Group VIII/18 elements compounds

These attempts to synthesize five-membered ring system failed due to the low reactivity of the CH2C1 group or the formation of unsuitable isomeric trans compounds. The addition of the SiH group to a C=CH group occurs as a cis addition when catalysts of group-VIII elements are present37) a trans olefine subsequently formed ... 

Catalytic Properties of Group VIII Elements and Their Compounds... 

Addition to the cyclopentadienyl ring or to the metal atom is a frequently used synthetic method, especially for the compounds of Group VIII elements. An example of an addition reaction (94) is... 

The majority of the RXj compounds crystallize with the AuCus type, which is widespread in the periodic system. The PuNis and CeNis structures are characteristic of the compounds with the group VIII elements, while the J AU3 compounds crystallize with the TiCus type which is a deformation of the NisSn type (Schubert, 1964). 

X-Ray studies confirm that platinum crystallites exist on carbon supports at least down to a metal content of about 0.03% (2). On the other hand, it has been claimed that nickel crystallites do not exist in nickel/carbon catalysts (50). This requires verification, but it does draw attention to the fact that carbon is not inert toward many metals which can form carbides or intercalation compounds with graphite. In general, it is only with the noble group VIII metals that one can feel reasonably confident that a substantial amount of the metal will be retained on the carbon surface in its elemental form. Judging from Moss s (35) electron micrographs of a reduced 5% platinum charcoal catalyst, the platinum crystallites appear to be at least as finely dispersed on charcoal as on silica or alumina, or possibly more so, but both platinum and palladium (51) supported on carbon appear to be very sensitive to sintering. 

The first published examples of hydrosilation, which appeared about 30 years ago, noted that they were observed to proceed by free-radical mechanisms initiated thermally (about 300°C) (J), by acyl peroxides (4), by azonitriles (5), by ultraviolet light (6), or by y radiation (7). The first hint that catalysts known to be effective for hydrogenation might also be effective for hydrosilation was found in a French patent (8) (1949) which stated that catalysts may be chosen from compounds and salts of the elements of Groups IIA, IVA, IB, and IIB of the periodic table and metals of Group VIII and certain of their salts. No example to demonstrate this was included in the patent. 

Although iron, cobalt, and nickel occur in the same triad in Group VIII., the three elements differ considerably in their ability to form addition compounds with ammonia. Iron forms few ammino-salts, most of which are unstable, and its tendency to complex-salt formation of the ammine type appears in the complex cyanides and not in the ammines themselves. 

Cobalt. The speciation of radiocobalt has been selected for discussion in this chapter because it exemplifies an element for which much information already exists regarding its stable chemical speciation, yet there are additional species which have become environmentally important as a result of the activities of the nuclear industry Cobalt, the middle member of the first triad of group VIII transition metals in the Periodic Table (iron, cobalt, nickel), is most stable in the divalent state when in simple compounds. Studies of radionuclide releases from nuclear power plants under tropical conditions in India seem to indicate that... 

We shall discuss the structures of intermetallic compounds in terms of the groups Ai, Aj, Bi, and B2 (see above), of which Aj and A2 comprise the A subgroup elements together with the Group VIII triads and Cu, Ag, and Au, while Bj and B2 include the more metallic B subgroup elements apart from those of IB. The reason for placing these latter elements with the transition metals rather than with the B subgroup elements will be apparent later. 

Risse and S. Breunig, Transition metal catalyzed vinyl addition polymerizations of norbor nene derivatives with ester groups, Makromol. Chem. 193, 2915 (1992) C. Mehler and M. Risse, Addition polymerization of norbornene catalyzed by palladium(2- -) compounds. A polymerization reaction with rare chain transfer and chain termination, Macromol. 25, 4226 4228 (1992) R.G. Schulz, Polym. Lett. 4, 541 (1966). C. Tanielian, A. Kiennemann, and T. Osparpucu, Influence de differents catalyseurs abase d elements de transition du groupe VIII sur lapol3mierisation du norbor nene, Can. J. Chem. 57, 2022 (1979) A. Sen and T. W. Lai, Catalytic polymerization of acetylenes and olefins by tetrakis(acetonitrile)palladium(II) ditetrafluoroborate, Organometallics 1, 415 (1982) C. Mehler and W. Risse, Pd(II) catalyzed polymerization of norbornene derivatives, Mak romol. Chem. Rapid Commun. 12, 255 (1991). 

It is customary in most chemical formulae of inorganic compounds (Chapter 2), to write metallic atoms first (if there are any), e.g. NaCl, sodium chloride, or common salt. In the Periodic Table, the metallic elements extend from the left almost three-quarters of the way across the table all the A groups, groups VIII, IB, IIB and the lower parts of groups IIIB-VIB. In spite of this, the non-metallic elements have the larger role to play in chemistry, as we shall see (Chapters 3-16). 

Ziegler polymerizations occur mostly in olefins and dienes (Table 19-1), although not every catalyst system is equally effective. As a rule of thumb, one can say that all Ziegler catalysts that polymerize a-olefins (CH2=CHR) also polymerize ethylene. The reverse of this rule, however, does not hold. Catalysts with transition elements of group VIII do indeed polymerize dienes, but not a-olefins whereas compounds with elements of groups IV-VI initiate polymerization in both a-olefins and dienes. 

In a series of RX compounds the CsCl type is found generally for the rare earths with the smallest atomic radius, while the other types are more dependent on the X element. With the group VIII metals, in going from Lu to La, the sequence CsCl-FeB-CrB is found, in other cases the sequence is CsCl-CrB-FeB. 

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