Weakly held metal

To determine the various species and their concentrations in soil, many selective, semiselective, and sequential extraction methods have been developed. Species associated with various components in soil can be extracted with varying effectiveness (see Chapters 11 and 12). Thus, metal cations that are in solution, exchangeable, weakly held, or associated with carbonate and with... 

It is noteworthy that surface carbon did not come from those CO molecules responsible for the HT peak but from sites that are able to disproportionate CO and correspond to the LT peak. Because the latter sites are important only on quite small particles, it is tempting to associate them with low coordination number surface metal atoms, the relative concentration of which increases rapidly as the particle size decreases below 2 nm (8). Thus, these atoms may be the sites responsible for the relatively weakly adsorbed state of CO. Results similar to our work were found on other Group VIII metals. In the case of a Ru/Si02 sample, Yamasaki et al. (9) have shown by infrared spectroscopy that the deposition of carbon occurs rapidly by CO disproportionation on the sites for weakly held CO. The disproportionation also occurred on a Rh/Al20 sample with 66% metal exposed so that appreciable concentrations of low coordination atoms are expected (10). 

The positive charge on the resulting metal ion is due to the atom possessing more nuclear protons than orbital electrons. The valence electrons are most distant from the nucleus thus, they are weakly held by the electrostatic attraction of the protons and, consequently, are easily stripped from atoms of the metals. 

The outer electrons of most metal atoms tend to be weakly held to the atomic nucleus. 

The C-M a bonds (where M = Si, Ge, Sn, Pb) in the fragment C-MR3 (R = alkyl, aryl) are powerful a donor orbitals the electrons in the C-M bond are weakly held and easily ionized, and, importantly, the C-M bonding electrons are polarized towards the carbon. As a result, the presence of group 4 metal substituents in organic compounds has long been recognized to have very profound effects on both the ground-state properties and reactivities of the compounds.10-13 The most well known of these effects is the silicon (3-effect,10 12 which is the... 

Phosphate, silicate, borate, arsenate, selenite, chromate, and fluoride are anions for which ligand exchange is important. Nitrate, chloride, bromide, and perchlorate are not held, while sulfate and selenate may be weakly held. As a consequence, leaching of nitrate and sulfate from soil in drainage water can be significant, but very little phosphate is lost in solution. Of the trace metals, Co, Cu, Ni, and Pb are strongly held on oxide surfaces by chemisorption, but the process is much less important for Cd and Zn. 

The meaning of the terms low or high spin as applied to transition metal complexes and the terms strong or weak held as applied to ligands... 

In this equation the Rate is the molar TOF of the reaction, moles of product formed/mole of metal catalyst/unit time. The terms in [ ] are the STO measured site densities given in moles of site/mole of metal. The specific site TOFs, A, B and C, have units of moles of product/mole of site/unit time. Of these factors, the site densities are available from an STO characterization of the catalyst and the Rate is determined for the specific reaction nm over the STO characterized catalyst. When a series of at least three STO characterized catalysts is used for the same reaction, run under the same conditions, the specific site TOFs can be calculated from the simultaneous equations expressed as in Eqn. 3.6. When this approach was used in the hydrogenation of cyclohexene over a series of seven Pt/CPG catalysts specific site TOF values for the Mr and MH sites were found to be 2.1, 18.2 and 5.2 moles of product/mole of site/second, respectively.21 Not surprisingly, that site with the weakly held hydrogen was the most active and that on which the hydrogen was strongly held was the least active. 

These results appear to indicate that the Mo(llO) surface cannot be regarded as accommodating only the most weakly held /3-CO phase, as was suggested by May and Germer 71) for the W(llO). The process of adsorption on the (110) face of these two metals appears, however, to be similar, although a complete comparison must await further study. 

Relations between coordination chemistry of single metal atoms and surface chemistry are illustrated by the interaction of H2 with either a Ru atom or a Ru02(l 10) single-crystal surface [26]. As shown in Fig. 1.6, H2 forms weakly held Ti -H2 complexes with transition metal atoms [27], while on Ru02(l 10) the H2 molecule is held in a similar way above the Ru atoms, where bond lengths, vibrational frequencies, and bond strengths are quite similar in both cases. However, the further reactivity is different While with the complex compound, dissociation of the... 

Most group A metal ions have the electronic configuration of the rare gases, and the interaction of the metal with a ligand is primarily electrostatic in nature. Because the charge density (oc charge radius) is small, the water molecules are weakly held and the rate of their loss is comparable with the diffusion-controlled value of around 10 —10 s. The complex formation rate-constant for an ion in this group is therefore approximately equal to the maximum possible value, unless chelation steps are important, and the stability of the complex is reflected in the dissociation rate constant. 

At this point we have considered the interaction of oxygen with metal complexes in solution and have presented ways in which coordinated dioxygen is transferred to substrates which may themselves be coordinated to the metal. In several instances we have seen that the oxidized substrate leaves the coordination sphere. For catalysis to occur this is a necessary step. Thus, those substrates which on oxidation become more weakly held ligands, can be displaced by more unoxidized substrate and a catalytic cycle, equation (58), is possible. If, on the other hand, the oxidized substrate is more strongly held than the original substrate, catalysis does not occur. 

Consider a "Chinese checkers" board. The marbles are placed in very shallow depressions but, as any player has experienced, the slightest motion of the board causes the marbles to leave thar depressions and roll to others. In a similarly way, the electrons in metallic solids are very weakly held a very slight input of energy causes the electrons to move fairly freely. 

The exchange occurs because the divalent and trivalent toxic metal anions or cations have an increased affinity for the charged sites on the surface of the resins, which originally were coated with weakly held monovalent anions or cations such as chloride, hydroxyl, sodium, or hydrogen ions. 

The fifth component is the stmcture, a material selected for weak absorption for neutrons, and having adequate strength and resistance to corrosion. In thermal reactors, uranium oxide pellets are held and supported by metal tubes, called the cladding. The cladding is composed of zirconium, in the form of an alloy called Zircaloy. Some early reactors used aluminum fast reactors use stainless steel. Additional hardware is required to hold the bundles of fuel rods within a fuel assembly and to support the assembhes that are inserted and removed from the reactor core. Stainless steel is commonly used for such hardware. If the reactor is operated at high temperature and pressure, a thick-walled steel reactor vessel is needed. 

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