Second-and Third-Row Transition Metals

Some second- and third-row transition metals are, for good reason, known as precious metals. These include silver, palladium, rhodium, iridium, osmium, gold, and platinum. As this is written, gold is over 900 per ounce and silver is over 15 per ounce. Some of the other metals such as rhodium, osmium, and rhenium are also extremely expensive. Most of the second- and third-row transition metals are found as minor constituents in ores of other metals. Consequendy, we will not enumerate the sources, minerals, or the processes by which these metals are obtained. Some of their most important properties are shown in Table 11.3. 

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Because several of the superalloys contain very little iron, they are closely related to some of the non-ferrous alloys. Some of the second- and third-row transition metals possess many of the desirable properties of superalloys. They maintain their strength at high temperatures, but they may be somewhat reactive with oxygen under these conditions. These metals are known as refractory metals, and they include niobium, molybdenum, tantalum, tungsten, and rhenium. 

However, with soft electron pair acceptors such as Pt2+, Ag+, and Ir+, phosphines are stronger Lewis bases than are NH3 and amines, so phosphines and arsines interact better with class B metals than do amines. Generally, phosphines and arsines form stable complexes with second- and third-row transition metals in low oxidation states. 

Much of what has been said so far in this chapter applies equally well to complexes of second- and third-row transition metals. However, there are some general differences that result from the fact that atoms and ions of the second- and third-row metals are larger in size than those of first-row metals. For example, because of their larger size (when in the same oxidation state as a first-row ion), ions of metals in the second and third rows form many more complexes in which they have a coordination number greater than 6. Whereas chromium usually has a coordination number of 6, molybdenum forms [Mo(CN)8]4 and other complexes in which the coordination number is 8. Other complexes of second- and third-row metals exhibit coordination numbers of 7 and 9. 

Substitution Studies of Second- and Third-Row Transition Metal Oxo Complexes... 

Experimental data for solvent exchange on octahedral second- and third-row transition metal ions are limited to the Ru2+/3+, Rh3+ and Ir3+ and to water and acetonitrile solvents (Table VIII (3,125-129)). 

The second- and third-row transition metal ions also form the same sequence of ions (113)... 

Examination of the reaction kinetics of the M+ + H2S reactions show that these reactions are not simple first-order reactions, that is, nonlinear slope for the rate of disappearance of M+ shown in Fig. 7 for Pt+. The non-first-order rate of disappearance of M+ suggests that there is more than one intermediate, possibly due to the presence of electronic excited states of the metal ions or intermediates with different interactions between the metal and H2S. The addition of H2S to Au+ is similar to the reaction of H2S with Ag+ and Cu+ (M+ — [MH2S]+ — [M(H2S)2]+), but is dissimilar to most of the second- and third-row transition metal ions. 

See, for example, J. H. Canterford and R. Colton, "Halides of the Second and Third Row Transition Metals. Wiley, New York, 1968. 

SUBSTITUTION STUDIES OF SECOND- AND THIRD-ROW TRANSITION METAL OXO COMPLEXES... 

Rate Constants and Activation Parameters for Water Exchange on Second and Third Row Transition Metal Ions... 

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