Chromium 2 structure

Chiral Catalysts Containing Group 6 Metals (Cr, Mo, and W). Although all the three metals have important role in organometallic chemistry (eg, carbonyl complexes), their catalytic properties are scarcely investigated in the past few years. The tricarbonyl(jj -arene)chromium structural unit (see Fig. 16)... 

Fig. 20. Chromium structural changes in CrAPO-5 during calcination.

The most prominent structural parameters in 42 and 43 are the Fe-Fe and Co-Co distances which present an even greater contrast to the chromium structure. The Fe-Fe (2.515(9)A) and Co-Co (2.8033(5)A) separations greatly exceed the... 

At the other end of the diagram (nearly pure chromium) and at 7(X) °C, only a small amount of nickel can be substituted into the body-centered cubic structure of the chromium. As the temperature rises to 1300 °C, however, about 20 mol % nickel can be acconunodated into the chromium structure. The region in between the two phases is called a two-phase r on. At these compositions, the two phases (nickel-rich face-centered cubic and chromium-rich body-ceutered cubic) exist together. The amount of each phase depends upon the composition of the alloy. 

As already described above it is possible to generate free-standing structures using a structured chromium layer on fused silica glass mask blank. This continuous blank as substrate for the chromium structures guarantees the holding together. However, this is not possible if a low cost mask, such as a structured metal foil or a machined brass sheet, is used. 

Chromium dioxide. Cr02 (HjO plus O2 on Cr03 at high temperature). Black solid with the rutile structure forming chromates(IV) in solid stale reactions. Used in magnetic lap>es. 

Cheng H S and Wang L S 1996 Dimer growth, structure transition and antiferromagnetic ordering in small chromium clusters Phys. Rev. Lett. 77 51... 

Veliah S, Xiang K H, Pandey R, Redo J M and Newsam J M 1998 Density functional study of chromium oxide dusters structure, bonding, vibrations, and stability Phys. Rev. B 102 1126... 

The chromates of the alkali metals and of magnesium and calcium are soluble in water the other chromates are insoluble. The chromate ion is yellow, but some insoluble chromates are red (for example silver chromate, Ag2Cr04). Chromates are often isomorph-ous with sulphates, which suggests that the chromate ion, CrO has a tetrahedral structure similar to that of the sulphate ion, SO4 Chromates may be prepared by oxidising chromium(III) salts the oxidation can be carried out by fusion with sodium peroxide, or by adding sodium peroxide to a solution of the chromium(IIl) salt. The use of sodium peroxide ensures an alkaline solution otherwise, under acid conditions, the chromate ion is converted into the orange-coloured dichromate ion ... 

Chromium forms a white solid, hexacarhonyl, Cr(CO)j, with the chromium in formal oxidation state 0 the structure is octahedral, and if each CO molecule donates two electrons, the chromium attains the noble gas structure. Many complexes are known where one or more of the carbon monoxide ligands are replaced by other groups of ions, for example [CrfCOlsI] . 

A chromium atom forms a neutral complex with carbon monoxide molecules and 1,10-phenanthroline molecules. The structure of the complex is ... 

A large number of organometallic compounds are based on transition metals Examples include organic derivatives of iron nickel chromium platinum and rhodium Many important industrial processes are catalyzed by transition metals or their complexes Before we look at these processes a few words about the structures of transition metal complexes are m order... 

UNS C 81500 (chromium—copper alloy) is used structurally where strength and hardness are required and UNS C 81700 (beryllium—copper alloy) is used stmcturaEy where high strength and hardness are required. 

Again, as with pyridopyrimidines, the main reaction is oxidation of di- or poly-hydro derivatives to fully aromatic structures, often merely by air or oxygen. In some cases the reagent of choice is mercury(II) oxide, whilst other reagents used include sulfur, bromine, chloranil, chromium trioxide-acetic acid, hydrogen peroxide, and potassium ferricyanide, which also caused oxidative removal of a benzyl group in the transformation (306) (307)... 

Carbon steels heated for prolonged periods at temperatures above 455°C (8.50°F) may be subject to the segregation of carbon, which is transformed into graphite. When this occurs, the structural strength of the steel will be affected. Killed steels or low-alloy steels of chromium and molybdenum or chromium and nickel should be considered for elevated-temperature seivices. 

Metal atoms tend to behave like miniature ball-bearings and tend to pack together as tightly as possible. F.c.c. and c.p.h. give the highest possible packing density, with 74% of the volume of the metal taken up by the atomic spheres. However, in some metals, like iron or chromium, the metallic bond has some directionality and this makes the atoms pack into the more open b.c.c. structure with a packing density of 68%. 

Poor Weldability a. Underbead cracking, high hardness in heat-affected zone. b. Sensitization of nonstabilized austenitic stainless steels. a. Any welded structure. b. Same a. Steel with high carbon equivalents (3), sufficiently high alloy contents. b. Nonstabilized austenitic steels are subject to sensitization. a. High carbon equivalents (3), alloy contents, segregations of carbon and alloys. b. Precipitation of chromium carbides in grain boundaries and depletion of Cr in adjacent areas. a. Use steels with acceptable carbon equivalents (3) preheat and postheat when necessary stress relieve the unit b. Use stabilized austenitic or ELC stainless steels. 

It is known that thin (-20 A) passive films form on iron, nickel, chromium, and other metals. In s ressive environments, these films provide excellent corrosion protection to the underlying metal. The structure and composition of passive films on iron have been investigated through iron K-edge EXAFS obtained under a variety of conditions, yet there is still some controversy about the exact nature of... 

If a sample of polycrystalline material is rotated during the sputtering process, the individual grains will be sputtered from multiple directions and nonuniform removal of material can be prevented. This technique has been successfully used in AES analysis to characterize several materials, including metal films. Figure 9 indicates the improvement in depth resolution obtained in an AES profile of five cycles of nickel and chromium layers on silicon. Each layer is about 50 nm thick, except for a thinner nickel layer at the surface, and the total structure thickness is about 0.5 pm. There can be a problem if the surface is rough and the analysis area is small (less than 0.1-pm diameter), as is typical for AES. In this case the area of interest can rotate on and off of a specific feature and the profile will be jagged. 

Stainless and heat-resisting steels containing at least 18% by weight chromium and 8% nickel are in widespread use in industry. The structure of these steels is changed from magnetic body centered cubic or ferritic crystal structure to a nonmagnetic, face-centered cubic or austenitic crystal structure. 

Type 304-19/10 (chromium nickel) provides a stable austenitic structure under all conditions of fabrication. Carbon (0.08% max.) is sufficient to have reasonable corrosion resistance without subsequent corrosion resistance for welded joints. Type 304 is used for food, dairy and brewery equipment, and for chemical plants of moderate corrosive duties. 

An excellent alternative route to 5,6-seco-acids of general structure (69) has been used by Knof to prepare B-norsteroids. Although the single-step chromium trioxide oxidation process described in section III-A is convenient when starting materials are readily available and relatively low yields are acceptable, the multistep Knof process may be preferred when high yields are important or when a sensitive functionality is present. 

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