Chromium oxyhalides

The two chromium oxyhalides are characterized by the stability of the Cr ion. The chloride gives Eo(CrOCl) = —1340 mV and the bromide, E(,(CrOBr) = —940 mV. As can be predicted, the Cl" ions stabilize the higher valency of chromium than the Br ions more effectively, as reflected by the more negative potential of reduction. No reoxidation peak appears, indicating that the electrochemical process is irreversible in the solvent used. 

Halide and oxyhalide complexes of elements of the titanium, vanadium and chromium sub-groups. 

Chromium(vi) Complexes.—These studies have been virtually confined to oxyhalide and oxide systems. 

Oxyhalides are another type of volatile inorganic precursors, which have been used in only a few studies, however. For instance, timgsten oxyfluo-ride (WO F ) and H2O have been used as precursors in the deposition of WO3 [48], while Cr02Cl2 together with CH3OH as an oxygen source have been used in the deposition of chromium oxide [49]. 

Ziegler-Natta Catalysts (Heterogeneous). These systems consist of a combination of a transition metal compound from groups IV to VIII and an organometallic compound of a group I—III metal.23 The transition metal compound is called the catalyst and the organometallic compound the cocatalyst. Typically the catalyst is a halide or oxyhalide of titanium, chromium, vanadium, zirconium, or molybdenum. The cocatalyst is often an alkyl, aryl, or halide of aluminum, lithium, zinc, tin, cadmium, magnesium, or beryllium.24 One of the most important catalyst systems is the titanium trihalides or tetra-halides combined with a trialkylaluminum compound. 

Exactly what is the analytical problem and what is the minimum analytical information needed to provide a reasonable answer In this connection it is well to categorize the type of analysis desired oxyhalides in drinking water, arsenic speciation in drinking water, speciation of chromium in plating baths, etc. 

Most commonly, the catalyst component consists of halides or oxyhalides of titanium, vanadium, chromium, molybdenum, or zirconium, and the cocatalyst component often consists of an alkyl, aryl, or hydride of metals such as aluminum, lithium, zinc, tin, cadmium, beryllium, and magnesium. The catalyst systems may be heterogeneous (some titanium-based systems) or soluble (most vanadium-containing species). Perhaps the best known systems are those derived from TiCl4 or TiCls and an aluminum trialkyl. 

Aside from CrFs, there is the hexafluorochromate(V) ion in the brick-red complexes Cs[CrF0] and NO2[CrF0]. The oxyhalides CrOXs (X = F, Cl) form a number of adducts with halide ions, namely, oxotetrahalochromates(V) of the composition [CrOX4] and oxopentahalochromates(V) of the type [CrOXs]The crystal structure of AsPh4[CrOCl4 shows the presence of five-coordinate chromium in a square pyramidal arrangement (Cr-0, 151.9 pm Cr-Cl, 224.0 pm). CrOCls also adds amines, for example, to give [CrOCls (bipy)]. 

Environmental directives and legislation about elemental speciation have been limited. Therefore analytical laboratories have often lacked the incentive to invest in the necessary technologies to perform chemical spedation. However, this situation is changing, and methods in effed include EPA Method 6800 for spedation of chromium, EPA Method 314 for perchlorate, EPA Method 317 for oxyhalide anions, EPA Method 8323 to detect organotin compounds, and several others. 

Another catalyst system found in the patent literature involves the deposition of halides such as zirconium tetrachloride, vanadium tetrachloride, titanium tetraiodide, or oxyhalides such as chromium oxychloride or vanadium oxychloride on a finely divided particulate inorganic substrate having surface hydroxyl groups. Among such solids are alumina, zirconia, silica (particularly a pyrogenic silica such as Cab-O-SiF ), or a carbon black such as channel black or furnace black. A toluene slurry of this material is added, under dry nitrogen, to a toluene solution of A -vinyl-pyrrolidone containing a small amount of triisobutylaluminum. After 24 hr at 80°C, a 25% yield of polymer is produced [73]. 

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