum−Hydride Bond to Form a Novel Dialkylhydroperoxo Pt(IV) Complex

The selective catalytic oxidation of alkanes using inexpensive and environmentally friendly oxidants is of immense interest to the chemical community and of great potential benefit to the world economy and ecology. Since the earliest reports that the oxidative addition of alkane C-H bonds to transition metal species can occur with high selectivities, substantial research effort has focused on the functionalization of the metal alkyl hydride products. The authors report that Pt(IV) dialkyl hydride complex reacts cleanly with dioxygen to produce a dialkyl Pt(IV) hydroperoxide species. Of potential relevance to the development of alkane oxidation systems is that the Pt(IV) dialkyl hydride reactant is analogous to compounds formed by intermolecular oxidative addition of alkane C-H bonds to Pt(II). The novel Pt(IV) product has been crystallographically characterized and displays an {eta}-hydroperoxide linkage.

Platinum-Catalyzed Acrylonitrile Hydrophosphination. P−C Bond Formation via Olefin Insertion into a Pt−P Bond

The acrylonitrile complexes Pt(diphos)(CH2CHCN) (diphos = dppe (1), dcpe (2); dppe = Ph2PCH2CH2PPh2, dcpe = Cy2PCH2CH2PCy2, Cy = cyclo-C6H11) are catalyst precursors and, for some substrates, resting states, during addition of P−H bonds in primary and secondary phosphines across the CC double bond of acrylonitrile (hydrophosphination). Oxidative addition of P−H bonds to related catalyst precursors gives the phosphido hydride complexes Pt(diphos)(PRR‘)(H) (diphos = dppe, R = H, R‘ = Mes* (20), R = R‘ = Mes (21); diphos = dcpe, R = H, R‘ = Mes* (22); Mes = 2,4,6-Me3C6H2, Mes* = 2,4,6-(t-Bu)3C6H2). Acrylonitrile does not insert into the Pt−H bond of these hydrides to give cyanoethyl ligands; the putative products, the phosphido complexes Pt(diphos)(CH2CH2CN)(PRR‘) (diphos = dppe, R = H, R‘ = Mes* (9), R = R‘ = Mes (10); diphos = dcpe, R = H, R‘ = Mes* (11)) were prepared independently and found to be stable to P−C reductive elimination. Instead, catalysis appears to occur by selective insertion of acrylonitrile into the Pt−P bond to yield the alkyl hydrides Pt(diphos)[CH(CN)CH2PRR‘](H), followed by C−H reductive elimination and regeneration of 1 or 2. This insertion was observed directly in model methyl phosphido complexes M(dppe)(Me)(PRR‘) (M = Pt, R = H, R‘ = Mes* (12), R = R‘ = Mes (13); M = Pd, R = H, R‘ = Mes* (17)), yielding M(dppe)[CH(CN)CH2PRR‘](Me), (14, 15, 18). Similarly, treatment of Pt(dcpe)(PHMes*)(H) (22) with acrylonitrile gives Pt(dcpe)[CH(CN)CH2PHMes*](H) (24) as a mixture of diastereomers; the isomeric Pt(dcpe)[PMes*(CH2CH2CN)](H) (25), which was prepared independently, was also observed during this reaction. Both 24 and 25 decompose in the presence of acrylonitrile to form Pt(dcpe)(CH2CHCN) (2) and PHMes*(CH2CH2CN) (3a). The C−H reductive elimination step was modeled by studies of Pt(dcpe)[CH(Me)(CN)](H) (26). Another isomer, Pt(dcpe)[CH(Me)(CN)](PHMes*) (29), which formally results from insertion of acrylonitrile into the Pt−H bond of 22, was formed by decomposition of complex 2 during catalysis. Complex 29 is inactive in catalysis but decomposes to partially regenerate the active catalyst 2. The cyanoethyl compounds Pt(dcpe)(CH2CH2CN)(PHMes*) (11), trans-Pt(PPh3)2(CH2CH2CN)(Br), and PMes2(CH2CH2CN) (23) were structurally characterized by X-ray crystallography.

