Dimetal compounds

The PR3 ligands transoid to the y-CR group in (lj are readily displaced by CO gas (1 bar), affording initially the dimetal compounds (1 ), which subsequently in solution yield the trimetal complexes (lj). Evidently compounds ( ) partially dissociate into [(n-C5H5)(0C)2W =CRJ and [Pt(C0)(PR3)], and the latter adds to undissociated (14) to give the complexes (15) (10). 

Many distibines and dibismuthines have lighter colors in solutions or melts than in the solid state. Crystals of these thermochromic distibines or dibismuthines consist of linear chains of the dimetal compounds with short intermolecular metal-metal contacts. Delocalization of electrons along the chains is possibly responsible for the bathochromic shift between fluid and solid phases. Usually, the /raor-conformation is adopted by the tetraorganodimetal compounds in the solid state. (CF3)4As2 shows the /ra r-conformation also in the gas phase. Photoelectrospectroscopic measurements on Me4Sb2 revealed the presence of gauche- (12%) and trans- (88%) conformed in the gas phase.52... 

The use of bulky organic groups permitted the isolation of several thermally stable compounds containing double bonds between arsenic, antimony, or bismuth.8 9 56 Examples of homonuclear dimetal compounds, that is, diarsenes,... 

With a view to examining whether bulky substituents at the amidine nitrogen atoms can prevent axial coordination in quadruply bonded dimetal compounds, Cotton and coworkers have studied the reactions of (Bu4N)2Re2Cl8 with PhN(CMe)NHPh and MeN(CPh)NHMe.83 The structures of the crystalline products can be represented as (34) and (35) respectively. 

The structure of 11a has been established by X-ray diffraction. The compounds 11 are 30-cluster valence-electron (CVE) dimetal species, and are therefore electronically unsaturated. The electron distribution within the W(fi-C)fth rings may be represented by various canonical forms. However, that shown, which implies that the W=C bond in the ( 5-C2B9H9R 2)(OC)2-W=CC6H4Me-4 moiety formally contributes three electrons to the rhodium center (16-electron valence shell), accords with other results from our laboratory involving electronically unsaturated dimetal compounds with bridging alkylidyne ligands 18). 

Treatment of [Mo(NCMe)(CO)2(ij7-C7H7)][BF4] with 2c in THF affords the dimetal compound [MoW(/i-CC6H4Me-4)(CO)2(f/7-C7H7)(f/5-C2B9H9-Me2)] (17). The latter species, like complex 15a, contains a B—H —Mo three-center two-electron bond (22). 

Allylmetal compounds can be prepared by the Pd-catalysed allylation of dimetal compounds. The phenylallylboronate 195 is prepared by the reaction of 2-phenylallyl acetate with bis(pinacolate)diboron (194) catalysed by ligandless Pd in DMSO [95], A good synthetic route to the allylsilane 197 is the reaction of Me3SiSiMe3 with geranyl trifluoroacetate (196) catalysed by ligandless Pd via transmetallation of the n-allylpalladium intermediate at room temperature in DMF [96],... 

Addition reactions of three kinds of main group metal compounds, namely R—M X (carbometallation, when R are alkyl, alkenyl, aryl or allyl groups), H—M X (hydrometallation with metal hydrides) and R—M —M"—R (dimetallation with dimetal compounds) to alkenes and alkynes, are important synthetic routes to useful organometallic compounds. Some reactions proceed without a catalyst, but many are catalysed by transition metal complexes. 

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

Another highly important class of unsaturated dimetal compounds is based on the general formula Cp2M2(CO)x (Cp = C5H5) and forms the subject of this article. In this class, perhaps the complexes Cp2M2(CO)4 1-3 attracted most early attention. Of these, 2 is most studied. Although not the first discovered, it is undoubtedly the easiest to make in large quantities, and this fact is not the least important reason for the attention it has received. 

Multimetallic sandwiches, including polymetallocenes, bis(fulvalene)-dimetal compounds, multi-decker sandwiches, and compounds in which two metals are bonded to a common unsaturated cyclic hydrocarbon are discussed in Section II. Sections III and IV are subdivided according to the metal group, and describe ligand-bridged bimetallics and metal clusters, respectively. Where appropriate, redox potentials have been included. Unless otherwise stated they are referenced versus the aqueous saturated calomel electrode (see). 

Twenty-seven dimetal compounds, bridged by one dimethylarsenide group but containing no direct metal-metal bond, were found to undergo irreversible oxidation and/or reduction (59), usually giving mononuclear... 

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