Nickel palladium-platinum triad

This set of results is at variance with those in the nickel-palladium-platinum triad, where palladium appears to be the most reactive species [11.3.2.1.3(i)], indicating the outstanding role played by palladium in substitution reactions. On the other hand, presence of the cyclopentadienyl ligand in the systems of reaction (a) could introduce a perturbation due to the possibility of a -> rj" mechanism, with n being an odd number < 5. 

The mechanism by which this low oxidation state is stabilized for this triad has been the subject of some debate. That it is not straightforward is clear from the fact that, in contrast to nickel, palladium and platinum require the presence of phosphines for the formation of stable carbonyls. For most transition metals the TT-acceptor properties of the ligand are thought to be of considerable importance and there is... 

Not only do the elements in each of the above triadic groups exhibit a more or less regular gradation in their properties, but. a certain amount of similarity is found to exist between a member of any one triad and the corresponding members of the other triads. Thus, for example, iron, ruthenium, and osmium have several interesting peculiarities in common so have cobalt, rhodium, and iridium, as well as nickel, palladium, and platinum. 

The Grouping of Elements into Triads—Atomic Weights of the Elements— General Properties of tho Elements—Comparative Study of Iron, Cobalt, and Nickel—Their Position in tho Periodic Table—Comparative Study of Iron, Ruthenium, and Osmium—Comparative Study of Cobalt, Rhodium, and Iridium—Comparative Study of Nickel, Palladium, and Platinum. 

In addition to the resemblance within the triads themselves, there is also a certain similarity between each element and the corresponding element in the other triads. Thus iron, ruthenium, and osmium have certain peculiar properties in common cobalt, rhodium, and iridium are somewhat alike, and nickel, palladium, and platinum present similar peculiarities. The resemblances in these vertical triads are especially striking between the last two members, but it is perhaps to be expected that the first member of such a group would differ somewhat from the other members. 

Detailed investigations on the kinetics and mechanisms of reactions of square planar palladium (II) complexes are largely lacking. However, enough data exist to show that the reactions of palladium (II) complexes are much faster than those of platinum (II), and that the two systems react by the same type of mechanism. Some of the data available are given in Table VIII along with the same information on platinum (II) and nickel (II) for comparison (3). The results show an approximate relative order of reactivity for analogous complexes of the triad as follows ... 

Numerous dmit chelates have been synthesized following that route. Although dmit chelates of common metal ions are known, considerable interest has been focused on those of d8 ions (nickel triad). Nickel chelates are listed in Tables 26 and 27. Table 28 summarizes all other salts of bis-chelates of dmit, including the other two elements of nickel triad, palladium and platinum. 

Oxidative additions are frequently observed with transition metal d8 systems such as iron(0), osmium(O), cobalt(I), rhodium(I), iridium(I), nickel(II), palladium(II) and platinum(II). The reactivity of d8 systems towards oxidative addition increases from right to left in the periodic table and from top to down within a triad. The concerted mechanism is most important and resembles a concerted cycloaddition in organic chemistry (Scheme 1.1). The reactivity of metal complexes is influenced by their... 

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. 

Platinum (Z = 78) is a whitish, noble metal found in group 10 (or VIIIB) of the periodic table, in the triad with and lying below nickel and palladium (see Periodic Table Trends in the Properties of the Elements). It can be found in elemental form in the mountains of Columbia, where the pre-Columbian natives used it. European chemists discovered and reported it in the mid-1700s. Like gold and silver, it has a positive reduction potential ... 

This monotonic decrease in the reactivity in the rates of substitution is the usual behaviour for triads of transition metal complexes, e.g. cobaIt(III) > rhodium(III) > iridium(III) and nickel(II) > palladium(II). However, some years later it was observed that in low oxidation state metal complexes the second row transition metal is the fastest to react, e.g. rhodium > cobalt > iridium and palladium > nickel > platinum. The most important commercial homogeneous catalysts use organometallic complexes of the second row transition metals. ... 

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