Chromium-organometallic compounds

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... 

Recently some information became available on a new type of highly active one-component ethylene polymerization catalyst. This catalyst is prepared by supporting organometallic compounds of transition metals containing different types of organic ligands [e.g. benzyl compounds of titanium and zirconium 9a, 132), 7r-allyl compounds of various transition metals 8, 9a, 133), 7r-arene 134, 185) and 71-cyclopentadienyl 9, 136) complexes of chromium]. 

Table 26. Selected bond enthalpy contributions, D (M-L) and (M-L) kJ mol-1, in organometallic compounds of chromium, manganese, iron cobalt and nickel and related compounds (An asterisk ( ) denotes the average value in a series)...

The spectrum of the organometallic compound dimesitylene chromium, [C Hj(CHs)3]2Cr, has relatively high amplitude. The location of the principal peak, 18 ev., corresponds to that of divalent chromium. 

There are many examples of organometallic compounds of chromium in oxidation states -II to III. 

Addition of hydrogen to terminate the chains is the most important and widely used industrial process to control molecular weight. Chain transfer with organometallic compounds (cocatalysts), which is basically an alkyl exchange, can take place with titanium-based and chromium-based systems ... 

The Oppenauer oxidation is a common side reaction during the condensation of organometallic compounds with aldehydes and ketones, something that very often comes as a surprise for the unaware chemist. This has been observed in condensations of diverse organometallic species, for example chromium,61 Zr62 and Mg63 organometallics. This side reaction... 

Arenes and heteroarenes which are particularly easy to metalate are tricarbo-nyl( 76-arene)chromium complexes [380, 381], ferrocenes [13, 382, 383], thiophenes [157, 158, 181, 370, 384], furans [370, 385], and most azoles [386-389]. Meta-lated oxazoles, indoles, or furans can, however, be unstable and undergo ring-opening reactions [179, 181, 388]. Pyridines and other six-membered, nitrogen-containing heterocycles can also be lithiated [59, 370, 390-398] or magnesiated [399], but because nucleophilic organometallic compounds readily add to electron-deficient heteroarenes, dimerization can occur, and alkylations of such metalated heteroarenes often require careful optimization of the reaction conditions [368, 400, 401] (Schemes 5.42 and 5.69). 

The works by Volpin and Shur [160] as well as by Shilov [161] on molecular nitrogen fixation via complexing with transition metals indicates nitrogen readily reacting with low-valence organometallic compounds of titanium, chromium, molybdenum, wolfram and iron. 

Strizhkov, B. V., Novikova, G. A., Nepryakhina, A. Gas-chromatographic determination of carbon in chromium metal films obtained by thermal decomposition of organometallic compounds by vacuum deposition. Zavodsk. Lab. 36, 667 (1970).— Anal. Abstr, 20, 3019(1971). 

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. 

Chromium complexes exist in a very wide range of formal oxidation states (-IV to +VI). The extremely low ones are stabilized by jr-acid ligands, notably carbon monoxide these are described in the article on organometallic compounds (see Chromium Organometallic Chemistry). The extensive coordination chemistry of Cr spans coordination numbers 3-7, but is dominated by octahedral complexes. Complexes of... 

---