Intermetallic cooperation

The authors presume that transient vinylidene intermediates are involved in the observed reactions. This hypothesis is consistent with the observed (Z)-selectivity of dimerization and the unusual facility with which complexes of type 58 form isolable vinylidenes. Intermetallic cooperation in bimetallic complexes has been recognized to facilitate vinylidene-mediated processes [23] however, the operative mechanism in the present case remains unknown. 

Taking into account the usual dynamic behavior in solution of mononuclear polyhydrides, the occurrence of these intermetallic hydride exchanges is not surprising, although its importance can not be underestimated. In fact, hydride transfer among metals has been found to be crucial in order to observe intermetallic cooperation in polynuclear catalysts. This subject will be analyzed in the... 

The possible pathways for intermetallic cooperation during catalysis are not restricted to dinuclear elementary steps such as those already mentioned. It has been shown that genuine dinuclear mechanisms can operate in unsaturated species where the metals are capable of sharing coordination vacancies. This is possible through processes of vacancy migrations between metals, which are the consequence of the high mobility of hydrides combined with the transmission of trans effects within the dinuclear frames. This phenomenon can be illustrated... 

Keywords Binuclear complexes, Bond activation. Homogeneous catalysis, Intermetallic cooperation. Iridium, Ligand migration, Trans effect... 

Despite the vast number of outstanding examples of enzymatic catalysis that rely on the collaboration of two or more vicinal metal centers hitherto disclosed, the design and development of efficient binuclear organometallic complexes able to enhance the performance of mononuclear catalyst by means of an intermetallic cooperative process remains widely unexplored [22-24]. In fact, the formation of bi- or polynuclear complexes has been often described as a catalyst deactivation pathway [25-30]. However, the availability of more electron density at the active site, extra coordination positions, and the possibility to develop more preorganized systems that allow for (enantio)selective reactions shows great promise for an improved catalytic performance [9]. 

The nature and origin of superconductivity was described in 1957 by John Bardeen, Leon Neil Cooper, and John Robert Schrieffer. Together they created the Bardeen Cooper Schrieffer (BCS) model. It occurs for many metals, alloys, intermetallic compounds, and doped semiconductors. The transition temperatures range from 92.5 K for Ybc CUjOg j, down to 0.001 K for the element Rh. And there are some materials that become superconducting only under high-pressure conditions. These materials all have to be extremely pure, even just one impurity in 10,000 atoms can severely affect the superconducting property. 

In this section we present experimental results on the temperature dependence of elastic constants for intermetallic rare-earth compounds in which magnetoelastic effects due to the presence of crystal fields are dominant. There are systematic studies of these effects for given structures across the rare-earth series. Examples are the rare-earth monopnictides, especially the rare-earth antimonides (RSb), the rare-earth dialuminides (RAlj) and rare-earth compounds with the CsCl structure. From such experiments one obtains the single-ion magnetoelastic coupling constants gj. across the series and in a few cases the quadrupolar coupling constant gf [eq. (38)] too. The case of a cooperative Jahn-TeUer effect will be treated separately in sect. 2.4.3. The examples presented here can be explained mostly with the single-ion strain susceptibility Xr [ <1- (35) instead of eq. 

Heavy Fermion superconductivity is found more frequently in intermetallic U-compounds than in Ce-compounds. This may be related to the different nature of heavy quasiparticles in U-compounds where the 5f-electrons have a considerable, though oibitally dependent, degree of delocalisation. The genuine Kondo mechanism is not appropriate for heavy quasiparticle formation as is the case in Ce-compounds. This may lead to more pronounced delocalised spin fiuctuations in U-compounds which mediate unconventional Cooper pair formation as discussed in sect. 2. The AF quantum critical point scenario invoked for Ce compounds previously also does not seem to be so important for U-compounds with the possible exception of UGe2. On the other hand AF order, mostly with small moments of the order 10 /Ub is frequently found to envelop and coexist with the SC phase in the B-T plane. 

Multimetallic catalysts, alloy catalysts, intermetallic compounds, fuel cell catalysts, colloidal intermediates, metal complexes, and metal clusters have received considerable attention [12-21] because the metal-metal cooperating bifunctional catalysts which can activate reactants simultaneously showed high catalytic activity and stereoselectivity under mild conditions [19]. In fact, there have been many bifunctional multimetallic catalysts in which multimetallic alloy- and electro-catalysts offer a way to fine-tune the catalytic properties of metals, atomic composition, and microstrucrnres [16-18, 20]. Cooperative multimetallic activation of oxidants via the multielectron transfer is also a common feature in biological oxidation catalysis [14]. Artificial multimetallic complexes with two or more metal atoms that contain... 

Whether a material is type-I or type-II is determined by the ratio of the penetration depth A (the distance into the surface a magnetic field can penetrate) to the coherence length (essentially the diameter of the Cooper pair). Type-II behavior occurs when A > y/2. Most pure materials are type-I, but adding impurities such as alloying with another metal increases A and decreases so most practical superconductors are alloys or intermetallic compounds. 

Many of the properties of superconductors have been successfully accounted for by the Bardeen-Cooper-Schrieffer (BCS) microscopic theory of superconductivity. However, the theory cannot predict the occurrence of superconductivity in materials, i.e., it does not tell us what atomic constituents should be put together and what crystal system is necessary in order to obtain materials exhibiting high critical temperatures 7. However, prior to the advent of the BCS theory a large body of information regarding the occurrence of the superconductive state in elements, alloys, and intermetallic compounds was accummulated, notably by Matthias and his... 

Ino] Inoue, H.R., Cooper, C.V, Favrow, L.H., Hamada, Y, Wayman, C.M., Mechanical Properties of Fe-Modified Z,l2-Type AlgTi , Mater. Res. Soc. Symp. Proc. High-Temp. Ordered Intermetallic Alloys IV, 213, 493-498 (1991) (Experimental, Meehan. Prop., 10)... 

This suggestion theoretically explains the metal and intermetallic compound superconductivity phenomenon (J. Bardin, L.N. Cooper and R. Schrieffer, Nobel Prize 1972), discovered earlier by H. Kamerling-Onnes (Nobel Prize, 1913). This phenomenon occurs only at very low temperatures (—20 K). However, superconductivity has been discovered in nonmetalhc, oxide-type chemical compounds with critical points of superconductivity up to ==140 K (in hquid nitrogen region) (so-called high-temperature super conductors, HTSC) (J.G. Bednorz and K.A. Muller, Nobel Prize, 1987). Intensive attempts to synthesize new materials of this kind are in progress. 

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