Hydride Complex Cp Research Articles

Expanding Thorium Hydride Chemistry Through Th²⁺, Including the Synthesis of a Mixed-Valent Th⁴⁺/Th³⁺ Hydride Complex.

The reactivity of the recently discovered Th(2+) complex [K(18-crown-6)(THF)2][Cp″3Th], 1 [Cp'' = C5H3(SiMe3)2-1,3], with hydrogen reagents has been investigated and found to provide syntheses of new classes of thorium hydride compounds. Complex 1 reacts with [Et3NH][BPh4] to form the terminal Th(4+) hydride complex Cp″3ThH, 2, a reaction that formally involves a net two-electron reduction. Complex 1 also reacts in the solid state and in solution with H2 to form a mixed-valent bimetallic product, [K(18-crown-6)(Et2O)][Cp″2ThH2]2, 3, which was analyzed by X-ray crystallography, electron paramagnetic resonance and optical spectroscopy, and density functional theory. The existence of 3, which formally contains Th(3+) and Th(4+), suggested that KC8 could reduce [(C5Me5)2ThH2]2. In the presence of 18-crown-6, this reaction forms an analogous mixed-valent product formulated as [K(18-crown-6)(THF)][(C5Me5)2ThH2]2, 4. A similar complex with (C5Me4H)(1-) ligands was not obtained, but reaction of (C5Me4H)3Th with H2 in the presence of KC8 and 2.2.2-cryptand at -45 °C produced two monometallic hydride products, namely, (C5Me4H)3ThH, 5, and [K(2.2.2-cryptand)]{(C5Me4H)2[η(1):η(5)-C5Me3H(CH2)]ThH]}, 6. Complex 6 contains a metalated tetramethylcyclopentadienyl dianion, [C5Me3H(CH2)](2-), that binds in a tuck-in mode.

Kinetics and Thermodynamics of H–/H•/H+ Transfer from a Rhodium(III) Hydride

The thermodynamics and kinetics of all three cleavage modes for Rh-H, the transfer of H(-), H(+), or H•, have been studied for the Rh(III) hydride complex Cp*Rh(2-(2-pyridyl)phenyl)H (1a). The thermodynamic hydricity, ΔG°H(-), for 1a has been measured (49.5(1) kcal/mol) by heterolytic cleavage of H2 with Et3N in CH3CN. The transfer of H(-) from 1a to 1-(1-phenylethylidene)pyrrolidinium is remarkably fast (kH(-) = 3.5(1) × 10(5) M(-1) s(-1)), making 1a a very efficient catalyst for the ionic hydrogenation of iminium cations. The pKa of 1a in CH3CN has been measured as 30.3(2) with (tert-butylimino)tris(pyrrolidino)phosphorane (12), and the rate constant for H(+) transfer from 1a to 12 has been estimated (kH(+) = 5(1) × 10(-4) M(-1) s(-1)) from the half-life of the equilibration. Thus, 1a is a poor H(+) donor both thermodynamically and kinetically. However, 1a transfers H• to TEMPO smoothly, forming a stable Rh(II) radical Cp*Rh(2-(2-pyridyl)phenyl)• (14a) that can activate H2 at room temperature and 1 atm. The metalloradical 14a has a g value of 2.0704 and undergoes reversible one-electron reduction at -1.85 V vs Fc(+)/Fc in benzonitrile, implying a bond-dissociation enthalpy for the Rh-H bond of 1a of 58.2(3) kcal/mol--among the weakest Rh(III)-H bonds reported. The transfer of H• from 1a to Ar3C• (Ar = p-(t)BuC6H4) is fast, with kH• = 1.17(3) × 10(3) M(-1) s(-1). Thus, 1a is a good H(-) and H• donor but a poor H(+) donor, a combination that reflects the high energy of the Rh(I) anion [Cp*Rh(2-(2-pyridyl)phenyl)](-).

Understanding the Reactivity Difference of Isocyanate and Isothiocyanate toward a Ruthenium Silylene Hydride Complex

The detailed reaction mechanisms of the C═O hydrosilylation of isocyanate and the C═S bond cleavage of isothiocyanate mediated by the neutral ruthenium silylene hydride complex Cp*(CO)(H)Ru═Si(H){C(SiMe3)3} have been investigated with the aid of density functional theory calculations. Through the investigation, the difference in reactivity between isocyanate and isothiocyanate toward the ruthenium silylene hydride complex has been examined and discussed. The different bond strengths and π-accepting abilities of C═O and C═S and the different degrees of affinity of O and S toward the Si center in the silylene ligand contribute to the different reactivities of the isocyanate and isothiocyanate substrates observed experimentally.

