Oxygen late transition metals

The late transition metals, either Co or Ni, bond to four apical oxygen atoms of two vertex-shared 5 units (Fig. 17.9a), two each to layers both above and below, and also bond to two bridging pyrazine ligands in a trans fashion. The al-... 

Figure 6.28. Schematic illustration of the change in local electronic structure of an oxygen atom adsorbing on the late transition metal rhodium, the DOS of which is shown on the right-hand side. The interaction of the oxygen 2p orbital with the sp band of the transition metal is illustrated through interaction with the idealized free-electron...

Late transition metals are found on the right of the Periodic Table (e.g. Ni, Pd, Pt, Co, Fe, Ru, Rh, Cu active in low oxidation states) and are more tolerant of oxygen functionalities in the monomer and the polymer. This behavior stems from the preference of late transition metals for soft ligands (polarizable ligands, typically... 

A major limitation of such Group IVB metallocene catalysts is that they are air- and moisture-sensitive and not tolerant to heteroatom-containing monomers. In the case of heteroatom-containing monomers the unbonded electron pairs on the heteroatom, such as oxygen, preferentially coordinate to the Lewis acid metal center in place of the carbon-carbon double bond. Some so-called middle and late transition metal organometallics are more tolerant to the presence of such heteroatoms and can be used as effective cocatalysts. These include some palladium, iron, cobalt, and nickel initiators. 

As an introduction to the problem of understanding trends in adsorption energies on metal surfaces, consider the adsorption of atomic oxygen on a range of late transition metal surfaces. Figure 4.1 shows calculated energies as a function of the... 

Figure 4.42. The turnover frequencies for the low-temperature WGS reaction as a function of adsorption energies of oxygen and carbon monoxide. The positions of the step sites on noble and late transition metals are shown. As observed experimentally only copper appears to be a suitable pure metal catalyst for the process. Adapted from [139].

As seen from Fig. 10.11, the value of (3-8) in Lai xSrxCo03 falls off with decreasing oxygen activity much more rapidly than for the other compounds shown. The general trend at which the perovskites become nonstoichiometric follows that of the relative redox stability of the late transition metal ions occupying the B-site, i.e. Cr " > Fe > Mn > Co ". The reductive nonstoichiometry of the cobaltites increases further by partial B-site substitution with copper and nickel. 

The late transition metal-containing perovskites exhibit high electronic conductivities. In the materials which receive prime interest for oxygen delivery applications, the electronic contribution at high temperature of operation is usually predominant. The values for e.g., Lai rxCoi.yFey03 at 800°C in air... 

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