Non transition-metal systems

Mixed clusters NH3/H20 (139-141), NH3/MeOH (61), and NH3/Me2CO (142) have been reacted with bare metal ions and in general the transition metal ions preferred coordination to ammonia whereas the non-transition metal ions such as Mg+ and Al+ were nonselective, showing some similarity to condensed-phase systems. 

In 1988, Cava and co-workers also prepared (88a) a quaternary oxide, Ba/K/Bi/O, and observed superconductivity at -28 K. This compound was the first "non-transition metal" oxide with a Tc above the legendary "alloy record" of 23 K. Further studies indicated (88a) that the optimum composition for "high temperature" superconductivity in this system was Ba0 6K0 4BiO3 x, having a Tc of 30.5 K (Figure 17). The samples were multiphase, and the superconducting fraction varied from 3 to 25%. Superconductivity for the rubidium-substituted compound was observed at -28.6 K. 

Through a co-assembling route, mesostructured lamellar molybdenum sulfides are formed hydrothermally at about 85 °C using cationic surfactant molecules as the templates. The reaction temperature and the pH value of the reaction system are important factors that affect the formation of the mesostructured compounds. The amount of the template and that of the S source are less critical in the synthesis of the compounds. For the three as-synthesized mesostructured materials, the interlayer distance increases linearly with the chain length of the surfactant. Infrared and X-ray photoelectron spectroscopy reveals that the individual inorganic layers for the three compounds are essentially the same both in composition and in structure. The formal oxidation state of the molybdenum in the materials is +4 whereas there exist S2 anions and a small amount of (S-S)2 ligands in the mesostructures. The successful synthesis of MoS-L materials indicates that mesostructured compounds can be extended to transition metal sulfides which may exhibit physico-chemical properties more diverse than non-transition metal sulfides because of the ease of the valence variation for a transition metal. 

The arylene-vinylene systems include a vast array of hybrid systems owing to their synthesis from classical organic means [311-314]. However, new systems continue to surface that are not viable other than from non-transition metal-mediated pathways. Just a few of the many examples obtained from the various synthetic routes mentioned previously in the PPV section are depicted below in Scheme 64. 

Quantum Chem. Symp., 24, 295 (1990). Simple Non-Empirical Calulations of the Zero-Field Splitting in Transition Metal Systems. I. The Ni(II)-Water Complexes. 

As the active metal-carbon bond assumes more covalent character, there will be a greater tendency for hemolytic cleavage and radical-type polymerizations. This becomes favorable when the alkyl is attached to a transition metal in one of its highest valence states or to a non-transition metal of Group IV or V. One can expect such catalysts to initiate polymerizations by both the conventional simple free radical and coordinated radical mechanisms. Stereospecificity generally suffers in these systems because both mechanisms are operative and because radical addition to a double bond is less selective for producing a head-to-tail polymer structure. 

However, because of the pathologieal behaviour ofXa for some ( magnetic ) transition-metal systems (see below) and because the other potentials include correlation in a more explicit manner and are parameter-free (except for parameters needed to fit electron-gas results), there seems to be little reason to retain the Xa potential, except perhaps for the sake of compatibility with previous calculations. Since the VWN or the potential represents the local limit, it should be used as the standard of reference. If an LSD-VWN problem has been aceurately solved, any remaining errors may be attributed to non-local effects (Section II.B). 

Present views concerning the operation mechanism of ZN catalysts are not conclusive. Cossee [288, 289] assumes that, in the first step, donor-acceptor interaction occurs between the transition metal and the monomer. A a bond is formed by the overlap of the monomer n orbital with the orbital of the transition metal. A second n bond is formed by reverse (retrodative) donation of electrons from the orbital of the transition metal into the antibonding 7T orbital of the monomer. In the following phase, a four-centre transition complex is formed with subsequent monomer insertion into the metal-carbon bond. This, in principle, monometallic concept is criticized by the advocates of the necessary presence of a further metal in the active centre. According to them, the centre is bimetallic. Monometallic centres undoubtedly exist on the other hand, technically important ZN catalysts are multicomponent systems in which each component has its specific and non-negligible function in active centre formation. The non-transition metal in these centres is their inherent component, and most probably the centre is bimetallic. Even present ideas concerning the structural difference in centres producing isotactic and atactic polymers are not united. 

Few materials show up the limitations of the two extreme viewpoints of magnetic moment formation in transition metal systems ( localized or itinerant ) more than do their intermetallic compounds. In some compounds, e.g., the (non-integral) magnetic moment may vary from one type of site to another and the moment associated with a particular transition metal atom is often different in its different compounds. The interest in the wide variety of properties exhibited by intermetallic compounds stems as much from the opportunity they offer for the understanding of magnetism in metallic systems at a fundamental level as from the possibility of producing materials of technological importance. 

Pseudo-binary compounds An(X1, X2)3 X1,2 = non-transition metals Several quasi-binary An(X1, X2)3 systems (preserving the AuCu3 structure of the parent phases) were studied with the aim to follow the development of electronic properties across the onset of magnetic ordering, the latter pointing always to a certain degree of 5f-electron localization. 

Rare earth elements form intermetallic compounds with non-transition metals or transition metals. Such RI compounds exist in a variety of crystalline structures which were described in detail by Taylor (1971) and Wallace (1973). The review articles of Buschow (1977a, 1979) and landelli and Palenzona (1979) contain more recent information about the crystalline structure of new RI compounds. We refer the reader to these articles for details. More recently it has been found that amorphous alloys can be prepared from R-atoms and both non-transition and transition metals at the same concentration of elements found in RI compounds (Cochrane et al., 1979). These alloys therefore serve as useful systems for comparison purposes. 

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