Structures and Reaction Pathways in Rhodium(I)-Catalyzed Hydrogenation of Enamides: A Model DFT Study

The potential energy profile of Rh(I)-catalyzed hydrogenation of enamides has been studied for the simple model system [Rh(PH3)2(α-acetamidoacrylonitrile)]+ using a nonlocal density functional method (B3LYP). Intermediates and transition states along four isomeric pathways for dihydrogen activation have been located, and pathways for interconversion between isomeric reaction pathways have been explored. The general sequence of the catalytic cycle involves coordination of H2 to [Rh(PH3)2(α-acetamidoacrylonitrile)]+ to form a five-coordinate molecular H2 complex, followed by oxidative addition of the coordinated molecular hydrogen to form a dihydride complex, [RhH2(PH3)2(α-acetamidoacrylonitrile)]+. This dihydride is converted into an alkyl hydride by a migratory insertion reaction. Reductive elimination of the hydrogenated acetamidoacrylonitrile completes the catalytic cycle. No computational support for alternate H2 activation pathways, such as direct conversion of H2 and [Rh(PH3)2(α-acetamidoacrylonitril...

Kinetics of photochemical alkane dehydrogenation catalyzed by Rh(PMe 3) 2(CO)Cl: implications concerning the C–H bond activation step

The mechanism of photochemical alkane dehydrogenation catalyzed by Rh(PMe 3) 2(CO)Cl has been further probed with an emphasis on characterizing the initial C–H activation step and understanding the effect of added CO on selectivity. While pure cyclooctane and pure cyclohexane are dehydrogenated at the same rate (same quantum yields), cyclooctane shows much greater reactivity in mixtures of the two solvents. The product ratio (cyclooctene:cyclohexene) is highly dependent upon the partial pressure of CO, ranging from 12 in the absence of CO, to 75 in the limit of high CO pressure (>ca. 400 torr). The kinetic isotope effect for the dehydrogenation of c–C 6H 12/c–C 6D 12 is also found to be dependent upon CO pressure, ranging from 10 in the absence of CO to 4.2 under high CO pressure. The results support our earlier conclusion that the intermediate responsible for C–H activation is ground state [Rh(PMe 3) 2Cl]. It is also concluded that inhibition of the reaction by CO operates primarily via addition of CO to the intermediate alkyl hydrides, (R)(H)Rh(PMe 3) 2Cl. Addition of CO prior to C–H bond addition is apparently not a kinetically significant process, even under high CO pressure.

Ligand effects on the rates of protonolysis and isotopic exchange for platinum(II) alkyls

The protonolysis/deuterolysis of complexes L2PtRX (L = phosphorus ligand, R = alkyl group, X = anionic ligand) has been investigated as a mechanistic probe of the reverse reaction, activation of alkanes by Pt(II). Trans-(Et3P)2PtMeX (X− = triflate (OTF−, F−,NO3−) solvolyze in acidic CD3OD, forming [trans-(Et3P)2PtMe(CD3OD)]+ which reacts slowly with DOTf at room temperature liberating CH3D. In dichloromethane, trans-(Et3P)2PtMe(OTf) reacts with HOTf at low temperatures (−70 to −20°C) to give (Et3P)2PtMe(H) (OTf)2 in rapid equilibrium with the reagents, while at higher temperatures rapid methane loss is preceded by extensive deuterium incorporation (with DOTf) into the Pt(II) methyl group. Upon treatment with acid in CD3OD, trans-(Et3P)2PtMeX (X= Cl, Br) also undergo H/D exchange before elimination of methane, while trans-(Et3P)2PtMel, (depe)Pt(CH3)2 (depe = 1,2-bis(diethylphosphino) ethane) and cis-[(MeO)3P]PtMeCl do not. The α hydrogens of trans-(Et3P)2PtRCl (R = Me, Et, Bz) exchange with deuterium in CD3OD/DOTf with rates following the order Bz < Me < Et, while no exchange is observed in the protonolysis of rans-(Et3P)2Pt(CH2CMe3)Cl which yields (CH3)3CCH2D. These trends are interpreted in terms of effects on relative stabilities of key intermediates, Pt(IV) alkyl hydrides and Pt(II) alkane sigma complexes.

