Quadruple bonds reactions

The molybdenum(II) carboxylates, Mo2(02CR)4, are key derivatives containing molybdenum-molybdenum quadruple bonds.1 First described by Wilkinson and coworkers in the early 1960s,2 they remain the key starting materials in (Mo2)4 + chemistry and they have been the objects of numerous physical and theoretical studies.1 The most commonly used method of preparation is the reaction between molybdenum hexacarbonyl and a carboxylic acid in an inert solvent such as 1,2-dichlorobenzene (DCB) [eq. (I)].3 Excellent yields of Mo2(02CR)4 can be obtained by... 

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). 

The reaction of WCU with K2C8H8 in THF gives a reasonably air-stable material that was shown to be [W2(C8Hg)3]. The W—W bond length of 2.375 A, although long, is consistent with the presence of a quadruple bond.329 However, it is recognized that this compound (and the related chromium and molybdenum) is electronically different from other quadruply bonded species. 

EtSSEt (forming [Mo2Cl4(/i-SEt)2(dmpe)2]).162 Cleavage of Mo—Mo quadruple bonds has provided a useful synthetic route to several mononuclear compounds7 and, of special significance, are the reactions of alkyl and aryl isocyanides with Mo2+ compounds.163... 

A number of excellent reviews have been written that address various aspects of metal-metal bonding. In particular, Cotton has contributed several eminent early reviews commensurate with his pioneering efforts in this area (54, 59, 60). More recently he reviewed quadruple bonds (56), molybdenum-molybdenum bonds (57), and multiple bond studies in his laboratory (55) in separate articles. Earlier reports discuss multiple metal-metal bonds in the context of general metal—metal bonding schemes and clusters at a time when the area was more limited in scope and therefore amenable to an encompassing survey (13, 128, 129, 164, 188, 189, 252). The redox behavior of metal-metal bonds has been reviewed by Meyer (193), and Walton has recently described ligand-induced redox reactions of strong multiple metal—metal bonds (253). 

The reaction of Mo(CO)6 and acetic acid is still the most convenient preparative route known for condensing two monomeric units into a compound containing a quadruple bond. In fact, facile conversion of Mo2(02CCH3)4 to chloro species occurs in concentrated hydrochloric acid to form the Mo2 Clj ion (36), which is isostructural with the Re2Clg ion. The eclipsed configuration of Mo2Clg" and the molybdenum atom separation of 2.14 A are entirely consistent with a quadruple bond in the anion as expected (33). 

The reactions discussed in this section include those in which monomeric reagents are converted into dimeric species containing quadruple bonds. Later sections of this review consider reactions in which substitution or redox processes occur with retention, alteration, or cleavage of the metal—metal multiple bond. In very general terms, it is the initial formation of the quadruple bond that is common to the reactions below, and the number and variety of... 

The origin of the majority of rhenium—rhenium quadruple bonds can be traced directly to the parent Re2Clg anion. Several routes to the octachloro-dirhenate(III) ionic complex have been compared (42). The method of choice is the reaction of trirhenium nonachloride with molten diethylammonium chloride (12). Yields of up to 65% are attainable when the reaction mixture is extracted with hydrochloric acid and the anion is precipitated as the salt of a large cation (i.e., tetrabutylammonium cation). 

BM) forms with a quantum yield of 0.017 0.005 when Re2Cl " is irradiated at 313 nm. Cleavage of the quadruple bond to form ReCl4(CH3CN)2 is followed by rapid conversion to the neutral monomer by means of acetonitrile replacement of chloride. No reaction occurs in the absence of irradiation at reflux, and the 88 excited state is not responsible for the observed photochemistry, since 632.8 nm radiation does not lead to cleavage of the rhenium dimer (135). Flash-photolysis studies suggest that the dominant photochemical process involves internal conversion to a transient 65 excited... 

The dimers have turned out to be a very interesting and useful class of metalloporphyrins which was extended to dimers containing two divalent metal ions with Rh-Rh single (see Sect. 3.3), Re-Re triple, and Mo-Mo or W-W quadruple bonds [49,222-224]. These species react with many donor and acceptor molecules (see the following Sects. 3.22 and 3.3). The reactions with typical neutral donor ligands may be seen in Scheme 1, the oxidation reactions in Scheme 2. 

The quadruple bond can undergo a variety of interesting reactions, as outlined... 

Some reaction types of dimetal compounds containing a M=M quadruple bond. 

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). 

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