Zwitterionic half-sandwich Rh and Ir complexes containing a diphosphine nido-carborane ligand: synthesis, structure transformation and application in H2 activation

Several novel zwitterionic half-sandwich complexes and pseudocloso metallacarboranes based on 1,2-(PPh(2))(2)-1,2-C(2)B(10)H(10) were successfully prepared, and further study proved that some of them can activate H(2) to form the metal hydride complexes Cp*M(H)(7,8-(PPh(2))(2)-7,8-C(2)B(9)H(10)) (M = Rh, Ir).

Hydrogen bonding to carbonyl hydride complex Cp*Mo(PMe3)2(CO)H and its role in proton transfer

The interaction of the carbonyl hydride complex Cp*Mo(PMe(3))(2)(CO)H with Brønsted (fluorinated alcohols, (CF(3))(n)CH(3-n)OH (n = 1-3), and CF(3)COOH) and Lewis (Hg(C(6)F(5))(2), BF(3).OEt(2)) acids was studied by variable temperature IR and NMR ((1)H, (31)P, (13)C) spectroscopies in combination with DFT/B3LYP calculations. Among the two functionalities potentially capable of the interaction - carbonyl and hydride ligands - the first was found to be the preferential binding site for weak acids, yielding CO...HOR or CO...Hg complexes as well as CO...(HOR)(2) adducts. For stronger proton donors ((CF(3))(3)COH, CF(3)COOH) hydrogen-bonding to the hydride ligand can be revealed as an intermediate of the proton transfer reaction. Whereas proton transfer to the CO ligand is not feasible, protonation of the hydride ligand yields an (eta(2)-H(2)) complex. Above 230 K dihydrogen evolution is observed leading to decomposition. Among the decomposition products compound [Cp*Mo(PMe(3))(3)(CO)](+)[(CF(3))(3)CO.2HOC(CF(3))(3)](-) resulting from a phosphine transfer reaction was characterized by X-ray diffraction. Reaction with BF(3).OEt(2) was found to produce [Cp*Mo(PMe(3))(2)(CO)BF(4)] via initial attack of the hydride ligand.

Hydrometalation of fluoroallenes

Tetrafluoroallene () and 1,1-difluoroallene () react under mild conditions with group 6 and 7 transition metal hydride complexes Cp(CO)(3)MoH (), CO(5)MnH (), Cp(2)WH(2) (), and Cp(2)MoH(2) () to give the new fluoropropenyl complexes , , , , , and , which were characterized by single crystal X-ray diffraction. Complexes and undergo thermally induced 1,3-fluorine migration, whereas and show hindered rotation around the carbon metal single bond.

Neutral and Anionic Silyl Hydride Derivatives of the Tantalum Imido Fragment Cp*(DippN)Ta (Cp* = η5-C5Me5; Dipp = 2,6-iPr2C6H3). Reactive σ-Bonds and Intramolecular C−H Bond Activations Involving the Silyl Ligands

