Chromium, Molybdenum and Tungsten Oxides

Chromium(III) oxide, Cr203, is used as a protective coating on metal surfaces and can be prepared by the chemical vapor deposition of Cr(acac)3 (acac 6) in an oxygen atmosphere [44]. Chromium hexacarbonyl, Cr(CO)6 (9) also can be converted into 

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Studies on the behavior of chromium, molybdenum and tungsten oxide-doped monolithic carbon aerogels in the isomerization of 1-butene have been carried out by the group of Moreno-Castilla [38]. Samples were prepared by polymerization of a resorcinol-formaldehyde mixture which contained metallic salts of the elements and then the polymer was carbonized at both 500 and 1000 °C. The catalyhc performance of the samples was studied at temperatures ranging from 50 to 425 °C. 

In the case of metal-containing PAN the spectral picture is qualitatively the same (as compared with the initial PAN) however, ftie nature of tiie metal carbonyl introduced into the polymer somewhat influences the character of file transformations. By FTIR, ESCA and ESR, it was found that under thermal treatment in air at 220 C, first metal carbonyl complexes decompose (by FTIR spectroscopy tungsten and molybdenum carbonyl complexes lose carbon monoxide at 150-160 C, chromium carbonyl ones at 110°C) and chromium, molybdenum and tungsten oxide particles form. 

Table 23.2 Oxidation states and stereochemistries of compounds of chromium, molybdenum and tungsten...

Adam and Lohray122 have used thianthrene 5-oxide (88) as a mechanistic probe in oxidations with transition metal peroxides. They oxidized 88 with various diperoxo complexes of chromium, molybdenum and tungsten and formulated a plausible mechanism on the basis of the products formed, 89 and 90. 

I.3.4.2.6. Compounds with Unusual Double Bonds 1,3-Dipolar cycloaddition of l-chloro-2-phenyl-2-trimetkylsilyl-l-phosphaethene with nitrile oxides, followed by elimination of Me SiCl, results in 3,5-diphenyl-l,4,2-oxaphosphazole 190 (356). Chromium, molybdenum, and tungsten pentacarbonyls of 3,5-diphenyl-).3-phosphinins react with nitrile oxides to give the corresponding 1,3-dipolar cycloadducts, at the P = C bond, see 191 (Ar = Ph, Mes) (357). 

Molybdenum is in the middle of the triad elements of group 6. These three metals (from periods 4, 5, and 6) are chromium, molybdenum, and tungsten, which, in their pure states, are relatively hard, but not as hard as iron. They are silvery-white as pure metals, and they have similar oxidation states. Their electronegativity is also similar—Cr = 1.6, Mo = 1.8, and W = 1.7—which is related to their reactivity with nonmetals. 

Molybdenum oxidizes at high temperatures but not at room temperatures. It is insoluble in acids and hydroxides at room temperatures. At room temperatures, all three metals (chromium, molybdenum, and tungsten) resist atmospheric corrosion, which is one reason chromium is used to plate other metals. They also resist attacks from acids and strong alkalis, with the exception of chromium, which, unless in very pure form, will dissolve in hydrochloric acid (HCl). 

Their strong reducing nature is a special characteristic of the penta-carbonyl metalates(-II) and decacaibonyl dimetalates(—I) of chromium, molybdenum, and tungsten that distinguishes them from the other carbonyl metalate anions of the 3d metals, e.g., [Co(CO)J. The oxidation of the mononuclear species with water... 

Fe2(CO)9, and Fe3(CO)12, respectively. Similar disproportionations occurred with Ni(CO)4 and Co2(CO)8 which gave anionic species such as [Ni2(CO)6]2, [Ni3(CO)8]2, [Co(CO)4] , etc., upon treatment with ammonia or other amines. In contrast to the carbonyls of iron, nickel and cobalt, those of chromium, molybdenum and tungsten reacted with pyridine and 1,2-ethylenediamine to afford substitution products of the general composition M(CO)6 (py) (n = 1, 2, and 3) and M(CO)4(en) with the metal remaining in the oxidation state zero [25], Mainly as the result of this work, Hieber became convinced that the metal carbonyls should be regarded as true coordination compounds, and the coordinated CO should not be considered a radical but a monodentate ligand like NH3, pyridine, etc. He held this view despite the criticism by several of his contemporaries [3, 19] and was very pleased to see that in most textbooks published after 1940 this view had been accepted. 

G. A. McDermott, A. M. Domes, and A. Mayr, Synthesis of Carbyne Complexes of Chromium, Molybdenum, and Tungsten by Formal Oxide Abstraction from Acyl Ligands, Organometallics 6, 925-931 (1987). 

Methyl-3,5-diphenyl-l//-thiopyran 1-oxide forms red air-stable complexes with carbonyls of chromium, molybdenum and tungsten. (78CB1709). X-Ray diffraction measurements on these compounds confirm the non-planarity of the thiabenzene oxide nucleus and reinforce the evidence for high inversion barriers at sulfur. These complexes are found in isomeric forms with either the sulfur-oxygen bond axial to the half-chair conformation adopted by the ring, or the S- methyl group axial the complexes cannot be interconverted. 

In 1872 Mendeleeft pointed out that there was no place in the Periodic Table for a trivalent element of atomic weight 120, and drew attention to the similarity of uranium to chromium, molybdenum, and tungsten he therefore suggested that the atomic weight should be doubled, so that uranium could be placed below these elements in the table. He also formulated the oxides, by analog with those of the other elements in the group, as follows uranous oxide UOg, urano-... 

An efficient method for the formal abstraction of oxide from acyl ligands was developed in our laboratory 44). Oxalyl halides react directly with the pentacarbonylmetal acyl complexes of chromium, molybdenum, and tungsten to form the fra ,s-alkylidyne(halo)tetracarbonyl complexes [Eq. (5)]. Other suitable Lewis acids are COCI2, CI3COCI, CI3COCOCI, and... 

Chromium, molybdenum, and tungsten are the transition metals in Group 6 of the periodic table. Chromium metal has the outer electronic configuration. W54,yl and forms compounds in oxidation states II- to VI+.1 The extensive organometallic chemistry of Cr is covered in the accompanying Comprehensive Organometallic Chemistry series, and this review is generally restricted to the coordination complexes (oxidation states 0 to VI+). 

When the donor character of the amino substituent at the transition-metal-carbene carbon atom is reduced, it should be possible to influence the thermal stability and reactivity in favor of the desired cyclopropanation process. Indeed, pyrrol-1 -ylcarbene complexes 18 of chromium, molybdenum and tungsten do exhibit the desired reactivity. In the last step, the pyrrole ring of 19 can be converted to the NH2 function in 20 after oxidative cleavage with ozone.In this respect, the pyrrole heterocycle represents a synthetic equivalent of the amino function. 

Epoxidation with hydroperoxides is the basis for the large-scale indirect production of propylene oxide by a process that has been called the Oxirane or Halcon processes. Early work was reported by Smith in a patent issued in 1956 [457], which described soluble heteropoly acids containing transition metals such as chromium, molybdenum, and tungsten that could be employed as homogeneous catalysts for the reaction of olefins with organic hydroperoxides and hydrogen peroxide. 

Reaction of 1 with the metal carbonyls of chromium, molybdenum, and tungsten in dimethylformamide (DMF) gives air-stable, binuclear complexes of t5q>e [(COlsMLMCCOls] (9). These proved to be useful starting species for generating (via oxidative decarbonylation reac-... 

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