The Mechanism of a C-H Bond Activation Reaction in Room-Temperature Alkane Solution

Chemical reactions that break alkane carbon-hydrogen (C–H) bonds are normally carried out under conditions of high temperature and pressure because these bonds are extremely strong (∼100 kilocalories per mole), but certain metal complexes can activate C–H bonds in alkane solution under the mild conditions of room temperature and pressure. Time-resolved infrared experiments probing the initial femtosecond dynamics through the nano- and microsecond kinetics to the final stable products have been used to generate a detailed picture of the C–H activation reaction. Structures of all of the intermediates involved in the reaction of Tp*Rh(CO) 2 (Tp* = HB–Pz 3 *, Pz* = 3,5-dimethylpyrazolyl) in alkane solution have been identified and assigned, and energy barriers for each reaction step from solvation to formation of the final alkyl hydride product have been estimated from transient lifetimes.

Preparation of (1,4,7-triazacyclononane)Rh(hydrocarbyl)3 Compounds and Their Derivatives. Strong Donor Labilization of RhCl Bonds toward Alkylation and Preparation of Unusually Stable Alkyl Hydride Complexes of Rhodium

(tacn)Rh(R)3 compounds (tacn = 1,4,7-triazacyclononane; R = Me, Et, Ph, vinyl) have been prepared in 65−86% yields by treatment of (tacn)RhCl3·H2O with more than a 7-fold molar quantity of RLi in THF, followed by protonation by methanol. The product before protonation is (Li3tacn)RhR3 (Li3tacn = 1,4,7-trilithio-1,4,7-triazacyclononane) which can be isolated. The alkylation is complete in a few minutes to a couple of hours, depending on R, which compares with 3−4 days for CnRhCl3 with MeLi (Cn = 1,4,7-trimethyl-1,4,7-triazacyclononane) and suggests that the presumably first-formed (Li3tacn)RhCl3 is highly activated toward Cl- dissociation by the strong donor effect of the three LiNR2 groups in the rhodium coordination sphere. Protonation of (Li3tacn)RhR3 by methanol gives the neutral products (tacn)RhR3. X-ray structure determinations of (tacn)RhEt3 and (tacn)RhPh3 have been carried out. Protonolysis of (tacn)RhR3 (R = Me, Et, Ph) by 2 HSO3CF3 (HOTf) affords (tacn)RhR(OTf)2. Dissolution of the latter in D2...

Diastereoselective Intramolecular C−H bond Activation by Optically Active Tris(pyrazolyl)hydroborate Complexes of Rhodium

Results are reported of photolyses of rhodium dicarbonyl complexes of C3-symmetric TpMenth and TpMementh ligands that illustrate the capability of these scorpionates to exhibit a high degree of regio- and stereocontrol in reactions involving intramolecular attack of ligand substituent C−H bonds. The starting material TpMenthRh(CO)2 was found to contain an η2-TpMenth ligand in the solid state by X-ray crystallography but was shown to be an interconverting mixture of η2 and η3 forms in toluene solution by IR and NMR spectroscopy. A mechanism for the fluxionality based on one previously suggested for achiral systems was supported by line shape analysis of VT-NMR data. Irradiation of TpMenthRh(CO)2 under a N2 purge in a variety of solvents resulted in the generation of an 85:15 mixture of diastereomeric alkyl hydrides resulting from intramolecular cyclometalation reactions involving the methyl substituents on the ligand isopropyl group. Analysis of two-dimensional NMR data (DQ-COSY, NOESY, HMQC, TOCSY spectra...