The syntheses and reactivities of alkyl, silyl, and hydride complexes of the type Cp*(DippN)TaRR‘ (Cp* = η5-C5Me5; Dipp = 2,6-iPr2C6H3; R = silyl, alkyl; R‘ = alkyl, hydride) are described. The compounds Cp*(DippN)Ta(SiR3)Cl (SiR3 = Si(SiMe3)3 (1), SiPh3 (2), SiHMes2 (3); Mes = 2,4,6-Me3C6H2), prepared from Cp*(DippN)TaCl2 and (THF)nLiSiR3, react with H2 to yield the dimeric hydride [Cp*(DippN)TaCl(μ-H)]2 (4). The reaction of 4 with (THF)3LiSi(SiMe3)3 (2 equiv) gave the 16-electron silyl hydride complex Cp*(DippN)Ta[Si(SiMe3)3]H (6), which is Lewis acidic and reversibly binds halide ions to afford anionic Ta(V) complexes of the type A+{Cp*(DippN)Ta(X)[Si(SiMe3)3]H}- (A = Li(THF)3, X = Cl (5); A = NBu4, X = Br (7) and I (8)). For complexes 5, 7, and 8, NMR and X-ray data suggest the presence of weak three-center interactions involving Ta, Si, and H. Silyl hydride 6 reacts with xylyl isocyanide (xylyl = 2,6-dimethylphenyl) and acetonitrile to give the corresponding η2-silaimine and azomethine insertion products, respectively, while treatment with acetone yields the stable isopropoxide product arising from insertion into the Ta−H bond. When complex 6 is treated with ethylene or diphenylacetylene, elimination of HSi(SiMe3)3 occurs, along with formation of a five-membered tantalacycle resulting from coupling of the unsaturated substrate at Ta. Hydrogenolysis of 6 yields HSi(SiMe3)3 and a dimeric dihydride [Cp*(DippN)TaH(μ-H)]2 (14). At room temperature, 6 rearranges (t1/2 = 8.5 h) to an unusual alkyl hydride species resulting from C−H bond activation, Cp*(DippN)Ta[CH2Si(SiMe3)2SiMe2H]H (15). This complex exhibits a γ-agostic Si−H interaction with the metal center. An attempt to prepare Cp*(DippN)Ta(SiHMes2)H produced the alkyl hydride product Cp*(DippN)Ta[η2-CH2(2-SiH2Mes-3,5-Me2C6H2)]H (20), which apparently results from decomposition of the expected silyl hydride. The alkyl hydride complex [Cp*(DippN)TaMe(μ-H)]2 (25) was prepared by the hydrogenolysis of Cp*(DippN)Ta[Si(SiMe3)3]Me (23), whereas Cp*(DippN)Ta(CH2CMe3)H (26) was obtained by treatment of 4 with NpMgCl. Complex 26 possesses an α-agostic C−H interaction, and the corresponding deuteride 26-d slowly scrambles deuterium into the methylene positions.

Hydride Transfer from (η5-C5Me5)(CO)2MH (M = Fe, Ru, Os) to Trityl Cation: Different Products from Different Metals and the Kinetics of Hydride Transfer

Reactions of the metal hydride complexes Cp*(CO)2MH (M = Fe, Ru, Os) with Ph3C+BF4- in CH2Cl2 were studied. Hydride transfer from Cp*(CO)2FeH to Ph3C+BF4- gives Cp*(CO)2FeFBF3. Hydride transfer from the ruthenium hydride Cp*(CO)2RuH to Ph3C+BF4- produces the bridging hydride complex {[Cp*(CO)2Ru]2(μ-H)}+BF4-, indicating that Cp*(CO)2RuH exhibits significant nucleophilicity in addition to hydridic reactivity of the Ru−H bond. The osmium hydride Cp*(CO)2OsH reacts with Ph3C+BF4- in CH2Cl2 to give a mixture of Cp*(CO)2OsFBF3 and [Cp*(CO)2Os(ClCH2Cl)]+BF4-. The kinetics of these hydride transfer reactions were monitored by stopped-flow methods, leading to the second-order rate law −d[Ph3C+BF4-]/dt = k[Ph3C+BF4-][MH]. The temperature dependence of the rate constants was determined for the iron hydride, the osmium hydride, and the osmium deuteride Cp*(CO)2OsD. Activation parameters for hydride transfer from Cp*(CO)2FeH are ΔH⧧ = 2.6 ± 0.1 kcal mol-1 and ΔS⧧ = −22.1 ± 0.4 cal K-1 mol-1; for Cp*(CO)2OsH the activ...

A Convenient Method for the Synthesis of Zirconocene Hydrido Chloride, Isobutyl Hydride, and Dihydride Complexes Using tert-Butyl Lithium

