Transition metal compounds bonding mechanisms

Sigma-bonded transition metal complexes are able to polymerize a range of vinyl monomers, the only limitation being that the monomer should not have groups that react chemically with the transition metal compound. An important observation is that styrene and its derivatives are polymerized by the sigma complexes. In this respect they differ from the jr-allyl compounds that show no reactivity at all toward these monomers. A reasonable explanation for this is that the mechanism of the initiation is different... 

Comparative data of the mechanism of polymerization on active centers containing non-transition metals. Olefin insertion into metal-carbon bond is also a key step not only for catalytic polymerization but also for olefin oligomerization with organoaluminium compounds. The latter process includes the same steps as the catalytic polymerization in the presence of transition metal compounds... 

In Ziegler-Natta catalysts comprising transition metal compounds and coinitiators such as alkylaluminium, the active species consists of metal-carbon bonds formed by the alkylation of the transition metal with the coinitiator. Initiation occurs by monomer insertion to the metal-carbon bond. Therefore, labelling of coinitiator may provide an opportunity to investigate the structure of end-groups and thus the initiation mechanism. 

Hietkamp S, Stufkens DJ, Vrieze K (1979) Activation of C-H bonds by transition metals V. A study of the mechanism of metalation reactions of benzyl- and rneta-fluorobenzylphosphines with Rh(I), Ir(I), Pd(II), and Pt(II) compounds. J Organomet Chem 168 351-361... 

The mechanisms of the usual organic reactions are now clearly established, and the reactions are classified as ionic, radical, and molecular. More detailed classifications have also been made. The mechanisms of many reactions involving non-transition metal compounds are clear enough for example, in the Grig-nard or Reformatsky reaction, the first step is the irreversible oxidative addition of alkyl halides to form Mg-carbon or Zn-carbon bonds, in which the carbon is considered to be a nucleophilic center or carbanion which reacts with various electrophiles. 

This chapter is devoted to pseudopotential calculations of molecular excited states, however, some words should be said about spin-orbit effects on the groimd state when dealing with the reactivity of open-shell systems. Even though, the reactivity of transition metal compounds has been extensively studied (see for example Ref [115]), spin-orbit effects are quite rarely taken into accoimt. This can be imderstood from the fact that in all the considered reactions, such as for example inert bond activation through the classical oxidative addition-reductive elimination mechanism, both the reactants and the products are closed-shell systems. In this case, spin-orbit contributions on the... 

As mentioned above such simple conclusions are not easily drawn in the case of transition metal compounds because of charge transfer into d as well as into outer metal orbitals. In addition, bonding mechanisms may be significantly different in different coordinations. A meaningful comparison between M and later transition elements is also difficult to draw. One or more of oxidation state, spin state, structure, or coordination geometry are generally different. It does appear, however, that the Ad and 5d covalency is similar in chlorides and bromides of the elements Pd and Pt for which significant comparisons are available. 

A mixture of metallic, covalent and ionic components prevails in the bonding of transition metal carbides, nitrides, and carbonitrides. The metallic character is shown by the high electrical conductivities of these compounds. The bonding mechanism has been described extensively by a variety of approaches for calculating the density of states (DOS) and hence the electron density in f.c.c. transition metal carbides, nitrides, and oxides [11]. In the DOS of these compounds there is a minimum at a valence electron concentration (VEC) of 8, which corresponds to the stoichiometric composition of the group ivb carbides TiC, ZrC, and HfC. Transition metal carbides have a lower DOS at the Fermi level than the corresponding transition metal nitrides, hence the electrical properties such as electrical and thermal conductivity and the superconducting transition temperature, T, are lower than those of the nitrides. 

Polymer Chain Growth. The essential characteristic of Ziegler-Natta catalysis is the polymerization of an olefin or diene using a combination of a transition-metal compound and a base-metal alkyl cocatalyst, normally an aluminum alkyl. The function of the cocatalyst is to alkylate the transition metal, generating a transition-metal-carbon bond. It is also essential that the active center contains a coordination vacancy. Chain propagation takes place via the Cossee-Arlman mechanism (23), in which coordination of the olefin at the vacant coordination site is followed by chain migratory insertion into the metal-carbon bond, as illustrated in Figure 1. 

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