The molybdenum dinitrogen complex trans-[Mo(N2)2(dppe)2] (2) reacted with 2−2.5 equiv of various β-ketonitriles at room temperature to afford the (nitrido)(nitrile-enolato) complexes trans-[Mo(N)(NCCR1COR2)(dppe)2] (4; R1 = H, R2 = 4-MeOCOC6H4, 4-ClC6H4, 4-Tol, 4-MeOC6H4, 2-C4H3O, 2-C4H3S, Pr i ; R1 = CN, R2 = Me, Ph; dppe = Ph2PCH2CH2PPh2) via the C⋮N triple bond cleavage of the nitriles on the molybdenum center. On the other hand, the reaction of complex 2 with 2 equiv of pivaloylacetonitrile at room temperature led to the isolation of the (alkylideneamido)(nitrile-enolato) complex trans-[Mo(NCHCH2COBu t )(NCCHCOBu t )(dppe)2] (5k), which further underwent the cleavage of the CN double bond of the alkylideneamido ligand to give the corresponding (nitrido)(nitrile-enolato) complex trans-[Mo(N)(NCCHCOBu t )(dppe)2] together with 4,4-dimethyl-1-penten-3-one. Furthermore, treatment of 2 with large excess amounts of 4-chlorobenzoylacetonitrile followed by anion metathesis with [NHEt3][OTf] (OTf = OSO2CF3) resulted in the isolation of the cationic (imido)(nitrile-enolato) complex trans-[Mo{NCH2CH2CO(C6H4Cl-4)}{NCCHCO(C6H4Cl-4)}(dppe)2][OTf] (6c +[OTf]-). The solid-state structures of 4h·1.5C2H4Cl2 (R1 = CN, R2 = Me), 5k·C6H6, and 6c +[OTf]- were determined by single-crystal X-ray analyses. The detailed NMR analysis of the reaction of 2 with aroylacetonitriles revealed that the (alkylideneamido)(nitrile-enolato) complexes trans-[Mo(NCHCH2COR)(NCCHCOR)(dppe)2] (5) act as the key intermediates for the C⋮N triple bond fission, and the rate constants for the conversion of the complexes 5 into the nitrido complexes 4 showed good correlation with the Hammett σ p or σ a constants for the aroyl substituents, where positive ρ values were obtained (ρ p, 1.42; ρ a, 0.41). A reaction mechanism for the nitrido complex formation is proposed, which includes (1) the substitution of a dinitrogen ligand in 2 with a β-ketonitrile molecule, (2) the fast protonation of the nitrile ligand by a second β-ketonitrile molecule leading to the formation of complex 5, (3) the relatively slow proton shift from the α-position of the carbonyl group to the amido carbon in the alkylideneamido ligand to form an enolated imido ligand, and (4) the fast elimination of a vinyl ketone from the imido ligand giving the nitrido complex 4 as the final product.

The reaction of trans-[Mo(N 2 ) 2 (dppe) 2 ] (1; dppe ) Ph 2 PCH 2 CH 2 PPh 2 ) with excess amounts of HCtCLi‚en (en ) H 2 NCH 2 CH 2 NH 2 ) afforded the fivecoordinate Mo(0) mono(acetylene) complex [Mo-(HCtCH)(dppe) 2 ] (2). Treatment of 2 with 2 mol of [Cp 2 Fe][BF 4 ] (Cp ) η 5 -C 5 H 5 ) in THF gave the Mo(II) acetylene complex trans-[MoF(HCtCH)(dppe) 2 ][BF 4 ], while that in THF-MeCN followed by anion metathesis with NaOTf (OTf ) OSO 2 CF 3 ) led to the formation of the metallacyclopropene complex [Mo(η 3 -CHCHPPh 2 -CH 2 CH 2 PPh 2 -C,C′,P)(MeCN) 2 (dppe)][OTf] 2 .The chemical bonding and transformation of fourelectron-donating alkyne ligands at early-transitionmetal centers have been a subject of extensive research activity. 2 A wide variety of six-coordinate d 4 metal (most typically Mo(II) and W(II)) complexes with four-electrondonating alkyne ligands have been synthesized and found to exhibit intriguing reactivities. 2,3 Four-coordinate d 6 complexes of the types ML(alkyne) 3 4-8 and ML 2 (alkyne) 2 9-11 (L ) monodentate ligand) have also been reported. In contrast, five-coordinate d 6 mono-(alkyne) complexes with the general formula ML 4 -(alkyne) have been surprisingly scarce; only a few examples, including the chromium complexes [Cr(CO) 2 -{P(OMe) 3 } 2 (RCtCPh)] (R ) Ph, H), 12 have been described in the literature. Our extensive studies on lowvalent molybdenum and tungsten complexes have demonstrated that the Mo(0) center in the complex trans-[Mo(N 2 ) 2 (dppe) 2 ] (1; dppe ) Ph 2 PCH 2 CH 2 PPh 2 ) is not only effective for the activation of dinitrogen 13,14 but

Reactions of [Cp*Ir(SH) 2 (PMe 3 )] (1; Cp* ) η 5 -C 5 Me 5 ) with [CpTiCl 3 ] (Cp ) η 5 -C 5 H 5 ) and [TiCl 4 (thf) 2 ] in the presence of triethylamine afforded the sulfidobridged titanium-iridium heterobimetallic complexes [CpTiCl(µ 2 -S) 2 Ir(PMe 3 )Cp*] (2) and [TiCl 2 (µ 2 -S) 2 Ir-(PMe 3 )Cp*] (3), respectively.

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