Preparation of zirconocene hydrido chloride complexes has been accomplished via addition of 1 equiv of tert-butyllithium to the corresponding zirconocene dichlorides. In this manner, both Cp*Cp‘ ‘Zr(H)Cl and Cp‘ ‘2Zr(H)Cl (Cp* = C5Me5, Cp‘ ‘ = 1,3-(SiMe3)2C5H3) were prepared in high yield. Further addition of tBuLi to Cp*Cp‘ ‘Zr(H)Cl affords the isobutyl hydride complex Cp*Cp‘ ‘Zr(CH2CHMe2)(H), which upon addition of 1 atm of dihydrogen undergoes rapid hydrogenolysis yielding the monomeric dihydride Cp*Cp‘ ‘ZrH2. In the absence of dihydrogen, the zirconocene dihydride undergoes reversible C−H activation of a cyclopentadienyl trimethylsilyl group, affording the “tuck-in” derivative Cp*(η5-1-SiMe3C5H3-3-(η1-SiMe2CH2))ZrH. Addition of 2 equiv of tBuLi to the ansa-zirconocene iPr2Si(η5-3-SiMe3C5H3)(η5-3,4-(SiMe3)2C5H2)ZrCl2 results in the corresponding zirconocene isobutyl hydride complex, which upon exposure to H2 affords the dimeric dihydride [iPr2Si(η5-3-SiMe3C5H3)(η5-3,4-(SiMe3)2C5H2)ZrH2]2. In a similar procedure the substituted indenyl complexes rac-(1-CMe3−C9H6)Zr(H)Cl and rac-(1-CMe3−C9H6)Zr(CH2CHMe2)(H) have been prepared. This synthetic methodology allows for the preparation of metallocene alkyl hydrides and dihydrides inaccessible by more traditional routes.

Pentamethylcyclopentadienyl Zirconium and Hafnium Polyhydride Complexes: Synthesis, Structure, and Reactivity

The half-sandwich zirconium and hafnium N,N-dimethylaminopropyl complexes Cp*M[(CH2)3NMe2]Cl2 (Cp* = η5-C5Me5, M = Zr, 1; Hf, 2) and Cp*M[(CH2)3NMe2]2Cl (M = Zr, 3; Hf, 4) were synthesized by mono- or dialkylation of Cp*MCl3 with the corresponding alkyl-lithium and Grignard reagents. Hydrogenolysis of the monoalkyl species resulted in the formation of the polyhydride complexes Cp*3M3(μ-H)4(μ-Cl)2Cl3 (M = Zr, 5; Hf, 6) and Cp*MCl3. A crystal structure determination of Cp*3Hf3H4Cl5 (6) revealed a fully asymmetric trinuclear structure with three widely differing Hf···Hf distances. Reaction of Cp*3M3H4Cl5 with PMe3 resulted in fragmentation of the cluster and ligand redistribution to give Cp*MCl3(PMe3) and the dimeric hydride complexes Cp*2M2(μ-H)3Cl3(PMe3) (M = Zr, 7; Hf, 8), structurally characterized for M = Zr. The trinuclear polyhydride Cp*3Hf3H4Cl5 reacts with 2,6-xylylisocyanide to give three distinct products, a μ-enediamide complex, [Cp*HfCl2]2[μ-xyNCHCHNxy] (11, xy = 2,6-dimethylphenyl), which was s...

Effect of Ancillary Ligation on the Relative Bond Disruption Enthalpies of Ru−H and Ru−Cl Bonds in Cp(PR3)2RuX (PR3= PMe3, PMe2Ph, PMePh2, PPh3; X = H, Cl)

The ruthenium chloride and hydride complexes Cp(PR3)(2)RuH {X = Cl; PR3 = PMe3 (1), PMe2Ph (2), PMePh2 (3), PPh3 (4); X = H; PR3 = PMe3 (5), PMe2Ph (6), PMePh2 (7), PPh3 (8)} were studied by spectroscopy and solution calorimetry. The structures of 2 and 3 are reported and complete the structural characterization of the series 1-4. In this series, the Ru-Cl distance (2.449 +/- 0.007 Angstrom) remains constant, while the Ru-PR3 distance increases in the order 1 6 > 7 > 8. Enthalpies of reaction for the hydride/chloride metatheses of 5-8 with chlorocarbons (CHCl3 or CCl4) were studied by solution calorimetry and allowed for the determination of Ru-Cl bond disruption enthalpies (BDEs) relative to the corresponding Ru-H BDEs. [BDE(Ru-Cl) - BDE(Ru-H)] values covered a 6 kcal/mol range with an average of 21.4 +/- 2.5 kcal/mol. The magnitude of [BDE(Ru-Cl) - BDE(Ru-H)] correlated with the number of methyl groups on the phosphine and was dependent on the electronic and steric properties of the phosphine.

