Chromium compounds bonding

Low Oxidation State Chromium Compounds. Cr(0) compounds are TT-bonded complexes that require electron-rich donor species such as CO and C H to stabilize the low oxidation state. A direct synthesis of Cr(CO)g, from the metal and CO, is not possible. Normally, the preparation requires an anhydrous Cr(III) salt, a reducing agent, an arene compound, carbon monoxide that may or may not be under high pressure, and an inert atmosphere (see Carbonyls). 

Acute and Chronic Toxicity. Although chromium displays nine oxidation states, the low oxidation state compounds, -II to I, all require Special conditions for existence and have very short lifetimes in a normal environment. This is also tme for most organ ochromium compounds, ie, compounds containing Cr—C bonds. Chromium compounds that exhibit stabiUty under the usual ambient conditions are limited to oxidation states II, III, IV, V, and VI. Only Cr(III) and Cr(VI) compounds are produced in large quantities and are accessible to most of the population. Therefore, the toxicology of chromium compounds has been historically limited to these two states, and virtually all of the available information is about compounds of Cr(III) and/or Cr(VI) (59,104). However, there is some indication that Cr(V) may play a role in chromium toxicity (59,105—107). Reference 104 provides an overview and summary of the environmental, biological, and medical effects of chromium and chromium compounds as of the late 1980s. 

Chromium compounds as catalysts, 188 Chromium oxide in catalytic converter, 62 Chromium oxide catalysts, 175-184 formation of active component, 176,177 of Cr-C bonds, 177, 178 propagation centers formation of, 175-178 number of, 197, 198 change in, 183, 184 reduction of active component, 177 Clear Air Act of 1970, 59, 62 Cobalt oxide in catalytic converter, 62 Cocatalysts, 138-141, 152-154 Competitive reactions, 37-43 Copper chromite, oxidation of CO over, 86-88... 

Apparently, there is not much advantage in using bond enthalpy contributions to discuss bonding energetics in a series of similar complexes. As already stated, we could have selected any value for Z)//,°(Cr-CO) + DH (Cr-CO) + Z)//j (Cr-CO) and then derived chromium-arene bond dissociation enthalpies in Cr(CO)3(arene) compounds, all based on the same anchor. The trend would not be affected by our choice. Nevertheless, besides emphasizing that the absolute values so obtained should not be regarded as bond dissociation enthalpies, the bond enthalpy contribution concept attempts to consider a pertinent issue in molecular energetics the transferability of bond enthalpies. 

The strong jt-bonding character in compounds of pentacarbonyl (group-VI metal) (0) with thiocarbene can be seen from the metal-carbon bond distances. For chromium compounds they were found to be ... 

The stoichiometric transfer of allenylidene ligands from one metal fragment to another metal center has been scarcely documented, the only examples known involving the allenylidene transfer from chromium compounds [Cr(=C=C= CR R )(C0)5] (R R = aryl, amino or alkoxy groups) to [W(C0)5(THF)] [9d]. DFT calculations indicate that the reaction proceeds by an associative pathway, the initial reaction step involving the coordination of W(CO)5 to the Cc(=Cp bond of the allenylidene ligand in the chromium precursor. 

Many other unsaturated organic compounds can form 7r complexes with transition metals. A substance that is in some ways analogous to ferrocene is the complex of two benzene molecules with chromium metal, called dibenzene-chromium. The bonding involves zerovalent chromium and the tt electrons of... 

Finally, the a,/ -unsaturated carbene complex may be generated in situ by alkyne insertion into a chromium-carbene bond of a saturated chromium carbene leading to a chromium vinyl carbene (equivalent to intermediate (f )-D in the mechanism of the benzannulation reaction, see Section 8.2.1, Scheme 3), which may undergo subsequent benzannulation with a second equivalent of the alkyne [43a]. This strategy was subsequently applied to the synthesis of (Z)-enediynes and related compounds [43b], and to that of substituted benzofurans (see also Section 8.5) [43c, 43d]. 

Representative data are shown in Fig. 10.5 for aluminum joints bonded with an epoxy film adhesive and a standard chromate-containing primer. Up until recently, standard corrosion-resistant primers contained high levels of solvent, contributing to high levels of volatile organic compounds (VOCs) and chromium compounds, which are considered to be carcinogens. As a result, development programs have been conducted on waterborne adhesive primers that contain low VOC levels and little or no chrome. Data are presented on several of these primers in Tables 10.8 and 10.9. 

Thermolysis of the pentacarbonyl chromium compounds 151 affords tet-racarbonyl chelate complexes by loss of a cis CO group,20 in the same fashion that has been found for ortho-alkoxy phenylcarbene complexes181 and similar products.182 The C=C triple bond of alkoxy(l-alkynyl)carbene complexes 1 is more reactive than that of amino(l-alkynyl)carbene complexes. (CO)5Cr = C(NHMe)—C=CPh was shown to add ethanol very slowly to give (CO)sCr = C(NHMe)-CH=C(OEt)Ph in 79% yield.21... 

Table 7 Cleavage of Double Bonds to Carbonyl Compounds by Hexavalent Chromium Compounds... 

This class of organometaUics originates from chemistry of the chromium-carbon bond developed only within the last two decades. Illustrative of this fact is the observation that a comprehensive reference published in 1982 makes no mention of compounds of chromium(III) bearing both cyclopentadienyl and alkyl ligands. Within this class are... 

Chromium compounds increase the activity of platinum catalysts by increasing the electron densities of the active sites. jhe addition of ferrous sulfate, which promotes the hydrogenation of carbonyl groups, and zinc acetate, which inhibits the hydrogenation of double bonds, to platinum gives a catalyst system capable of effecting the selective hydrogenation of an unsaturated aldehyde to an unsaturated alcohoP - (Eqn. 11.12). ... 

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