Poor metals

Fast sulphon black F ( C.I.26990). This dyestuff is the sodium salt of 1-hydroxy-8-( 2-hydroxynaphthylazo) -2- (sulphonaphthylazo) -3,6-disulph onic acid. The colour reaction seems virtually specific for copper ions. In ammoniacal solution it forms complexes with only copper and nickel the presence of ammonia or pyridine is required for colour formation. In the direct titration of copper in ammoniacal solution the colour change at the end point is from magenta or [depending upon the concentration of copper(II) ions] pale blue to bright green. The indicator action with nickel is poor. Metal ions, such as those of Cd, Pb, Ni, Zn, Ca, and Ba, may be titrated using this indicator by the prior addition of a reasonable excess of standard copper(II) solution. 

Initially, it was thought more likely that the electron poor metal atom would be involved in the electrophilic attack at the alkene and also the metal-carbon bond would bring the alkene closer to the chiral metal-ligand environment. This mechanism is analogous to alkene metathesis in which a metallacyclobutane is formed. Later work, though, has shown that for osmium the actual mechanism is the 3+2 addition. Molecular modelling lends support to the 3+2 mechanism, but also kinetic isotope effects support this (KIEs for 13C in substrate at high conversion). Oxetane formation should lead to a different KIE for the two alkene carbon atoms involved. Both experimentally and theoretically an equal KIE was found for both carbon atoms and thus it was concluded that an effectively symmetric addition, such as the 3+2 addition, is the actual mechanism [22] for osmium. 

There are several possible reasons why the overall rate of electron transfer is slow. Occasionally, it is because the electrode is a poor conductor, such as semiconductors like silicon or poor metals such as tungsten (see SAQ 2.6). Fabricating an electrode from metals such as platinum, gold, or from metallic conductors such as graphite or glassy carbon, will circumvent that possibility. 

The metallics are often called other metals and begin an arrangement on the periodic table in zigzag steps. (You may view this dark zigzag line that divides the metallics from metalloids on a copy of the Periodic table.) For the other metals or metallics, this zigzag line runs run from aluminum to gallium to indium to tin to thallium to lead and then ends with bismuth. Elements left of the zigzag are also called poor metals. ... 

The rare combination of two electron-poor metals in heterodimetallic complexes has been found in reactions of dialkynylmetallocenes with suitable metallocene precursors. Thus, reaction of Zr(f -C4H6)Cp2 with Hf(C=CPh)2Cp2 affords Cp2Zr (/x.-C2Ph)2 HfCp2 (222-Zr/Hf). Similar reactions of VCp2 with... 

Much simpler than the template-controlled generation of p-functionalized isocyanides is their direct use. 2-Hydroxyalkyl isocyanide 59, where the nucleophilic group and the isocyanide are linked together, spontaneously cyclizes upon activation of the isocyanide by coordination to an electron-poor metal center under... 

Haymore (4) has pointed out that the noncyclic ligand has many degrees of freedom with respect to rotations about single bonds, whereas the cyclic analog has fewer since the ends are connected. Thus, the cyclic chelate should be greatly favored from an entropy consideration. If correct, this explanation allows one to postulate that the poor metal complexing properties observed for non-cyclic ethers is a result of small... 

The alkyne-to-vinylidene tautomerization processes on various transition metal centers have also been discussed. Three different pathways for the formation of vinylidene from p -acetylene on electron-rich transition metals were the most theoretically studied. Most studies suggested that the favorable pathway proceeded via an intermediate with an agostic interaction between the metal center and one C—H bond followed by a 1,2 hydrogen shift (the bl+b2 pathway shown in Scheme 4.5). The reverse process, the vinylidene-to-p -acetylene tautomerization, was also discussed. It was found that complexes with electron-poor metal centers were able to mediate the reverse process. 

For linear isocyanides at an electron-poor metal center such as Cr(CO)s. Isocyanotriphenylborate. 

A similar derivative has been proposed on the basis of spectral studies on a mixture of BeMe2 and AlMe3, but this derivative appears to be far less stable (14). This is thought to be the result of steric interactions that occur between the bridge and terminal groups. It also may be in part a result of poor metal-metal interaction in this type of system, but, as suggested earlier, this is a very speculative point. 

Belertser et al (1988) have observed that the electrical resistivity of amorphous chromium films at liquid-helium temperatures jumps from a value (10 3 O cm) characteristic of a poor metal by a factor 103, when the hydrogen content is increased sufficiently to increase the lattice constant by 10%. The transition is not abrupt, and is thought by these authors to be of Anderson type. They claim that it is the first time such a transition has been observed in a solid, and that it is similar to that in expanded mercury vapour (Section 4). 

It will be seen that although normal (or thermal ) electrochemical reactions can be sustai ned at low current densities using semiconductors (Le., they act as electron-poor metals), they real ly do not come onto center stage until their photoelectrochemistry is studied (see Chapter 10). Thus, semi conductor electrodes are responsive to light when metals are almost unreactive to it. 

In addition to interacting with the Lewis acid center of the C02 molecule, these same low-valent metal complexes may also interact with the carbon-oxygen 7t-bonds in C02, in much the same way as olefins interact with electron-rich complexes. Finally, the oxygen atoms in C02 may be expected to show weak electron donating ability, possibly coordinating to a very electron-poor metal, although this mode of coordination of C02 is not presently known. 

The poor metals among the BCNOs usually include aluminum, gallium, indium, thallium, tin, lead, and bismuth. The metalloids are boron, silicon, germanium, arsenic, antimony, tellurium, and polonium. The nonmetals are carbon, nitrogen, oxygen, phosphorus, sulfur and selenium. These groups are not official, and chemists sometimes disagree on whether a particular element like boron should be called a metal or a metalloid. 

The BCNO elements are found in Groups 13 through 16 of the periodic table. The BCNO groups are a mixture of poor metals,... 

Poor metals The metal-like elements of the BCNO groups poor metals act like metals but are less reactive than the alkali and alkaline earth metals but more reactive than the transition metals. 

Poor metal surface—oil or powdery residue evident... 

LaFe4Sbn is a poor metal or heavily doped semiconductor with good thermoelectric properties above room temperature (700-1000 K) (Sales et al., 1996, 1997). Only polycrystalline samples have been investigated. The room temperature resistivity is about 0.5 m 2cm de-... 

The y-agostic interaction is such that the electron pair in the C, -He bond of the alkyl substituent at the metal is partly donated to the electron-poor metal this may play an important role in making more simple the insertion of a coordinated olefin into the metal-alkyl bond. This hypothesis has been tested in some instances [346-350], showing metal-hydrogen interaction via an oc-agostic bond. 

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