Molybdenum complexes quadruple bonds

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 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). 

Dinuclear molybdenum(II) molecules4,6 also contain quadruple bonds.5 The acetate Mo2(02CCH3)4 is also a very useful starting material for the preparation of other complexes of lower-valent molybdenum.5-7 The preparations of Mo2(02CCH3)4 and Mo2(02CC6H5)4 are therefore described. 

M1M) + Complexes (2). In scores of dinuclear molybdenum compounds, and to a lesser extent ditungsten compounds, a central unit has a M-M quadruple bond of configuration a2 1 2. 

Oxidation of molybdenum hexacarbonyl with acetic acid produces the carboxylate-bridged quadruple-bonded dimers, as shown inequation (35). This complex can undergo a number of substitution reactions that leave the metal metal bond intact. ... 

Dimeric molybdenum complexes bearing metal-metal quadruple bonds have been prepared in better yield than previously reported conventionally. The tetra-carboxylate complexes Mo2(p-02CR)4 (R = Me, Et, Ph) were synthesized by heating Mo(CO)6 and the corresponding carboxylic acid at 180-200 °C for 15-32 min." The treatment of a MeOH/CH2Cl2 solution of Cu(BF4)2 H20 and 2,2 -dipyridyldisulfide at 140 °C for 2 h led to the formation of the polymeric Cu complex [Cu9(C5H5NS)8(SH)g](BF4) in 55% yield and the Cu complex [ Cu(dps)2)2(p-S)] +, dps = 2,2 -dipyridylsulfide, in 4% yield. In this unusual reaction, C-S and S-S bonds are cleaved simultaneously. 

An example of the model complexes that point the way to potentially fascinating polymers in the future is given by the tetranuclear species 6.21 [49]. Remarkable square arrays of molybdenum-molybdenum quadruple bonds with oxalate spacers have also been prepared by simple assembly procedures, as illustrated by the formation of the complex Mog(oxalate)4(N-N)g (6.22) (N-N = di(p-methoxyphenyl)forma-midinate) (Fig. 6.11) from [Mo2(N-N)2(NCMe)2] and [oxalate] " [52]. 

There have been many compounds discovered which resemble the [Re Xj,] " ions in possessing extremely short M—M distances, eclipsed conformations and, presumably. quadruple metal-metal bonds. The isoelectronic molybdenum(ll) species, [Mo Clijj , is known and both Re(lll) and Mo(Il) form a large series of carboxylaie complexes of formulas Re2(RC02)2X4, Re2(RC02)4X2. and Mo2(RC02)4 ... 

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