Group VIII Nickel Triad

Palladium.—number of papers have appeared dealing with the many facets of allyl rearrangement in A -allyl complexes of palladium. The criteria for tackling these often complicated systems have already been outlined in this section in Volume 2 of these Reports and need no further mention here. For the sake of convenience the reports will be dealt with in two categories, intrmnolecular rearrangements and rearrangements promoted by substitution. 

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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 place hydrogen as the first element in the first period, along with helium. When helium was discovered, Mendeleev put it in the second period. We put the triads of iron, cobalt, and nickel ruthenium, rhodium and palladium and osmium, iridium, and platinum in group VIIIB, in the middle of the table. Mendeleev put them in group VIII. We also have two long groups, the lanthanides and actinides, that were a headache for Mendeleev. 

In Group VIII, each position instead of being filled by a single element is occupied by a group of three elements. Thus there appear in triads iron, cobalt, and nickel ruthenium, rhodium, and palladium and osmium, iridium, and platinum. In this group there is no subdivision into families, but all the members are heavy metals. 

The most appropriate position for nickel in the Periodic Table is, as explained in Chapter I, the end of the first horizontal series of triads in Group VIII. An atomic weight greater than that of cobalt, namely, 58-97, but less than that of copper, namely, 63-57, is thus to be expected. 

Cobalt, in its properties, is an excellent intermediary between iron and nickel, and, moreover, it is clearly a suitable element to constitute the first of the central vertical triads of Group VIII, namely, Co, Eh, and Ir. Hence, if the Periodic Law holds absolutely, the atomic weight of cobalt should exceed that of iron, but not that of nickel. Either, therefore, the atomic weight of cobalt is slightly too high or that of nickel is slightly too low. 

Two trends are evident from these results first, the selectivity decreases with increasing atomic number down each triad of Group VIII (palladium or nickel falling out of line), and second, for the noble metals the selectivity increases from left to right across the Group. 

The running order in this chapter is essentially similar to that used in the preceding one. The main section, dealing with metal-carbon Group VIII triads in the order, iron, cobalt, and nickel, and is followed by a shorter section on carbene and carbyne complexes. Oxidative addition or reductive elimination reactions are included only when they lead directly to the formation or rupture of metal-carbon bonds. 

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