Nickel sandwich compound

TC-Cyclopentadienyl Nickel Complexes. Nickel bromide dimethoxyethane [29823-39-9] forms bis(cydopentadienyl)nickel [1271 -28-9] upon reaction with sodium cyclopentadienide (63). This complex, known as nickelocene, 7T-(C3H3)2Ni, is an emerald-green crystalline sandwich compound, mp 173°C, density 1.47 g/cm. It is paramagnetic and slowly oxidi2es in air. A number of derivatives of nickelocene are known, eg, methylnickelocene [1292-95-4], which is green and has mp 37°C, and bis( 7t-indenyl)nickel [52409-46-8], which is red, mp 150°C (87,88). 

The sandwich compounds, feiTocene and nickelocene, FefCg 115)2 and Ni(C5H5)2, so called because die metal atom is sandwiched between the organic radicals, C5H5, have also been used to prepare iron and nickel deposits. 

The removal of metals with the asphaltenic sulfur is observed in Figure 4. This response is consistent with an asphaltene model in which vanadium and nickel are buried as porphyrins or sandwich compounds (9). The slightly higher removal of vanadium reflects a general tendency for vanadium to deposit on the catalyst more readily than nickel. 

On the other hand, there are indications that our expectations must not be set too high. A great variety of chemically extraordinary interesting transition metal organometallics has been prepared during the past decades. I think in particular of the types known as "sandwich compounds", exemplified by ferrocene, bisbenzenechromium, bis 7-allyl nickel and cyclobutadienemetal complexes ... 

T. Kuhlmann, S. Roth, J. Roziere, and W. Siebert, Polymeric ( j5, j,-2,3-Dihydro-l, 3-diborolyl)nickel, the First Multilayer Sandwich Compound, Angew. Chem. Int. Ed. Engl. 25, 105-106 (1986). 

D. J. Brauer, and C. Kruger, The Stereochemistry of Transition Metal Cyclooctatetrae-nyl Complexes Di-h3,h3 -cyclooctatetraenedinickel, a Sandwich Compound with Two Enveloped Nickel Atoms, J. Organomet. Chem. 122, 265-273 (1976). 

In the valence shell electron count for the complex (Ti -C3Ph3)NiCl(py)2 (py = pyridine) (Fig. 4.1), the cyclopropenyl hgand C3Ph3 supplies three electrons, the chlorine atom one electron, and each pyridine ligand supplies two electrons. Added to the 10 electrons in the valence shell of the nickel atom, this generates a total of 18 electrons. In the case of the sandwich compound dibenzenechromium, (q -C/H jiCr (Fig. 4.2),six electrons from each benzene ligand may be added to the six electrons in the metal valence shell to generate the total of 18 electrons. 

Both CgB2 and CgB -based systems have been used to construct multidecKer sandwich compounds.Reaction of bls(allyl) nickel and... 

Co-condensation reaction of the vapors of l,3-di-rcrt-butylimidazol-2-ylidene and nickel, palladium, or platinum gives the coordinatively unsaturated 14-electron sandwiches [L M] (M=Ni, Pd, Pt) of the carbene type (990M3228). Palladium(O) carbene complexes can also be prepared by the direct interaction of l,3-R2-imidazol-2-ylidenes (R=/-Pr, r-Bu, Cy, Mes) (L) with the palladium(O) compound [Pd(P(o-Tol)3)2] (OOJOM(595)186), and the product at the first stage is [(L)PdP(o-Tol)3l, and then in excess free carbene [PdL ]. 

Hydrated nickel(II) salts in the presence of the ligand p3 react with both white phosphorus and yellow arsenic, breaking up the structure of the tetraatomic P4 and As4 molecules to form triatomic cyclo-triphosphorus and cyc/o-triarsenic species. These fragments act as trihapto ligands yielding double sandwich complexes (equation 99).292 The compounds are air-stable both in the solid state and in solution. Upon reduction with NaBH the complexes of equation... 

In a sense the tr-allyl compounds of the transition metals can be regarded as the simplest of the sandwich molecules. Bis(jr-allyl)nickel, the best known of such complexes, has been shown by x-ray crystallography (104,105) to have a staggered arrangement of tr-allyl moieties and hence a C2h molecular conformation. The electronic structure of the ground state of bis(jr-allyl)nickel has been investigated by both semiempirical (47) and ab initio (274,275) methods, and a semiempirical computation has been performed on bis(7r-allyl)palladium (47). 

Ferrocene is only one of a large number of compounds of transition metals with the cyclopentadienyl anion. Other metals that form sandwich-type structures similar to ferrocene include nickel, titanium, cobalt, ruthenium, zirconium, and osmium. The stability of metallocenes varies greatly with the metal and its oxidation state ferrocene, ruthenocene, and osmocene are particularly stable because in each the metal achieves the electronic configuration of an inert gas. Almost the ultimate in resistance to oxidative attack is reached in (C5H5)2Co , cobalticinium ion, which can be recovered from boiling aqua regia (a mixture of concentrated nitric and hydrochloric acids named for its ability to dissolve platinum and gold). In cobalticinium ion, the metal has the 18 outer-shell electrons characteristic of krypton. 

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