Of chromium and tungsten

The lithiated Cr(CO)3 derivatives of thiophene may react with organometal-lic species. Their interaction with hexacarbonyls of chromium and tungsten... 

Iminocarbene complexes of chromium and tungsten are useful isolable synthetic equivalents to nitrile ylides having the advantage that the range of 1,3-dipo-larophiles is not limited to electron-acceptor substrates and can be extended to electronically neutral as well as to electron-rich systems [56] (Scheme 18). 

The potential of Fischer carbene complexes in the construction of complex structures from simple starting materials is nicely reflected in the next example. Thus, the reaction of alkenylcarbene complexes of chromium and tungsten with cyclopentanone and cyclohexanone enamines allows the di-astereo- and enantioselective synthesis of functionalised bicyclo[3.2.1]octane and bicyclo[3.3.1]nonane derivatives [12] (Scheme 44). The mechanism of this transformation is initiated by a 1,4-addition of the C -enamine to the alkenylcarbene complex. Further 1,2-addition of the of the newly formed enamine to the carbene carbon leads to a metalate intermediate which can... 

As in the case of chromium and tungsten, manganese carbonyl adducts of Me2SO have been used as catalysts for the polymerization of vinyl chloride (248). Preparative studies have allowed the isolation of complexes of the type [MnCCpHjMeXCOlXRaSO)] [RjSO = (CH2)4SO, (CH20)2S0 see ref. 255], and infrared (257) and mass spectral studies (154, 275) have appeared on these and related systems. 

The route could be extended to the syntheses of selenoketone complexes of chromium and tungsten (43) from the corresponding diarylcarbene complexes (e.g., 39 R = H, Me, OMe, Br, CF3) and PhNCSe. Later on, the use of tetraethylammonium or potassium salts of [ECN]- as the heteroatom sources turned out to be superior to R NCE.175 Thus it was even possible to synthesize a pentacarbonyl tellurobenzophenone complex of tungsten (44) from K+[TeCN]" and 39 (R = H).176... 

Fig. IO. Carbonyl complexes of chromium and tungsten with large y.

The results obtained indicate that with the possible exception of chromium and tungsten carbide, none of the materials studied is comparable to vitreous carbon. There appears to be no reason why chromium cannot be used in place of platinum for measurements in the near-positive potential range the voltam-mograms that are obtained are well defined and reproducible. Despite having... 

Cycloadditions of alkynyl carbenes. Alkynyl carbene complexes of chromium and tungsten undergo [2 + 2]cycloaddition at room temperature with a wide range of enol ethers. 

H. Fischer, and N. Szesni, 7i-Donor-Substituted Metallacumulenes of Chromium and Tungsten, Coord. Chem. Rev. 248, 1659-1677 (2004). 

The electrochemical oxidations of chromium and tungsten tricarbonyl complexes, respectively, were studied. 

The chemistry of (l-alkynyl)carbene iron complexes is different from that of chromium and tungsten compounds. [4+2] cycloaddition of cyclopentadiene to a (l-alkynyl)carbene iron complex 11,m (R = SiMe3, r-Bu, c-QHn, n-Pr, Ph) affords (l-alkenyl)carbene Fe(CO)4 complexes 55, but these are readily isomerized at 50°C to Fe(CO)2 complexes 57 by insertion of carbon monoxide into the Fe = C bond of an intermediate Fe(CO)3... 

The mass spectra of bimetallic carbonyl metal compounds with cyclic arsine ligands have been discussed (Table 10). Molecular ion peaks are present for the pentamethyl-cyclopentaarsine containing complexes of chromium and tungsten, (AsMe)5[M(CO)]2 (186,188). Their decomposition includes CO and/or M(CO)5 loss giving rise to the ions (AsMe)sM2(CO) + (n = 0-9), (AsMe)sM(CO)/ (n = 0-5), M As Me-" and MAs Me+ (m = 2-4), MjAs Me, M AsMe -" (m = 2-4), MAs (w = 2-5), MAsjCH and AsMOj The latter is the most abundant in the mass spectra . ... 

Molybdenum, like the common a-forms of chromium and tungsten, has a b.c.c. structure. In addition there are an h.c.p. j3-Cr and a f.c.c. / -W. The metals are silvery-white and rather soft when pure, but their alloys can be particularly hard, e.g. stellite which contains Co, Cr and W. The m.p. are very high, every metal being at the peak of its own transition series and the m.p. of tungsten being consequently higher than that of any other metal. 

Similarly, benzylidene(pentacarbonyl) complexes of chromium and tungsten react with cyclo-pentadiene, by stereoselective transfer of the benzylidene moiety to one of the C-C double bonds of the diene, to form coordinated e do-6-phenylbicyclo[3.1.0]hex-2-ene 9 with the metal-ligand unit complexing from the exo side. This method was applied to several other dienes. Reaction with cyclohexa-1,3-diene gave the corresponding cndo-7-phenylbicyclo[4.1.0]hept-2-ene complex. The cyclopropane-containing ligands obtained in this way were liberated from the complex to form the free hydrocarbon system. 

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