Transition-metal atoms

Morse M D 1986 Clusters of transition-metal atoms Chem. Rev. 86 1049... 

Some transition metal atoms combined with uncharged molecules as ligands (notahiv carbon monoxide. CO) have a formal oxidation state of 0. for example Ni + 4CO Ni"(CO)4. 

The one exception to this is the INDO/S method, which is also called ZINDO. This method was designed to describe electronic transitions, particularly those involving transition metal atoms. ZINDO is used to describe electronic excited-state energies and often transition probabilities as well. 

Since the coordination almost certainly involves the transition metal atom, there is a resemblance here to anionic polymerization. The coordination is an important aspect of the present picture, since it is this feature which allows the catalyst to serve as a template for stereoregulation. 

Transition metal atoms are distinguished from other atoms by their having partially filled 3d, Ad or 5d orbitals. Here we consider only metals of the first transition series. Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn, in which the 3d orbital is involved. 

The bonding between carbon monoxide and transition-metal atoms is particularly important because transition metals, whether deposited on soHd supports or present as discrete complexes, are required as catalysts for the reaction between carbon monoxide and most organic molecules. A metal—carbon ( -bond forms by overlapping of metal orbitals with orbitals on carbon. Multiple-bond character between the metal and carbon occurs through formation of a metal-to-CO TT-bond by overlap of metal-i -TT orbitals with empty antibonding orbitals of carbon monoxide (Fig. 1). 

In some materials, semiconductors in particular, interstitial atoms play a crucial role in diffusion. Thus, Frank and Turnbull (1956) proposed that copper atoms dissolved in germanium are present both substitutionally (together with vacancies) and interstitially, and that the vacancies and interstitial copper atoms diffuse independently. Such diffusion can be very rapid, and this was exploited in preparing the famous micrograph of Figure 3.14 in the preceding chapter. Similarly, it is now recognised that transition metal atoms dissolved in silicon diffuse by a very fast, predominantly interstitial, mechanism (Weber 1988). 

P4) is closely similar with P-P distances of 216 pm (smaller than for P4 itself, 221pm). Indeed, a whole series of complexes has now been established with the same structure-motif and differing only in the number of valency electrons in the cluster some of these are summarized in Table 13.11. The number of valence electrons in all these complexes falls in the range 30-34 as predicted by R. Hoffmann and his colleagues.Many other cluster types incorporating differing numbers of Group 15 and transition metal atoms are now known and have been fully reviewed. ... 

Metal-directed self-assembly of two- and three-dimensional synthetic receptors (macroheterocycles involving transition metal atoms, among them chelated atoms) 98CSR417. 

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. 

In the present work, we report on a new semi-empirical theoretical approach which allows us to perform spin and symmetry unconstrained total energy calculations for clusters of transition metal atoms in a co .putationally efficient way. Our approach is based on the Tight Binding Molecular Dynamics (TBMD) method. 

This is the case for K and Ca but not for subsequent transition metal atoms [31],... 

There are two basic differences of (sic) free atoms and chemically bound atoms. First, the more diffuse an AO, the stronger it is perturbed in molecular and condensed matter. The (n + )s AOs of the transition metal atoms, especially of the earlier ones, are not of primary importance for chemical bonding. Their relevance is comparable to that of the diffuse orbitals of main group elements ([34], p 653). 

Second, metal atoms carry some positive charge in the majority of their compounds. Transition metal cations have pure d configurations, in contrast to the mixed d-s configurations of free neutral transition metal atoms. There is the chemical rule that "s electrons fall down into the d level... 

It should be noted that we have confined ourselves to the simplest reaction intermediates, namely, complexes involving only one transition metal atom and two alkene molecules. If the possibility of two transition metal atoms is taken into account the following complexes seem most likely ... 

Optical activity from asymmetric transition metal atoms. H. Brunner, Angew. Chem., Int. Ed. Engl., 1971,10, 249-260 (88). 

Transition metal atoms in the synthesis of binuclear complexes. E. P. Kiindig, M. Moskvits and... 

The above results for ZnS, CdS, and HgS indicate that an additional electron in a transition-metal atom increases its radius by 0.03 A. 

Strictly speaking, a cluster complex, as generally considered in or-ganometallic chemistry, consists of a framework of more than two transition-metal atoms. However, in this Section, we shall ignore tra-... 

Moving from alkenes to alkynes, it was found that a variety of transition-metal atoms react with hexafluoro-2-butyne (HFB) to form new... 

In addition to complex-formation, the interaction of transition-metal atoms with organic substrates at low temperatures can result in rearrangement of the organic moiety without complexation. Two such reactions have already been briefly mentioned, namely, the polymerization of hexafluoro-2-butyne by Ge and Sn atoms (72) and the polymerization of styrene by Cr atoms (i 1). In this section we shall briefly summarize some of these transition-metal-atom-promoted, organic rearrangements. 

Transition-metal atoms have been shown to deoxygenate epoxides to alkenes (36). Chromium and titanium atoms emerged as the most effective species in this regard, abstracting over two equivalents of oxygen. By studying the reaction of a wide range of epoxides with chromium atoms, the reaction... 

An interesting reaction involves the cocondensation of transition-metal atoms with liquid methylphenylsiloxane polymers at -20 to 0°C... 

---