C−H Bond Activation in Aqueous Solution: Kinetics and Mechanism of H/D Exchange in Alcohols Catalyzed by Molybdocenes

The mechanism of the catalytic H/D exchange in alcohol substrates promoted by molybdocenes in D2O was shown to occur by C−H bond activation. Primary alcohols selectively exchanged α-hydrogens in aqueous solutions containing the catalyst precursor [Cp‘2Mo(μ-OH)2MoCp‘2](OTs)2, while some additional β-hydrogen exchange was observed in secondary alcohols. Tertiary alcohols did not undergo H/D exchange. Formation of chelate complexes such as the independently synthesized and crystallographically characterized glycolate complex [Cp‘2MoOCH2CH2OH](OTs) inhibited the H/D exchange in multidentate alcohols. The exchange reaction was shown to proceed by formation of ketone hydride molybdocene intermediate [Cp‘2Mo(OCR1R2)H]+, which can reversibly dissociate the ketone ligand. The molybdocene hydride complex resulting from ketone dissociation was identified by independent synthesis and crystallographic characterization of the hydride complex Cp‘2MoH(OTf). The H/D exchange reaction proceeds stepwise, with the active cat...

Silyl-Komplexe des Molybdäns und Wolframs. Synthese, Reaktivität und Struktur

The photochemical reaction of Cp 2MH 2 (M=Mo, 1; W, 2) with several hydridosilanes HSiR 3 produces the corresponding silyl hydride complexes Cp 2M(H)(SiR 3) (M=Mo: SiR 3SiEt 3, 3; SiCl 3, 4; Si(OEt) 3, 5; SiH 2(C 5Me 5), 6; SiH 2(C 5Me 4H) 2, 7; SiH(C 5Me 4H) 2, 8; SiH 2[2-(Me 2NCH 2)C 6H 4], 9; M=W: SiR 3SiCl 3, 10) by a reductive elimination/oxidative addition process. Analogous photolysis of 1 in the presence of (C 5Me 5) 2SiHCl or (C 5Me 5) 2SiH 2 does not yield the corresponding complexes Cp 2Mo(H)(SiR 3) (SiR 3SiCl(C 5Me 5) 2; SiH(C 5Me 5) 2). Treatment of 4 and 6, respectively, with LiAlH 4 leads to the hydridosilyl complex Cp 2Mo(H)(SiH 3) 11. Compounds 6–8 and 11 are expected to be single-source precursors for generation of molybdenum silicides. The hydride complexes 4, 5, and 6 are readily converted to the chloro compounds Cp 2Mo(Cl)(SiR 3) (SiR 3SiCl 3, 12; Si(OEt) 3, 13; SiH 2(C 5Me 5), 14). The reaction of 11 with CCl 4 yields complex 12 under exchange of each non-C-bound hydrogen atom. Spectroscopic data of the metallosilanes are discussed and compared with the ones of the parent silanes. An intramolecular N-donor coordination of the functionalized aryl substituent is indicated for 9 by means of 29Si NMR spectroscopy. 6 undergoes fast sigmatropic rearrangements within the Si–(C 5Me 5) fragment as proved by a variable temperature 1H NMR investigation. The structures of 6, 7, 8, and 12 have been determined by single-crystal X-ray diffractometry. Bonding parameters of these typical bent–sandwich complexes are discussed taking account of electronic and steric influences.

A Cationic Imido Complex of Permethyltantalocene: H2 and Carbon−Hydrogen Bond Activation, [2 + 2] Cycloaddition Reactions, and an Unusual Reaction with Carbon Dioxide That Affords Coordinated Isocyanate

Treatment of the imido hydride complex Cp*2Ta(NCMe3)H (Cp* = (η5-C5Me5)) with [Ph3C][B(C6F5)4] in tetrahydrofuran solution yields the cationic imido complex [Cp*2Ta(NCMe3)(THF)][B(C6F5)4] (1). Cation 1 reacts cleanly with H2 to yield [Cp*2Ta(NHCMe3)H][B(C6F5)4]. Carbon−hydrogen bond-activation reactions are observed with propyne or phenylacetylene to afford [Cp*2Ta(NHCMe3)(C≡CR)][B(C6F5)4] (R = CH3, C6H5). In the reaction with propyne, an initial mixture of the [2 + 2] cycloaddition product and C−H activation product is thermally driven to the C−H activation product. The heterolytic cleavage reactions for 1 may be rationalized in terms of the presence of both electrophilic and nucleophilic sites of reactivity in the same molecule. Intramolecular activation of a carbon−hydrogen bond of a Cp* methyl group to ultimately C−H activation of the carbon−hydrogen bonds of methane. An unusual reaction occurs with carbon dioxide: dealkylation of the imido group is observed, liberating isobutylene and yielding the i...

