The syntheses and single‐crystal X‐ray structures of a series of Mo–imido alkylidene N‐heterocyclic carbene (NHC) complexes (1–15) and of the first complexes containing bidentate NHC‐phenolate ligands (16–18) are reported. Mo(N‐2,6‐Me2‐C6H3)((1‐R‐phenethyl)‐3‐mesitylimidazolidin‐2‐ylidene)(CHR)(OTf)2 (R=CMe2Ph, 1) is the first enantiomerically pure Mo–imido alkylidene NHC catalyst. With [Mo(N‐2,6‐Me2‐C6H3)(IMes)(CHR)(CH3CN)(OTf)(CH3CN)+ B(ArF)4−] (7), turnover numbers up to 545 000 were achieved in the homometathesis (HM) of 1‐octene and 1‐nonene (≤95 % E). With 7 and 1‐nonene, a turnover frequency (TOF4 min) of 8860 min−1 was determined. Productivity and E/Z‐selectivity were correlated with catalyst structure. For 1, Mo(N‐3,5‐Me2‐C6H3)(IMesH2)(CHR)(OTf)2 (9) and Mo(N‐3,5‐Me2‐C6H3)(IMes)(CHR)(OTf)2 (10), productivity was correlated with the coalescence temperature of the two triflates, determined by variable‐temperature 19F NMR spectroscopy. The square‐planar conformer is postulated to be the most relevant for the catalyst activation.

New tungsten and molybdenum alkylidyne complexes bearing mono-, bi-, and tridentate N-heterocyclic carbenes (NHCs) have been synthesized. Formation of unprecedented structures in complexes bearing N-tert-butyl substituents on the imidazol(in)-2-ylidene was observed, leading to molybdenum complexes containing an abnormal carbene (Mo-4) and a bridging O,C,C-pincer ligand (Mo-10) and to a tungsten complex containing a cationic imidazolinium-tagged alkoxide forming an inner salt with an anionic tungsten center (W-5). Both the abnormal carbene binding in Mo-4 and the O,C,C-pincer-type structure of Mo-10 were confirmed by singlecrystal X-ray analysis, and the proposed structure of W-5 is supported by the single-crystal X-ray structure of a minor byproduct (W-8) formed during the synthesis of W-4, displaying the aforementioned inner-salt-like structure. The novel alkylidyne complexes were also investigated for their capability to form a previously postulated quasi-cationic species with a weakly coordinating anion (WCA) during the alkyne homometathesis of 1phenyl-1-propyne. Overall, incorporation of bidentate and strongly σ donating NHCs as well as introduction of better leaving groups did not lead to the expected increase in catalytic activity. Despite identical ligand spheres, changing from molybdenum to tungsten led to complete loss of activity in the bidentate systems.

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A route to cationic tungsten alkylidyne N-heterocyclic carbene (NHC) complexes of the general formula [W­(CR)­X2(NHC)­(tBuCN) y +B­(ArF)4 –] (R = tBu, C6H4-4-OMe; X = Br, Cl, OTf; NHC = 1,3-dimesityl-1,3-dihydro-2H-imidazol-2-ylidene, IMes; 1,3-bis­(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene, IDipp; 1,3-dimesityl-4,5-dichloro-1,3-dihydro-2H-imidazol-2-ylidene, IMesCl2; y = 1, 2; ArF = 3,5-bis­(trifluoro­methyl)­phenyl) is presented. Addition of 1 equiv of water to cationic tungsten benzylidyne and neopentylidyne NHC complexes results in the formation of the corresponding cationic tungsten oxo alkylidene complexes; however, only the resulting neopentylidene complexes were isolated as stable compounds and tested in selected olefin metathesis reactions. Interestingly, even though a strongly basic ligand in the form of the NHC is coordinated to the metal center, this bond is unaffected by the reaction with 1 equiv of water. The hydrolysis of cationic tungsten alkylidyne NHC complexes represents one pathway to cationic tungsten oxo alkylidene NHC complexes and enables the synthesis of complexes, which were so far not easily accessible by previously published procedures.

The tetracoordinated cationic molybdenum alkylidyne N-heterocyclic carbene (NHC) complexes [Mo(CC 6 H 4 -p-OMe)(IMes)(OCMewere synthesized from the pentacoordinated progenitor Mo(CC 6 H 4 -p-OMe)(IMes)-(OCMe(CF 3 ) 2 ) 2 (OTf) (Mo4). Complexes Mo4−Mo6 were evaluated for their ability to catalyze the self-metathesis of several internal alkynes. The presence of a triflate group facilitates formation of a cationic species while preformation of the cationic molybdenum center in molybdenum alkylidyne NHC complexes indeed results in a strong increase in catalyst productivity and activity, also in the presence of functional groups, compared to previously reported neutral congeners. The most striking feature of this class of tetracoordinate cationic complexes is the excellent catalytic activity in the alkyne metathesis of non-protic substrates, thereby supporting our previously published proposal of a tetracoordinate cationic active species in alkyne metathesis formed from the neutral, pentacoordinate molybdenum alkylidyne NHC progenitors. Catalyst productivity, expressed as turnover number, reached 20 000 in the self-metathesis of 1-phenyl-1-propyne (S1) using Mo(CC 6 H 4 -p-OMe)(1,3-dimesitylimidazol-2ylidene)(OCMe(CF 3 ) 2 ) 2 [B(Ar F ) 4 ] (Mo6) and 5-(benzyloxy)-2-pentyne (S2) at catalyst loadings as low as 0.005 mol %.