Transition metal classification

Relatively few complexes of the early transition metals with 1,1-dithiolato ligands have been prepared and characterized. This is consistent with their classification as "hard or "class a acceptors. Thus,... 

Phosphinidenes [1] are low-valent organophosphorus compounds that have attracted attention since the early 1980s when they were first discovered [2]. They are known in two classifications, one being the six-electron singly substituted phosphorus species (A) and the other in which the phosphorus atom carries an additional ri -stabifizing group, typically, but not necessarily, a transition metal group (B). Much has been learned about the reactivities of the complexed phos-... 

Busch DH (2005) First Considerations Principles, Classification, and History. 249 in press Bussiere G, Beaulac R, Belisle H, Lescop C, Luneau D, Rey P, Reber C (2004) Excited States and Optical Spectroscopy of Nitronyl Nitroxides and Their Lanthanide and Transition Metal Complexes. 241 97-118 Cadierno V, see Majoral J-P (2002) 220 53-77 Camara M, see Chhabra SR (2005) 240 279-315 Caminade A-M, see Majoral J-P (2003) 223 111-159 CantriU SJ, see Arico F (2005) 249 in press... 

For the reason of comparison and the development of new domino processes, we have created a classification of these transformations. As an obvious characteristic, we used the mechanism of the different bond-forming steps. In this classification, we differentiate between cationic, anionic, radical, pericyclic, photochemical, transition metal-catalyzed, oxidative or reductive, and enzymatic reactions. For this type... 

There are, however, also many examples of mixed domino processes , such as the synthesis of daphnilactone (see Scheme 0.6), where two anionic processes are followed by two pericydic reactions. As can be seen from the information in Table 0.1, by counting only two steps we have 64 categories, yet by including a further step the number increases to 512. However, many of these categories are not - or only scarcely - occupied. Therefore, only the first number of the different chapter correlates with our mechanistic classification. The second number only corresponds to a consecutive numbering to avoid empty chapters. Thus, for example in Chapters 4 and 6, which describe pericydic and transition metal-catalyzed reactions, respectively, the second number corresponds to the frequency of the different processes. 

In transition metal-catalyzed domino reactions, more than one catalyst is often employed. In Tietze s definition and the classification of domino reactions, no distinction has been made between transformations where only one or more transition metal catalyst is used for the different steps, provided that they take place in a chronologically distinct order. Poli and coworkers [13] differentiated between these processes by calling them pure-domino reactions (which consisted of a single catalytic cycle driven by a single catalytic system) or pseudo-domino reactions . The latter type was subdivided into ... 

It must be emphasized that the duodectet rule (4.6) initially has no structural connotation, but is based on composition only. Indeed, the compositional regularity expressed by (4.6) encompasses both molecular species (such as the metal alkyls) and extended lattices (such as the oxides and halides) and therefore appears to transcend important structural classifications. Nevertheless, we expect (following Lewis) that such a rule of 12 may be associated with specific electronic configurations, bond connectivities, and geometrical propensities (perhaps quite different from those of octet-rule-conforming main-group atoms) that provide a useful qualitative model of the chemical and structural properties of transition metals. 

Various catalytic systems for H202- and 02-based oxidations catalyzed by POMs have been developed. Typical examples are listed in Table 13.1. The systems can be classified into four groups according to the stmctures of POMs (1) mixed-addenda POMs, (2) transition-metal-substituted POMs, (3) POMs, and (4) lacunary POMs. In this chapter, liquid-phase homogeneous oxidations by POMs with H202 and 02 are described according to the above classification. 

There are three anions that may loosely claim to be nitrides. Pentazolides (salts of cyclic N ) will all be explosive. Some azides (salts of N3) fall just short of being explosive but all are violently unstable. The true nitrides, nominal derivatives of N3-, are more various. In addition to some ionic structures, there are polymeric covalent examples, and some monomeric covalent ones, while most of those of transition metals are best considered as alloys. Several are endothermic and explosive, almost all are thermodynamically very unstable in air with respect to the oxide. Many are therefore pyrophoric if finely divided and also may react violently with water and, more particularly, acids, especially oxidising acids. A few are of considerable kinetic stability in these circumstances. There is no very clear classification of probable safety by position in the periodic table but polymeric and alloy structures are in general the more stable. Individual nitrides having entries ... 