Ruthenium diphosphine complexes for catalysis; a general synthesis and direct comparisons with rhodium complexes

AbstractA method for the preparation of diphosphineruthenium complexes from a range of diphosphine precursors is described. The stable products, of general formula P2Ru(allyl)acac (P = phosphine ligand), do not catalyse the hydrogenation of alkenes but can easily be converted into catalytically active species by reaction with trimethylsilyl trifluoromethanesulfonate. The chemistry involved in this transformation is discussed. Catalysis of the hydrogenation of a range of alkenes, and direct comparison with related rhodium reductions, is described. Some mechanistic insights are gained through additions of D2 to alkenes and parallel hydrogenations on deuterated solvent, supporting the intervention of conventional dihydride and alkyl hydride intermediates which undergo exchange with solvent at both stages.

IR Flash Kinetic Spectroscopy of C-H Bond Activation of Cyclohexane-d0 and -d12 by Cp*Rh(CO)2 in Liquid Rare Gases: Kinetics, Thermodynamics, and Unusual Isotope Effect

Flash kinetic spectroscopy with infrared detection is used to prove C-H activation of cyclohexane-d[sub 0] and -d[sub 12] by intermediates generated upon ultraviolet irradiation of Cp*Rh(CO)[sub 2] (Cp* = C[sub 5](CH[sub 3])[sub 5]) in liquid rare gas (Rg = Kr or Xe) solution at low temperature (163-193 K). Upon UV photolysis, a new C-O stretching band (at 1946.5 cm[sup [minus]1] in Kr and at 1941.5 cm[sup [minus]1] in Xe) appears promptly, which we attribute to Cp*Rh(CO)(Rg). In the presence of hydrocarbon a second C-O stretching band (2002.5 cm[sup [minus]1] in Kr and 1998 cm[sup [minus]1] in Xe) grows in at the same observed rate as the disappearance of the 1946.5-cm[sup [minus]1] band. We attribute this second band to the alkyl hydride product, Cp*Rh(CO)(H)(C[sub 6]H[sub 11]). The concentration dependence of the observed reaction rate in Kr solution shows behavior consistent with a preequilibrium mechanism in which the initial Cp*Rh(CO)(Rg) complex equilibrates with a weakly bound Cp*Rh(CO)(alkane) complex (which has an IR carbonyl frequency unresolvable from that of the rare gas complex) followed by C-H activation of the latter. 53 refs., 13 figs., 1 tab.

Studies on molybdenocene derivatives: Reactions of [Cp 2Mo(η 2-NCMe)] and preparation of alkyl hydride complexes. Crystal structure of [Cp 2Mo(PMe 3)

Reaction of [Cp 2Mo(η 2-NCMe)] ( 1) with PMe 3 affords [CP 2Mo(PMe 3)] ( 2), which is readily alkylated go give [Cp 2MoR(PMe 3)]PF 6( 3a, R = Me; 3b, R = Et). The crystal structure of 2 shows tricoordination around the Mo atom. Cyclic voltammetry of 3a and 3b and the analogues [Cp 2MoR(CNMe)]I, ( 4a, R = Me; 4b, R = Et) and [Cp 2MoMe(PPh 3)]PF 6 ( 7) shows one reversible one-electron oxidation, but the potential is independent of R, in contrast to the complexes [Cp 2MoR 2]. Oxidative addition of Et 2S 2, Ph 2Se 2 or (PhCOO) 2 to 1 yields [Cp 2Mo(ER) 2] (E = O, S or Se). The molybdenocene alkyl hydrides [Cp 2Mo(H)Me] ( 5) and [Cp 2MoH(CH 2PPh 3)]I·THF ( 6) are prepared from [Cp 2MoHI]. Thermal decomposition of 6 as well as reaction Of [Cp 2MoMe 2] + with Ph 3C . provide evidence for transient formation of the methylidene complex [Cp 2MoH(CH 2)] +.

