Platinum triads

These six metals located beneath the iron triad on the periodic chart are very much alike. The first three are called the light platinum triad. The heavy platinum triad includes the other two and platinum itself. They are usually found together in nature and are used for similar things. All are shiny and beautiful and they do not tarnish or rust. 

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. 

Most of the transition elements do not react with strong acids, such as HCl and H2SO4. Some do have negative standard reduction potentials for the reaction M"+ -I- ne - M, and liberate hydrogen from hydrochloric acid. These include Mn, Cr, and the iron triad. Silver, gold, the palladium triad, and the platinum triad, the so-called noble metals, are especially inert to acids, both to the nonoxidizing species, such as hydrochloric and hydrofluoric acids, and to the oxidizing acids, such as nitric acid. 

The platinum-group metals (PGMs), which consist of six elements in Groups 8— 10 (VIII) of the Periodic Table, are often found collectively in nature. They are mthenium, Ru rhodium, Rh and palladium, Pd, atomic numbers 44 to 46, and osmium. Os indium, Ir and platinum, Pt, atomic numbers 76 to 78. Corresponding members of each triad have similar properties, eg, palladium and platinum are both ductile metals and form active catalysts. Rhodium and iridium are both characterized by resistance to oxidation and chemical attack (see Platinum-GROUP metals, compounds). 

Physical and Mechanical Properties. Whereas there are some similarities in the physical and chemical properties between corresponding members of the PGM triads, eg, platinum and palladium, the PGMs taken as a unit exhibit a wide range of properties (2). Some of the most important are summarized in Table 2. 

The nine elements, Fe, Ru, Os Co, Rh, Ir Ni, Pd and Pt, together formed Group VIII of Mendeleev s periodic table. They will be treated here, like the other transition elements, in vertical triads, but because of the marked horizontal similarities it is not uncommon for Fe, Co and Ni to be distinguished from the other six elements (known collectively as the platinum metals) and the two sets of elements considered separately. 

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... 

Studies of the compounds involving elements of the triad with platinum and gold are important both from the point of view of the structure of the compounds formed and their potential use as catalysts. The... 

Johann Wolfgang Dobereiner, 1780-1849, Professor of chemistry at Jena. His discovery of the triads was an important step toward the systematic classification of the chemical elements He wrote many books and papers on general and pharmaceutical chemistry, mineral waters, the manufacture of vinegar, and the use of platinum as a catalyst, The original of this portrait is in the City Museum at Jena. 

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 ... 

Detailed mechanistic studies with respect to the application of Speier s catalyst on the hydrosilylation of ethylene showed that the process proceeds according to the Chalk-Harrod mechanism and the rate-determining step is the isomerization of Pt(silyl)(alkyl) complex formed by the ethylene insertion into the Pt—H bond.613 In contrast to the platinum-catalyzed hydrosilylation, the complexes of the iron and cobalt triads (iron, ruthenium, osmium and cobalt, rhodium, iridium, respectively) catalyze dehydrogenative silylation competitively with hydrosilylation. Dehydrogenative silylation occurs via the formation of a complex with cr-alkyl and a-silylalkyl ligands ... 

Heteronuclear Clusters Containing Platinum and the Metals of the Iron, Cobalt, and Nickel Triads... 

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. 

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. 

---