Transition metal complexes metallocenes

Section 14 14 Transition metal complexes that contain one or more organic ligands offer a rich variety of structural types and reactivity Organic ligands can be bonded to a metal by a ct bond or through its it system Metallocenes are transition metal complexes m which one or more of the ligands is a cyclopentadienyl ring Ferrocene was the first metallocene synthesized Its electrostatic potential map opens this chapter... 

Metallocene (Section 14 14) A transition metal complex that bears a cyclopentadienyl ligand Metalloenzyme (Section 27 20) An enzyme in which a metal ion at the active site contributes in a chemically significant way to the catalytic activity... 

Metallocene (Section 14.14) A transition metal complex that bears a cyclopentadienyl ligand. 

Stable transition-metal complexes may act as homogenous catalysts in alkene polymerization. The mechanism of so-called Ziegler-Natta catalysis involves a cationic metallocene (typically zirconocene) alkyl complex. An alkene coordinates to the complex and then inserts into the metal alkyl bond. This leads to a new metallocei e in which the polymer is extended by two carbons, i.e. 

Very little is known as yet of the chemistry of cyclopentadienylthallium(I) and the related compounds listed in Table I. The parent compound gives tribromocyclopentane on treatment with bromine and the hexabromo derivative with potassium hypobromite 112). By far the most important use discovered so far for these organothallium(I) compounds is the preparation of metallocenes and cyclopentadiene-transition metal complexes. These preparations are, in general, characterized by manipulative simplicity and high yields, and details of the reactions reported thus far are summarized in Tables II-IV. 

A60. J. P. Candlin, K. A. Taylor, and D. T. Thompson, "Reactions of Transition-Metal Complexes. Elsevier, Amsterdam, 1968. A review of types of reactions of metal complexes (e.g., substitution, combination, redox) reactions with various reagents (e.g., hydrocarbons, halides, carbon monoxide, and isonitrile) and preparation of new stabilised organic systems (e.g., metallocenes, carbenes). Intended for research workers, consequently written at a fairly high level, with emphasis on organometallics. A61. H. J. Keller, NMR-Untersuchungen an Komplexverbindungen. Springer, Berlin, 1970. