Tungsten complexes quadruple bonds

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). 

In divalent tungsten complexes besides monomer configiuations, also dimeric configurations having a quadruple W-W bond are known. 

Compounds containing an Mo=Mo unit are well exemplified in Mo(ll) chemistry. In contrast, the chemistry of the W=W + unit is less extensive. Two reasons for this observation are that complexes containing a W2 core are more prone to oxidation than those based on Mo2 , and that suitable precursors to Mo2 -containing complexes are more abundant than those to analogous tungsten-containing compounds. A description of Mo=Mo and W=W quadruple bonds in terms of a,... 

Cra(QH8)Cp2] (11), formed from [CrCpa] and Naa[cot], has a double metal-metal bond bridged by a ring-opened Q ligand. Details of the preparation and X-ray structures of [MgCcotlj] (M = Mo or W), and of a neutron diffraction study on the tungsten complex, have been given. Quadruple metal-metal bonds are present, as in the isostructural chromium compound (see Vol. 6, p. 181 Vol. 7, p. 175). 

The quadruply metal-metal bonded complexes M2X4(L-L)2 (M = Mo, W, X = halide L-L = bridging tertiary phosphine) also show photochemical reactions with added substrates. When the complex W2CU(dppm)2 is photolyzed (A > 436 nm) in the presence of methyl iodide, the product is W2Cl4l(Me)(dppm)2. This product complex results from the oxidative addition across the tungsten-tungsten quadruple bond " ... 

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