Transition metal compounds structure

Rutile, and many other transition-metal compound structures, arc characterized by dense packing and high coordination numbers (numbers of nearest neighbors). Their bonding properties arc those of ionic solids, and many of the structures have been rationalized in terms of ionic radii and Madelung energies. 

Mention has already been made in this chapter of metallocene-catalysed polyethylene (see also Chapter 2). Such metallocene catalysts are transition metal compounds, usually zirconium or titanium. Incorporated into a cyclopentadiene-based structure. During the late 1990s several systems were developed where the new catalysts could be employed in existing polymerisation processes for producing LLDPE-type polymers. These include high pressure autoclave and... 

The simplest of the ir-bondcd Re-C compounds is the green, paramagnetic, crystalline, therm ly unstable ReMen, w ich, after WMe, was only the second hexamethyl transition metal compound to be synthe zed 11976). It reacts with LiMe to give the unstable, pyrophoric, Lii[ReMe(,, which has a square-antiprismatic structure, and incorporation of oxygen into the coordination sphere greatly H reases the stability, wit e,ss Re CMe, which is thermally stable up to 200 C, and Re "0[Pg.1068]

The general understanding of the electronic structure and the bonding properties of transition-metal silicides is in terms of low-lying Si(3.s) and metal-d silicon-p hybridization. There are two dominant contributions to the bonding in transition-metal compounds, the decrease of the d band width and the covalent hybridization of atomic states. The former is caused by the increase in the distance between the transition-metal atoms due to the insertion of the silicon atoms, which decreases the d band broadening contribution to the stability of the lattice. 

Numerous intercalation reactions are known in which one reactant enters the lattice of the other. Such behaviour is conveniently illustrated by reference to two recent studies. Lithium undergoes a low temperature (298 K) topochemical reversible reaction with transition metal compounds (e.g. TiS2, NbSe3) [1211] in which the host lattice structure may be partially retained (e.g. in Li TiS2, LijNbSe3). The reaction [1212]... 

Vibrational fine structure in the electronic spectra of transition metal compounds an experimental survey. M. Cicslak-Golonka, A. Bartecki and S. P. Sinha, Coord. Chem. Rev., 1980, 31, 251-288... 

We shall now look at a number of thermodynamic and structural variations in transition-metal compounds which owe their origins, at least in part, to the properties of open d shells. 

The dominant features which control the stoichiometry of transition-metal complexes relate to the relative sizes of the metal ions and the ligands, rather than the niceties of electronic configuration. You will recall that the structures of simple ionic solids may be predicted with reasonable accuracy on the basis of radius-ratio rules in which the relative ionic sizes of the cations and anions in the lattice determine the structure adopted. Similar effects are important in determining coordination numbers in transition-metal compounds. In short, it is possible to pack more small ligands than large ligands about a metal ion of a given size. 

First we consider the origin of band gaps and characters of the valence and conduction electron states in 3d transition-metal compounds [104]. Band structure calculations using effective one-particle potentials predict often either metallic behavior or gaps which are much too small. This is due to the fact that the electron-electron interactions are underestimated. In the Mott-Hubbard theory excited states which are essentially MMCT states are taken into account dfd -y The subscripts i and] label the transition-metal sites. These... 

Transition metal compounds with ligands of low electronegativity also show deviations, in spite of a d° electron configuration. For example, W(CH3)6 does not have the expected octahedral structure, but is trigonal-prismatic. 

In order to compare the structural options for transition metal compounds and to estimate which of them are most favorable energetically, the ligand field stabilization energy (LFSE) is a useful parameter. This is defined as the difference between the repulsion energy of the bonding electrons toward the d electrons as compared to a notional repulsion energy that would exist if the d electron distribution were spherical. 

I.B. Bersuker, Electronic Structure and Properties of Transition Metal Compounds. John Wiley and Sons, Inc., 1996. 

Combined quantum mechanics/molecular mechanics studies have demonstrated that 7r-7r stacking interactions are important in determining the structural features of polyarene transition metal compounds such as the two ds Pd complexes.96... 

Terminal methylene complexes are relatively rare—less than 10 such compounds have been isolated and about as many again have been characterized by spectroscopic techniques only. The methylene complexes previously reported fall into two groups, (i) neutral complexes of the early transition metals (e.g., Ti, Ta) and (ii) cationic complexes of the later transition metals (e.g., Re, Fe). The osmium complex 47 is important, then, as it is a new example extending the neutral group to the later transition metals. Compound 47 is the prototype for the series Os(=CHR)Cl(NO)(PPh3)2 and is one of only three terminal methylene complexes to be structurally characterized by X-ray crystallography (see Section IV,B). 

In sharp contrast to the intensely studied reactions of dipenteles with transition metal compounds, reactions with group 13 metal compounds are almost unknown. Only two diphosphine-borane bisadducts of Type C ([H3B]2[Me4P2], [H2(Br)B]2[Me4P2]) have been synthesized and structurally characterized65 but no diarsine, distibine or dibismuthine adducts. We, therefore, became interested in the synthesis of such compounds, focusing... 

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