The inherent hydrophobicity once thought to be typical of sulphides (Ravitz and Porter, 1933) is now thought to be restricted to sulphides such as molybdenite (Chander et al., 1975) and other minerals or compound with special structural feature (Gaudin et al, 1957b). Common commercial sulphide minerals, which are needed to recover in flotation, are normally composed of anion (S ) and heavy metal ions such as Cu, Cu, Pb, Zn, Hg, Sb, Bi transitive metal ion such as Fe, Co, Ni and noble and rare metal ions such as Ag, Au, Mo. On the basis of structural pattern or mode of linkage of the atoms or polyhedral imits in space, Povarennyk (1972) introduced a crystallochemical classification of sulphide minerals, which have six major patterns as shown in Table 1.1. 

Other metah—a classification given to seven metals that do not fit the characteristics of transition metals. They do not exhibit variable oxidation states, and their valence electrons are found only on the outer shell. They are aluminum, gallium, indium, tin, thallium, lead, and bismuth. 

The actinide solid state properties are to a large extent based on the properties of the 5f wave-functions. Central to the actinide solid state research has been the co-existence of evidence and of concepts pointing clearly to the recognition of light actinides as being elements in which a metallic bond is enhanced by the overlapping of 5 f wave-functions. The narrow band, itinerant character of the 5 fs is similar to the one d-shells have hence, the classification of these elements as 5 f-transition metals. 

We shall note, that the difficulties arise precisely when modelling is to be applied to molecules involving transition metal atoms mainly of the second half of the first transition row. Moreover, even among the TMCs formed by these atoms the problems are not uniformly distributed the normal chemical nomenclature does not provide here an adequate classification. 

Nitrides can be sub-divided into ionic, covalent and interstitial types.An alternate general classification of nitrides, based on bonding classification, as ionic, covalent and metallic has also been applied. Ionic or salt-like nitrides are formed by electropositive elements such as Li, Mg, Ca, Sr, Ba, Cu, Zn, Cd and Hg and possess formulae which correspond to those expected on the basis of the combination of the metal ion with ions. A range of covalent nitrides are known and are exhibited by less electropositive elements such as B, S, P, C and Si. Interstitial nitrides are formed by some transition metals and refer to compounds which can be described in terms of the occupancy of interstitial sites in close packed metallic structures by nitrogen atoms. Oxygen can also be accommodated within these structures and a range of oxynitrides are known to... 

The use of this classification might help to identify the ways a selected compound might participate in late transition metal catalyzed transformations, and might also help to establish potential reaction partners. Although Table 2 suggests that there is an abundance of potential reactions for a given substrate, we have to emphasize that certain classes are well represented in the synthetic literature (e.g. 3, 8, 11, 17), while for other classes there are only a very limited number of examples. 

Although most transition metal catalyzed processes are built up of similar steps, they are usually divided into categories (sometimes name reactions) by the synthetic chemists. This classification is usually made on the basis of their synthetic utility rather than on mechanistic considerations. This chapter gives an overview of the most commonly used reactions, briefly outlining their mechanism as well as the scope and limitation of substrates in these processes. 

The abundance of iron as a transition metal in proteins is enormous and the diversity of functions performed by iron containing proteins is large. In terms of classification we follow the scheme of our previous reviews in forming two major groups, heme iron proteins and non-heme iron proteins. The latter class is divided into two sub-groups. One comprises iron-sulfur proteins and the other consists of those non-heme iron proteins which are not iron-sulfur proteins i.e. involve coordination with nitrogen and/or oxygen. We commence with the latter. 

The present discussion has been restricted to oxides of the lighter metals of the periodic table. At present, owing to lack of data, it is difficult to judge whether the classification can be applied to the second and third row transition metals and also it is not clear to which classification p block metal oxides belong. Possibly a third classification metallic to covalent should be included, and this would enable relating oxides to each other in terms of a Ketelaar triangle 33. ... 

The classification follows the extent of overlap of the electron cloud of the quencher with the fluorescer molecule. The overlap of -orbital of 08>solvated ions of transition metals >/-orbitals of solvated rare earth ions. 

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