Lanthanide-transition metal compounds

Another distinct property of mixed complexes which we mention here is the possibility to observe two activation regimes of relaxation of magnetization. One of them corresponds to reversal of magnetization of individual ions, at higher temperatures, and the other is related to climbing over the barrier built from the exchange multiplets of the complex. The coexistence of these two relaxation regimes has been recently revealed in the Co Dy ,11 complex in a combined study 

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Since some properties of each sublattice, ei ecially the anisotropy of the lanthanide sublattice, as experimentally established, govern the behavior of the whole crystal of the magnet, the magnetic properties of the lanthanide sublattice will affect the sublattice of the transition metals, i.e., interactions between sublattices exist. The 4f electrons, however, have almost no direct bonds with the 3d electrons of the transition-metal sublattice, so the anisotropy of the 4f electrons initially transfers to the outer orbitals of 6s, 5d and/or 6p electrons of the lanthanide atoms, and these in turn interact with 4s and/or 4d electrons of the transition-metal sublattice, which is composed of and/or spd bands with the transition metal s 3d electrons. The interactions between the 4f and 3d electrons, therefore, are indirect. Figure la schematically shows the band structure in the lanthanide-transition-metal compounds. 

The discussion in this subsection will be based on the band structure of the lanthanide-transition-metal compounds as shown schematically in figs. la,b. If we think about the effect of the anisotropy of the inner 4f electrons of flie lanthanides (Sm or Nd) on the Fe sublattices, the 4f electrons should initially transfer their characteristics to 5d, 6s and 6p band electrons of the lanthanide atom, which in turn will transfer the information to 4s, 4p and 3d electrons of the Fe atoms, finally affecting the magnetic properties, for example the crystalline anisotropy, of the entire crystal. 

Mechanism of the intersublattice exchange coupling in lanthanide-transition metal compounds... 

Except for the factors mentioned above, such as the reactant ratio employed, variation of lanthanide and transition metal, crystallization conditions, and the presence of a secondary ligand, there are several other factors that can affect the controllable assembly of the lanthanide-transition metal-amino acid cluster compounds. 

Lanthanide iodide silicides, 200 Lanthanide metals, 200 Lanthanide nitrobenzoates, 200 Lanthanide—transition metal alloy hydrides, 201 Lassaigne test, 201 Lead salts of nitro compounds, 201 Lecture demonstrations, 202 Light alloys, 202 Lime fusion, 202 Linseed oil, 202 Liquefied gases, 203 Liquefied natural gas, 203 Liquefied petroleum gases, 203 Liquid air, 204 Liquid nitrogen cooling, 205 Lithium peralkyluranates, 205 Lubricants, 205 Lycopodium powder, 205... 

The second class consists of molecular compounds containing one or more silicon-transition-metal bonds. The first example, Me3SiFe(CO)2-(tjs-CbHb), was prepared in 1956 (359), but nearly 10 years then elapsed before other compounds were described (26, 94). These heralded many more, and now examples are known in which silicon is bonded to almost every transition metal (Fig. 1). Curiously, no Si-Ag compounds have been described there are also no reports of derivatives of lanthanides or actinides. Most work has involved Fe, Co, Pt, Mn, Re, Mo, Ru, and Ni, in roughly decreasing order of frequency. Almost all well-characterized molecular silicon-transition-metal compounds known at present are diamagnetic some possible exceptions are noted in Section II,F. The most recent comprehensive reviews of the area were published in 1973 (134) and 1974 (235), covering the literature until 1971 and 1972, respectively these contain details of earlier reviews. Other surveys of certain aspects have also appeared, two of them very recently (24, 25, 201). 

The examples discussed so far are all transition metal complexes. As we will see later (Chapters 4-9), most homogeneous catalytic processes are indeed based on transition metal compounds. However, catalytic applications of rare earth complexes have also been reported, although so far there has not been any industrial application. Of special importance are the laboratory-scale uses of lanthanide complexes in alkene polymerization and stereospecific C-C bond formation reactions (see Sections 6.4.3 and 9.5.4). 

A number of early transition metal compounds, e.g., Ti111 hydrides, have long been known as hydrogenation catalysts. Similarly, metallocene compounds of lanthanides and actinides can be extremely active, as a comparison of the turnover numbers of 1-hexene hydrogenations at 25°C (1 bar H2) show CpfLuH, 120,000 [Ir(COD)-(py)(PCy3)]PF6, 6400 [Rh(COD)(PPh3)2]PF6, 4000 RuHCl(PPh3)3, 3000 ... 

There are just few examples of authentic lanthanide complexes in the oxidation state zero. Bis(arene) complexes of the lanthanides (l,3,5- Bu3C6H3)2Ln (Ln = Sc, Y, La, Nd, Pr, Sm, Gd, Tb, Dy, Ho, Er, Lu) have been synthesized by cocondensation of metal vapors (see Metal Vapor Synthesis of Transition Metal Compounds) with 1,3,5-tri(ferf-butyl)benzene at 75 K. A sandwich structure with coplanar arene ligands has been proven by X-ray crystal structure analysis of the Gd and Ho complexes (Figure 86a). 

The most powerful magnets known to date are transition metal-rare earth intermetallics (42). The archetype of compounds of this kind is Nd2Fei4B, which exhibits a coercive field of the order of 10 kOe at room temperature (43). In molecular chemistry, the lanthanide-transition metal species are still rather rare, and only in a few cases have the magnetic properties been investigated in a thorough fashion. In Kahn... 

Although the preponderance of recent reports centres upon some inner transition metal ions (lanthanides), one prominent investigation features water exchange on a first row organo-transition metal compound. Organometallic aqua ions in which a transition metal ion... 

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