Bidentate ligands transition metals

The red tetrathiomolybdate ion appears to be a principal participant in the biological Cu—Mo antagonism and is reactive toward other transition-metal ions to produce a wide variety of heteronuclear transition-metal sulfide complexes and clusters (13,14). For example, tetrathiomolybdate serves as a bidentate ligand for Co, forming Co(MoSTetrathiomolybdates and their mixed metal complexes are of interest as catalyst precursors for the hydrotreating of petroleum (qv) (15) and the hydroHquefaction of coal (see Coal conversion processes) (16). The intermediate forms MoOS Mo02S 2> MoO S have also been prepared (17). 

Transition metal complexes containing bidentate phosphine ligands. W. Levason and C. A. McAuliffe, Adv. Inorg. Chem. Radiochem., 1972,14,172-253 (315). 

Transition Metal Complexes Containing Bidentate Phosphine Ligands W. Levason and C. A. McAuliffe... 

Dioximes are widely used bidentate ligands for the complexation of transition metal ions with the characteristic property to form complexes with a pseudo-macrocyclic structure if two or more of these ligands are chelating the same metal ion (119, Fig. 31) [163]. 

Extending the same concept of a planar chiral nucleophilic or basic heterocyclic Fe-sandwich complex, aza-ferrocenes 65 were prepared. The latter have also been successfully applied as bidentate ligands in transition metal catalysis [85]. 

On the other hand, the enantioselective 1,4-addition of carbanions such as enolates to linear enones is an interesting challenge, since relatively few efficient methods exist for these transformations. The Michael reaction of p-dicarbonyl compounds with a,p-unsaturated ketones can be catalysed by a number of transition-metal compounds. The asymmetric version of this reaction has been performed using chiral diol, diamine, and diphosphine ligands. In the past few years, bidentate and polydentate thioethers have begun to be considered as chiral ligands for this reaction. As an example, Christoffers et al. have developed the synthesis of several S/O-bidentate and S/O/S-tridentate thioether... 

Palladium(II) complexes possessing bidentate ligands are known to efficiently catalyze the copolymerization of olefins with carbon monoxide to form polyketones.594-596 Sulfur dioxide is an attractive monomer for catalytic copolymerizations with olefins since S02, like CO, is known to undergo facile insertion reactions into a variety of transition metal-alkyl bonds. Indeed, Drent has patented alternating copolymerization of ethylene with S02 using various palladium(II) complexes.597 In 1998, Sen and coworkers also reported that [(dppp)PdMe(NCMe)]BF4 was an effective catalyst for the copolymerization of S02 with ethylene, propylene, and cyclopentene.598 There is a report of the insertion reactions of S02 into PdII-methyl bonds and the attempted spectroscopic detection of the copolymerization of ethylene and S02.599... 

Monodentate (monometallic monoconnective) phosphor-1,1-dithiolato ligands are rare. Bidentate (monometallic biconnective) form chelate rings and three sub-types can be distinguished according to the degree of asymmetry (Scheme 2). The most asymmetric type (anisobidentate) occurs when a covalent bond is associated with a secondary bond this takes place mostly in main-group metal complexes. The second type is rare and is the result of the association between a covalent and a dative coordinate bond. The symmetric bidentate bonding (isobidentate) is found mainly in transition metal complexes. 

Distibines react as monodentate (type 5) or bridging bidentate (type 6) ligands through donation of the lone pairs of electrons to main group or transition metal centers. Fission of the Sb-Sb bond leads to complexes with bridging R2Sb ligands (type 7, 8 and 9) (Scheme 3). 

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