Bis(η 5-cyclopentadienyl)tungsten chemistry: Hydridation reactions of functional alkyl hydrides olefin complexes

Functional tungstenocene alkyl hydrides with ester or nitrile groups attached to the α-position of the alkyl group (e.g. Cp 2WH(CHMeCO 2Me) react with LiAlH 4 to give olefin complexes (e.g. Cp 2W(CH 2CHMe). Similarly, functional olefin complexes are reduced to give alkyl compounds. Thus, the acrylic ester complex Cp 2W(CH 2CHCO 2Me) affords the tungstenacyclobutane Cp 2W(CH 2) 3 and the fumaric ester complex Cp 2W[η 2CH(CO 2Me)CHCO 2Me] the new tungstenacyclopentane Cp 2W(CH 2) 4.

Insertion reactions of bis(η 5-cyclopentadienyl)-dihydridotungsten with activated olefins and with acetylenes: Tungstenocene alkyl hydrides, olefin and acetylene complexes

Tungstenocene dihydride 1 undergoes insertion of activated olefins at 120°C to give functional alkyl hydrides Cp 2WH(η 1-CHXCH 2R) (X = CO 2Me, R = H ( 2), CO 2Me ( 3), Ph ( 4); X = CN, R = H ( 5). In the case of acrylonitrile the olefin complex Cp 2W(η 2-CH 2CHCN) ( 8) may also be formed. Complexes 2 and 3 were oxidized with chloroform to give the corresponding alkyl chloro complexes 17 and 18. The reaction of 1 with activated acetylenes can produce olefin complexes Cp 2W(η 2-CH 2CHY) (Y = CO 2Me ( 6), COMe ( 7), but in the case of CF 3CCH alkenyl complexes Cp 2WX[η 1-C(CF 3)CH 2] (X = H ( 15), F ( 16) are formed. Complex 1 also reacts with diphenylacetylene to give an (itE)-stilbene complex Cp 2WH[η 2-( E)-CHPhCHPh] ( 9) and tolan complex Cp 2W(η 2-C 2Ph 2) ( 12); the ( Z)-stilbene complex 10 is observed as an intermediate. Phenylacetylene likewise gives a styrene complex 11. Both 12 and 9 can be reversibly protonated at the tungsten centre; the corresponding cations were characterized as PF 6 − derivatives 13 and 14. In the case of 4 the regiochemistry of the olefin insertion was established by an X-ray structural determination.

Reactions of alkyl and hydride derivatives of permethylzirconocene and permethylhafnocene with carbon monoxide. Synthesis and reactivity studies of aldehyde complexes of zirconium and hafnium

AbstractThe alkyl hydride derivatives Cp2*M(H)(CH2CHMe2) (Cp* = η5‐C5Me5, M = Zr, Hf) react with carbon monoxide at low temperature to yield the η2‐acyl hydride complexes CpM(H)(η2‐COCH2CHMe2) (1a, b), which on warming to room temperature under CO afford the carbonyl/isovaleraldehyde adducts CpM(CO)(η2‐OCHCH2CHMe2) (4a, b). The zirconium derivative 4a slowly loses its coordinated CO and rearranges to the enolate hydride CpZr(H)(OCHCHCHMe2) (3). By contrast CpHf‐(CO)(η2‐OCHCH2CHMe2) (4b) rearranges without loss of CO to the cyclic enediolate tautomer (5), most likely by a 1,2H shift from the cyclic acyl derivative . The coordinatively unsaturated η2‐isovaleraldehyde adduct of permethylzirconocene [CpZr(η2‐OCHCH2CHMe2)] is implicated in reactions of the acyl hydride CpZr(H)(η2‐COCH2CHMe2) (1a) with the trapping substrates ethylene, 2‐butyne, dihydrogen, and tert‐butylacetylene to yield (12), (13), CpZr(H)(OCH2CH2CHMe2) (14), and Cp2*Zr(CCCMe3)(OCH2CH2CHMe2) (15). Carbonylations of the hafnacyclopentane (9) and the alkenyl hydride CpHf(H)(trans‐CHCHCMe3) do not show direct evidence for η2‐cyclopentanone or η2‐aldehyde intermediates. The former does, however, proceed directly to the bicyclic enediolate complex 11 most likely by the subsequent carbonylation of the cyclopentanone adduct. The latter undergoes carbonylation first to the hafna‐oxacyclopentene , which reacts further with CO to yield hafnadioxo‐cycloheptadiene (19). Possible mechanisms for these processes are discussed. The general natures of this chemistry complement theoretical and experiment results from the Hofmann group presented in the preceding article17.

