Metal multiple-bonded complexes

Among the more interesting metal-metal multiple bonded complexes are the hexaalkoxo dinuclear tungsten and molybdenum complexes, fM,(OR),J [Pg.418]

Not included in the present review is the fascinating new chemistry which results from reaction between diazo compounds and low-valent transition-metal complexes bearing easily displaceable two-electron ligands as well as with metal-metal multiple bonds and metal hydrides whereby a variety of novel organometallic molecules could be obtained. This field has been covered, in accord with its rapid development, by successive reviews of Hermann 19 22) and Atbini23). 

In some cases, alkoxide displacements at a metal center proceed sequentially and at different rates so that, if redistribution processes do not intervene, mixed diketonates can be obtained by adjusting the ratios of reactants.190,191 Recently, bimetallic / -diketonates of the type M2(diketonate)2(OR)4 (M = Mo, W R = alkyl) have been prepared from diketones and M2(OR)6. X-Ray studies show that one diketonate is attached to each metal atom so that unbridged M=M units are retained although rapid rotation about this multiple bond can occur.192 Diketonate complexes with metal-metal multiple bonds are rather uncommon, one other example being C2v CK-Mo2(Me-COCHCOMe), (02 CMe)2.193... 

Major advances in organometallic chemistry during the last years have been achieved in the area of silicon-metal multiple bonding and silicon with low coordination numbers. For late transition metals, new complexes have been synthesized such as silanediyl (A), silene (B), silaimine (C), disilene (D), silatrimethylenemethane (E), silacarbynes (F), cyclic silylenes (G), silacyclopentadiene (H) and metalla-sila-allenes (I) (Figure 3). 

The use of bulky monodentate m-terphenyl ligands in the stabilisation of d-block organometallic compounds is surveyed. Importantly, these ligands have facilitated the isolation of hitherto unknown species containing low-coordinate centres and metal-metal multiple bonds. This review reports on these advances with emphasis on the synthesis, structural characterisation and, where possible, reactivity studies of complexes featuring metal-carbon bonds between m-terphenyl ligands and the transition metals. 

While metal-metal multiple bonds are occasionally cleaved by a-donors, these reactions are sufficiently unpredictable that little advantage can yet be taken of them to design synthetic routes to desired monomeric complexes. In contrast, the facile... 

The existence of multiple bonds between metal atoms in discrete complexes was first demonstrated by Cotton and coworkers in the early 1960 s (1 ). Since that time, several hundred papers have appeared dealing with the synthesis and structural and spectroscopic (2.) characterization of complexes with multiple bonds between metal atoms. However, there have been relatively few systematic investigations of the reactivity of the metal-metal multiple bonds. 

Finally, in analogy with organic chemistry, "polymerization" of metal-metal multiple bonds may lead to clusters as illustrated in eqs. 12 and 13. To date, the only definitely characterized oligomerization reaction of this type has been reported by McCarley et al. (8j(eq. 14), although Chisholm et al. ( ) have observed that Mo2(0Et)6 dimerizes to a tetranuclear complex of unknown structure and that the 0-H bond of isopropanol oxidatively adds to the WeW bond of W2(i-Pr0)6 to give a tetranuclear complex with an "open" as opposed to a closed, or cluster, structure (10). Also, some evidence has been presented that Cp2Mo2(C0)it may form unstable tetrahedrane intermediates (eq. 12) (. 

REACTIONS OF LOW VALENT METAL COMPOUNDS WITH COMPLEXES CONTAINING METAL—METAL MULTIPLE BONDS. 

In a novel use of photolytic addition of H2 across the metal-metal multiple bond, the two-H-atom-bridged double-bonded complex shown below has been prepared (equation 12). 

NMR resonances of the alkylidyne carbon atom and selected IR absorptions for new carbyne complexes are given in Table 11. In other studies, the Mo and NMR spectra of molybdenum and tungsten alkylidyne complexes were determined by Enemark and co-workers (23). The metal atoms in the alkylidyne complexes were found to be less shielded than those in nitrido complexes but more shielded than those in compounds containing metal-metal multiple bonds. 

A selective method of forming a bond between two different metals is by reaction of an electron-rich complex with either a stable, unsaturated metal complex or a metal complex containing an easily displaeeable ligand. The unsaturated metal eomplex may have a formal metal-metal multiple bond, a multiple bond between a metal and a nonmetal, or a eoordinatively unsaturated metal center. 

The addition of metal nucleophiles to complexes containing metal-metal multiple bonds is an excellent method of forming larger clusters. One of the most widely used is H20s3(CO)io ... 

A rational, and general, approach to complexes containing heteronuclear bonds between Ni or Pt and a second transition metal is by addition of a complex containing a metal-carbon or metal-metal multiple bond to a coordinatively unsaturated Ni(0) or Pt(0) species, thus producing a di- or tri-metallacyclopropane or -propene. These reactions proceed in good yield under mild conditions, and the products are quite predictable. Reactions (l)-(n) give some examples ... 

The reactions of alkynes and alkenes with metal-metal multiple bonds in dinuclear complexs have important catalytic implications, but less attention is paid to their insertions into metal-metal single bonds. A 1965 publication comprehensively reviewing insertions of compounds of metals and metalloids involving unsaturated substrates lists insertions into metal-metal bonds prior to 1965. 

Hill proposed an unambiguous electron-counting notation for metallaboratranes (M -> B). This notation follows the tradition often encountered in denoting compounds with metal-metal multiple bonds and allows a correct assignment of oxidation number in these complexes.94... 

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