Transition 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... 

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-... 

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... 

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... 

Pincer ligands, that is, tridentate Hgands that enforce meridional geometry upon complexation to transition metals, result in pincer complexes which possess a unique balance of stability versus reactivity [3]. Transition-metal complexes of bulky, electron-rich pincer ligands have found important appHcations in synthesis, bond activation, and catalysis [4, 5]. Among these, pincer complexes of Pr-PNP (2,6-bis-(di-iso-propylphosphinomethyl)pyridine), Bu-PNP (2,6-bis-(di-terPbutyl-phosphinomethyl)pyridine), and PNN ((2-(di-tert-butylphosphinomethyl)-6-diethyl-aminomethyl)pyridine), PNN-BPy (6-di-tert-butylphosphinomethyl-2,2 -bipyridine) ligands exhibit diverse reactivity [6-8]. These bulky, electron-rich pincer ligands can stabilize coordinatively unsaturated complexes and participate in unusual bond activation and catalytic processes. 

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). ... 

Pd-cataly2ed reactions of butadiene are different from those catalyzed by other transition metal complexes. Unlike Ni(0) catalysts, neither the well known cyclodimerization nor cyclotrimerization to form COD or CDT[1,2] takes place with Pd(0) catalysts. Pd(0) complexes catalyze two important reactions of conjugated dienes[3,4]. The first type is linear dimerization. The most characteristic and useful reaction of butadiene catalyzed by Pd(0) is dimerization with incorporation of nucleophiles. The bis-rr-allylpalladium complex 3 is believed to be an intermediate of 1,3,7-octatriene (7j and telomers 5 and 6[5,6]. The complex 3 is the resonance form of 2,5-divinylpalladacyclopentane (1) and pallada-3,7-cyclononadiene (2) formed by the oxidative cyclization of butadiene. The second reaction characteristic of Pd is the co-cyclization of butadiene with C = 0 bonds of aldehydes[7-9] and CO jlO] and C = N bonds of Schiff bases[ll] and isocyanate[12] to form the six-membered heterocyclic compounds 9 with two vinyl groups. The cyclization is explained by the insertion of these unsaturated bonds into the complex 1 to generate 8 and its reductive elimination to give 9. 

With an atomic number of 28 nickel has the electron conflguration [Ar]4s 3c (ten valence electrons) The 18 electron rule is satisfied by adding to these ten the eight elec Irons from four carbon monoxide ligands A useful point to remember about the 18 electron rule when we discuss some reactions of transition metal complexes is that if the number is less than 18 the metal is considered coordinatively unsaturated and can accept additional ligands... 

Despite the fact that transition metal complexes have found wide application in the synthesis of carbo- and heterocycles, [3+3] cyclisation reactions mediated or assisted by transition metals remain almost unexplored [3, 86]. However, a few examples involving Fischer carbene complexes have been reported. In all cases, this complex is a,/J-unsaturated in order to act as a C3-synthon and it reacts with different types of substrates acting as C3-synthons as well. 

All mechanisms proposed in Scheme 7 start from the common hypotheses that the coordinatively unsaturated Cr(II) site initially adsorbs one, two, or three ethylene molecules via a coordinative d-7r bond (left column in Scheme 7). Supporting considerations about the possibility of coordinating up to three ethylene molecules come from Zecchina et al. [118], who recently showed that Cr(II) is able to adsorb and trimerize acetylene, giving benzene. Concerning the oxidation state of the active chromium sites, it is important to notice that, although the Cr(II) form of the catalyst can be considered as active , in all the proposed reactions the metal formally becomes Cr(IV) as it is converted into the active site. These hypotheses are supported by studies of the interaction of molecular transition metal complexes with ethylene [119,120]. Groppo et al. [66] have recently reported that the XANES feature at 5996 eV typical of Cr(II) species is progressively eroded upon in situ ethylene polymerization. 

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