Transition metal complexes tungsten

Although trialkyl- and triarylbismuthines are much weaker donors than the corresponding phosphoms, arsenic, and antimony compounds, they have nevertheless been employed to a considerable extent as ligands in transition metal complexes. The metals coordinated to the bismuth in these complexes include chromium (72—77), cobalt (78,79), iridium (80), iron (77,81,82), manganese (83,84), molybdenum (72,75—77,85—89), nickel (75,79,90,91), niobium (92), rhodium (93,94), silver (95—97), tungsten (72,75—77,87,89), uranium (98), and vanadium (99). The coordination compounds formed from tertiary bismuthines are less stable than those formed from tertiary phosphines, arsines, or stibines. 

Schmidt reaction of ketones, 7, 530 from thienylnitrenes, 4, 820 tautomers, 7, 492 thermal reactions, 7, 503 transition metal complexes reactivity, 7, 28 tungsten complexes, 7, 523 UV spectra, 7, 501 X-ray analysis, 7, 494 1 H-Azepines conformation, 7, 492 cycloaddition reactions, 7, 520, 522 dimerization, 7, 508 H NMR, 7, 495 isomerization, 7, 519 metal complexes, 7, 512 photoaddition reactions with oxygen, 7, 523 protonation, 7, 509 ring contractions, 7, 506 sigmatropic rearrangements, 7, 506 stability, 7, 492 N-substituted mass spectra, 7, 501 rearrangements, 7, 504 synthesis, 7, 536-537... 

Silene-transition metal complexes were proposed by Pannell121 for some iron and tungsten systems, and such species were observed spectroscopically by Wrighton.122,123 Thus intermediates such as 33 have been proposed in the preparation of carbosilane polymers from hydrosilanes,124 both as intermediates in the isotope scrambling observed to occur in similar ruthenium hydride systems125 126 and in the 5N2 addition of alkyllithium species to chlorovinylsilanes.47... 

Transition metal complexes which react with diazoalkanes to yield carbene complexes can be catalysts for diazodecomposition (see Section 4.1). In addition to the requirements mentioned above (free coordination site, electrophi-licity), transition metal complexes can catalyze the decomposition of diazoalkanes if the corresponding carbene complexes are capable of transferring the carbene fragment to a substrate with simultaneous regeneration of the original complex. Metal carbonyls of chromium, iron, cobalt, nickel, molybdenum, and tungsten all catalyze the decomposition of diazomethane [493]. Other related catalysts are (CO)5W=C(OMe)Ph [509], [Cp(CO)2Fe(THF)][BF4] [510,511], and (CO)5Cr(COD) [52,512]. These compounds are sufficiently electrophilic to catalyze the decomposition of weakly nucleophilic, acceptor-substituted diazoalkanes. 

The living polymerization prindples developed using the titanacene metall-acyclobutane catalysts have been extended to ROMP reactions catalyzed by other transition-metal complexes. These new systems include tantalacyclobutane complexes and preformed tungsten(VI) carbene complexes. 

Since the epoxidation step involves no formal change in the oxidation state of the metal catalyst, there is no reason why catalytic activity should be restricted to transition metal complexes. Compounds of nontransition elements which are Lewis acids should also be capable of catalyzing epoxidations. In fact, Se02, which is roughly as acidic as Mo03, catalyzes these reactions.433 It is, however, significantly less active than molybdenum, tungsten, and titanium catalysts. Similarly, boron compounds catalyze these reactions but they are much less effective than molybdenum catalysts 437,438 The low activity of other metal catalysts, such as Th(IV) and Zr(IV) (which are weak oxidants) is attributable to their weak Lewis acidity. 

Section V describes the chemistry of 7r-allyl halocarbonyl complexes, mainly of the type [MX(CO)2(NCMe)2(i73-allyl)] and their derivatives. In 1995, Brisdon and Walton8 reviewed transition-metal complexes containing butadienyl ligands their review contains relevant material. Section VI is concerned with the relatively unexplored area of 7j4-diene halocarbonyl complexes of molybdenum(ll) and tungsten(II). 

I should like to propose that all the types of reactions which have been established for mononuclear transition metal complexes will also occur for dinuclear transition metal complexes, and furthermore, that the latter will show additional modes of reactivity which are uniquely associated with the metal-metal bond. In this article, I shall support this proposal by illustrations taken from the reactions of dinuclear compounds of molybdenum and tungsten, two elements which enter into extensive dinuclear relationships (J ). 

The design of transition metal complexes capable of C—F bond activation for the functionalization of fluorocarbons has attracted attention recently. It has been known for several years that oxidative addition of an aromatic C—F bond takes place at tungsten(O) to yield stable tungsten(II) metallacycles, the cleaved carbon and fluorine atoms both finishing up bound to the metal centre (Eqn. (2)) [34-36]. 

Bildstein et al. have previously shown that Fc functionalised NHC (annulated and nonan-nulated) can be successfully employed to synthesise a broad range of transition metal complexes [166]. These include palladium, tungsten and mercury. Interestingly, two different species of mercury NHC complexes could be synthesised and structurally characterised (see Figure 4.49), with the dinuclear complex being the precursor for the mononuclear one. 

Cluster Compounds Inorganometallic Compounds Containing Transition Metal Main Group Elements Heterogeneous Catalysis by Metals Oxidation Catalysis by Transition Metal Complexes Polyoxometalates Tungsten Organometallic Chemistry. Tungsten Proteins. 

The metal vapor technique, in which a metal is vaporized from a resistively heated tungsten container under high vacuum and is cocondensed with a potential ligand at -125 to -196°C, had proven useful in the synthesis of a variety of unusual low-valent transition metal complexes (67-71). With lanthanide metals, this method not only has generated low oxidation state species, but it has also provided the opportunity to study zero-valent lanthanide chemistry on an atomic/molecular basis for the first time. These studies have been important in identifying new directions in organolanthanide chemistry. 

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