Synthesis and Characterization of New Alkyl−Carbon Dioxide Compounds and the First Neutral Acyl−Carbonyl Niobocene Complexes

The olefin−hydride complexes Cp‘2Nb(H)(η2-RHCCH2) (Cp‘ = η5-C5H4SiMe3; R = H (3), C6H5 (4, endo isomer)) were prepared by the alkylation reactions of [Cp‘2NbCl]2 (1) with the appropriate Grignard reagents RMgX (R = CH2CH3, CH2CH2C6H5) followed by a stereoselective β-elimination from the intermediate alkyl complexes Cp‘2Nb(CH2CH2R). Complexes Cp‘2Nb(H)(η2-RHCCH2) (R = C6H4CH3 (5), C6H4OCH3 (6)) were prepared as a mixture of endo and exo isomers by reaction of Cp‘2NbH3 (2) with the corresponding olefin. Furthermore, reactions of CO with 3 and 4, and the reactions of CO2 with 3−6, afforded the alkylniobocene complexes Cp‘2Nb(CO)R (R = CH2CH3 (7), CH2CH2C6H5 (8)) Cp‘2Nb(η2-CO2)(R) (R = CH2CH3 (9)), CH2CH2C6H5 (10), CH2CH2C6H4CH3 (11), CH2CH2C6H4OCH3 (12)). The reactivity of 9 and 10 toward the strong Lewis acid B(C6F5)3 was also studied; in a first step the adducts Cp‘2Nb(η2-CO2−B(C6F5)3)(CH2CH2R) (R = H (13), C6H5 (14)) were formed and subsequently evolved to give the oxo−alkyl complexes Cp‘2Nb(O−B(C6F5)3)(CH2CH2R) (R = H (15), C6H5 (16)) with the loss of CO. Finally, reactions of 3 and 4 with CO under appropriate conditions gave the neutral acylniobocene complexes Cp‘2Nb(CO)(η1-C(O)CH2CH2R) (R = H (17), C6H5 (18)), which were alternatively prepared from the reactions of 7 and 8 with CO. The different complexes were characterized by spectroscopic methods.

Hydrogen-transfer reactions which generate new imine, imido, and trimethylenemethane complexes of tantalum

The reactions of Cp*TaMe3C1 (Cp* = qs-CsMeS) with a variety of alkali-metal alkoxide, alkylamide, and alkyl reagents have been examined. Reaction with LiNMe2 produces Cp*Ta(NMez)Me3, which decomposes at 25 OC to an imine (or metallaazirane) complex, Cp*Ta(CH2NMe)Me2. The decomposition is a first-order, unimolecular process with a large kinetic isotope effect (kH/kD = 9.7). Monoalkylamides (LiNHR) react with Cp*TaMe3C1 to form imido complexes Cp*Ta(NR)Mez. Reaction of Cp*TaMe3C1 with lithium diisopropylamide forms a bridging methylene complex, Cp*Me2Ta(pCH2)(p-H)zTaMezCp*. The alkoxide compounds Cp*Ta(OR)Me3 (R = Me, CHMe2, CMe3) are very stable and decompose only over 100 OC. Alkyl complexes are stable only if the alkyl group does not have j3-hydrogens. Treatment of Cp*TaMe3C1 with (2-methylally1)magneium bromide affords an unstable tantalum 2-methylallyl compound, which decomposes cleanly to the trimethylenemethanec omplex Cp*TaMeZ(q4-C(CH2),)T. he rates of hydrogen abstraction or elimination processes in this system correlate with the nature of the atom bound to tantalum: for reactions involving a /3-hydrogen transfer the order is C > N > 0, while the facility of a-hydrogen abstraction reactions appear to decrease in the reverse order N > C. These reactivity patterns appear to reflect the variance in T a x , Ta-N, and Ta-O bond energies in this series. Hydrogenation of the imido compounds (Cp*Ta(NR)Me2) in the presence of phosphine ligands yields new examples of imido hydride complexes Cp*Ta(NR)H,(L) (L = PMe3, PMe2(C6Hs); R = CMe3, CHzCMe3). A moderately stable alkyl hydride complex, Cp*Ta(CH2NMe)Me(PMe3)H, has also been prepared.