Molybdenum complexes triple bonds

Molybdenum imido alkylidene complexes have been prepared that contain bulky carboxylate ligands such as triphenylacetate [35]. Such species are isola-ble, perhaps in part because the carboxylate is bound to the metal in an r 2 fashion and the steric bulk prevents a carboxylate from bridging between metals. If carboxylates are counted as chelating three electron donors, and the linear imido ligand forms a pseudo triple bond to the metal, then bis(r 2-carboxylate) species are formally 18 electron complexes. They are poor catalysts for the metathesis of ordinary olefins, because the metal is electronically saturated unless one of the carboxylates slips to an ri1 coordination mode. However, they do react with terminal acetylenes of the propargylic type (see below). 

One of the first isolated and structurally characterized phosphido complexes [(t-Bu Ph"N)3Mo=P] (la) was obtained from the reaction of a molybde-num(III) complex with P4 in 79% yield (Eq. 1) [8]. The molybdenum starting material [(f-Bu Ph"N)3Mo] is planar, and the three unpaired electrons are localized on the Mo atom [10], engendering an ideal environment for the reduction of white phosphorus and formation of the triple bond, la can transfer the phosphido ligand to another complex [(t-BuPhN)3Moj via a het-erocumulene intermediate to give [(t-BuPhN)3Mo=Pj (lb) [11] (Eq. 2). 

The analogous reaction of 3 with W2(OrBu)6 gave a brown solution, from which a yellow, microcrystalline material could be isolated. Based on spectroscopic data the latter was formulated as [Cy7Si70i2]2W2(p-H)(0tBu) (158). Cleavage of the Mo = Mo triple bond in 157 occurred upon treatment of this compound with NO (two equivalents) to afford the dimeric nitrosyl molybdenum silsesquioxane complex [Cy7Si70i2]2Mo2(NO)2 (159). °°... 

Of considerable interest are reactions in which the metal-metal bond order changes. So far, there are no known examples in which a W—W triple bond is converted to a W—W quadruple bond, a reaction which is known for molybdenum.289 However, several reactions in which the W—W triple bond is oxidized are known. The preparation of alkoxo clusters of WIV have been mentioned (Section 37.5.4.1) and the reaction in which W=W reacts with RC CR or RC=N to give mononuclear complexes containing the W=CR and WssN units have been discussed (Section 37.3.3). 

There are two well-characterized examples of a naked carbon atom bound by a triple bond to a metal center (Fig 14.3.8). The molybdenum carbide anion [CMo N(R)Ar 3]- (R = C(CD3)2(CH3), Ar = C6H3Me2-3,5), an isoelectronic analog of NMo N(R)Ar 3, can be prepared in a multistep procedure via deprotonation of the d° methylidyne complex HCMo N(R)Ar 3. The Mo=C distance of 171.3(9) pm is at the low end of the known range for molybdenum-carbon multiple bonds. In the diamagnetic, air-stable terminal ruthenium carbide complex Ru(=C )C12(LL/)(L = L = PCy3, or L = PCy3 and L = l,3-dimesityl-4,5-dihydroimidazol-2-ylidene), the measured Ru-C distance of 165.0(2) pm is consistent with the existence of a very short Ru=C triple bond. 

Stable mononuclear transition-metal germylene complexes with a formal triple bond between the Ge and metal atoms (Cr, Mo, W, 56-62) have been synthesized by a combination of salt-elimination and decarbonylation reaction (equation 44)150,151. The decarbonylation process for the Cr and W intermediates 61 and 62 can involve either a thermal or a photochemical reaction the molybdenum germylyne complexes 56 and 57 were obtained directly, even at low temperatures. 

The reactions and nature of multiple bonds between two metal atoms form a lively area of research and debate (1,2). Many examples of triple bonds between metals are known, including the extensively studied molybdenum or tungsten alkoxide, M2(OR)6, and amido, M2(NR2)6, complexes (3). Classic examples of quadruple bonds are epitomized by [Re2Cl8]2 (4) and molybdenum acetate, Mo2(OCOMe)4 (5). 

Despite Schrock s extensive work on tantalum, molybdenum, and tungsten car-byne complexes (e.g., 47, 49, 50, 51, and 52) [96, 97, 99, 100], it must be recalled, however, that they were not the first compounds with a metal—carbon triple bond. 

E. O. Fischer, G. Kreis, C. G. Kreiter, J. Muller, G. Huttner, and H. Lorenz, trans-Halo[alkyl(aryl)carbyne]tetracarbonyl Complexes of Chromium, Molybdenum and Tungsten. A New Complex Type with Transition Metal-Carbon Triple Bond, Angew. Chem. Int. Ed. Engl. 12, 564-565 (1973). 

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