Metal complexes, of unsaturated

Transition metal complexes of unsaturated 1,2-dithiolates (metal dithiolenes) have attracted much attention because of their interesting structural and redox properties.169 Molybdenum dithiolene complexes have featured prominently170 in these studies and have special significance following the suggestion171,172 that the molybdenum-containing cofactor of the oxomolybdoen-zymes (Section 36.6.7) incorporates a molybdenum complex of an unsymmetrically substituted alkene-1,2-dithiolate. 

Structural Parameters for Selected Transition Metal Complexes of Unsaturated Molecules 1... 

Recent Advances in the Chemistry of Transition Metal Complexes of Unsaturated Bidentate Sulfur Donor Ligands (Metal Dithienes)... 

Nucleophilic additions to 18-electron, cationic transition metal complexes of unsaturated hydrocarbons have been particularly well studied. Davies, Green, and Mingos have developed guidelines to predict the most kinetically favorable position for nucleophilic... 

While up to 1990 all attempts to isolate a stable N-heterocyclic carbene failed, metal complexes of unsaturated imidazol-2-ylidenes were known as early as 1968. The first complexes of this type were obtained by in situ deprotonation of imidazolium salts using mercury(ll) acetate or dimethylimidazolium hydridopentacarbonylchromate(-II) followed by coordination of the carbene to the metal center (Scheme 1.3). Shortly thereafter, the stabilization of the saturated imidazolin-2-ylidene in a metal complex was described by Lappert who treated electron-rich entetraamines of type 6=6 with coordinatively unsaturated transition metal complexes to obtain complexes with imidazolin-2-ylidene ligands (Scheme 1.3). ... 

It has been observed, however, that the enantioselectivity of reactions of tartrate ester modified allylboronates with metal carbonyl complexes of unsaturated aldehydes are significantly improved compared with the results with the metal-free, uncomplexed aldehydes72. Two such examples involve the (benzaldehyde)tricarbonylchromium complex and the hexacarbonyl(2-... 

Analogously, the SAPC catalyzed hydroformylation reaction was carried out using other water-soluble metal complexes of Pt and Co. Pt complexes in the presence of an Sn co-catalyst underwent hydrolysis of the Pt-Sn bond, which led to lower reaction selectivity. With the corresponding Co catalyst, good hydroformylation selectivities and conversions could be achieved, provided excess phosphine was used. Other authors performed hydrogenation of a,(3-unsaturated aldehydes using SAPC, and Ru and Ir water-soluble complexes. 

The first metal-olefin complex was reported in 1827 by Zeise, but, until a few years ago, only palladium(II), platinum(Il), copper(I), silver(I), and mercury(II) were known to form such complexes (67, 188) and the nature of the bonding was not satisfactorily explained until 1951. However, recent work has shown that complexes of unsaturated hydrocarbons with metals of the vanadium, chromium, manganese, iron, and cobalt subgroups can be prepared when the metals are stabilized in a low-valent state by ligands such as carbon monoxide and the cyclopentadienyl anion. The wide variety of hydrocarbons which form complexes includes olefins, conjugated and nonconjugated polyolefins, cyclic polyolefins, and acetylenes. 

The thermodynamics of the oxidative addition process tends to be favored by increased electron density at the metal centre, hence the focus on later transition metal derivatives. Furthermore, as discussed above, it is believed that M—N n-bonds to the later transition metals are of significance only if the transition metal complex is unsaturated. Saturated late transition metal amides (parent or substimted) often exhibit the so-called n-conflict (see above) so that the nitrogen centre displays no n-bonding to the metal and retains its lone pair character and basicity. 

An important part of the inorganic chemist s contribution has been the preparation and characterization of transition metal complexes with unsaturated boron rings as ligands. They have also settled the geometry of a number of compounds using X-ray methods. 

As noted in the introduction, in contrast to attack by nucleophiles, attack of electrophiles on saturated alkene-, polyene- or polyenyl-metal complexes creates special problems in that normally unstable 16-electron, unsaturated species are formed. To be isolated, these species must be stabilized by intramolecular coordination or via intermolecular addition of a ligand. Nevertheless, as illustrated in this chapter, reactions of significant synthetic utility can be developed with attention to these points. It is likely that this area will see considerable development in the future. In addition to refinement of electrophilic reactions of metal-diene complexes, synthetic applications may evolve from the coupling of carbon electrophiles with electron-rich transition metal complexes of alkenes, alkynes and polyenes, as well as allyl- and dienyl-metal complexes. Sequential addition of electrophiles followed by nucleophiles is also viable to rapidly assemble complex structures. 

Hydrosilylation is by far the most important route for obtaining monomers and other precursors to fluorinated polysiloxanes. Hydrosylilation80 is the addition of silicon hydride moiety across an unsaturated linkage using transition metal complexes of platinum or rhodium such as Speier s catalyst, hexachloroplatinic acid in isopro-... 

Polymerisations and copolymerisations of heterounsaturated and heterocyclic monomers in the presence of coordination catalysts constitute a distinct group of coordination polymerisation processes. Considering the nature of the coordination bond of the a type between the monomer heteroatom (beyond carbon monoxide [60]) and the metal atom, the complexes formed differ essentially from the re complexes of unsaturated hydrocarbon monomers with transition metals. 

Transition-metal complexes of the more highly unsaturated seven-mem-bered allene 1,2,4,6-cycloheptatetraene have been isolated in two forms depending on the metal and its ligands an allene form (329), which is a complex of the parent cycloheptatetraene 327,134 and a carbene form (330/331), which is a complex of the controversial cycloheptatrienylidene 328.4135136 The carbene form corresponds to the allyl cation (320) that was suggested as an intermediate/transition state for fluxionality of 318. Scheme 42 lists all such complexes that have been prepared to date. The allene form is the ground state for all Pt(0) complexes (343, 344, 345, 347)82,83,130 137 138 and one dibenzannelated W(II) complex139 (346), whereas the carbene form is the ground state for all Fe(II)+ complexes (332-336),140-143 all Ru(II)+ complexes (337-340),144 one Pt(II)+ complex (342),137 and one W(0) complex (341).139... 

Al.1.6 Making Metal Complexes of Mono-Olefins and other Unsaturated Organics... 

This review of Ru-based Lewis acids centers on in-situ procedures in which the metal activates a substrate by forming a n-bond to a Lewis basic atom of the reacting substrate. Particular attention will be paid to stereoselective and catalytic reactions. We exclude from this survey the vast area of chemistry of transition metal complexes of jt-bound unsaturated ligands, as details of these are described in other chapters of this book. 

R. Mason and G. Wilkinson Structure and bonding in transition metal complexes of some cyclic unsaturated ligands, pp. 233-239 (15). 

The recent and continuing surge of interest in transition-metal complexes of o ,/3-unsaturated /3-keto amines (/3-amino ketones), especially with respect to their stereochemical properties, has resulted in increased efforts to secure general synthetic procedures for these complexes. Methods of preparation of these complexes have been reviewed recently. Although most of... 

Metal-complex coordination, unsaturation of the metal center, the templateshaped cavity, and the architecture of surface matrix overlayers integrate together... 

Reactions of metal hydrides with other metal complexes can cause formation of a metal-metal bond in several ways. A terminal metal hydride can act as a Lewis base to form a two-center, three-electron bond. This reaction works well with unsaturated or weakly stabilized complexes (see 9.2.5.5) or with some metal cabonyls. This method is used to make mixed-metal complexes of the early transition elements ... 

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