Organometallic nitrosyl chemistry. 33. New types of remarkably stable alkyl hydride complexes of tungsten

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTOrganometallic nitrosyl chemistry. 33. New types of remarkably stable alkyl hydride complexes of tungstenPeter. Legzdins, Jeffrey T. Martin, Frederick W. B. Einstein, and Richard H. JonesCite this: Organometallics 1987, 6, 8, 1826–1827Publication Date (Print):August 1, 1987Publication History Published online1 May 2002Published inissue 1 August 1987https://doi.org/10.1021/om00151a041RIGHTS & PERMISSIONSArticle Views110Altmetric-Citations16LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (299 KB) Get e-AlertsSupporting Info (1)»Supporting Information Supporting Information Get e-Alerts

Hydrogen transfer and C–H activation in a dihydroiridium amido-alkene complex

Dihydrogen addition to the alkene complex (5) is regiospecific and stereoselective; the dihydride mixture is converted into alkyl hydrides and ultimately into a chelation-controlled C–H insertion product (10).

Solution structures of iridium alkyl hydrides pertaining to asymmetric hydrogenation

Alkyliridium hydrides (3a) and (5) derived from diastereoisomerically pure enamide complexes, have the opposite configuration at Cα, but the same configuration at Ir.

Ansa-metallocene derivatives : X. Hydrogenolysis of the zirconium-alkyl bond in bridged and unbridged alkyl zirconocene derivatives. Evidence for direct and ring-mediated hydrogen transfer reactions

The rate of hydrogenolytic alkane liberation from permethylzirconocene neopentyl halide compounds, (C 5(CH 3) 5) 2Zr(X)CH 2C(CH 3) 3, X = F, Cl, Br), is greatly reduced if the ring ligands are interconnected by an ethylene bridge, as in C 2H 4(C 5(CH 3) 4) 2Zr(X)CH 2C(CH 3) 3. Hydrogenolysis of the corresponding permethylzirconocene neopentyl hydride compounds (X = H), on the other hand, is too fast for kinetic measurements at room temperature, even with the ethylene-bridged derivative. These observations, and the inverse kinetic isotope effect observed for reaction with D 2, are interpreted with the assumption that H 2-induced alkane liberation from permethylzirconocene alkyl halides proceeds via an indirect ring-mediated hydrogen transfer reaction which is feasible only with freely rotating ring ligands; hydrogenolysis of permethylzirconocene alkyl hydrides, on the other hand, apparently occurs without such limitation, by direct H 2-to-alkyl hydrogen transfer.

Coupling of bridging phosphido ligands with alkyl, hydride, and carbene ligands to give bridge elimination reactions

Described are some examples of the degradation of phosphido bridges via coupling with alkyl hydrides and carbene ligands. The experiments outlined demonstrate that the phosphide bridges are not inert and that they can participate in complex reactions. Such bridge elimination reactions may limit the utility of phosphide-bridged complexes in homogenous catalysis.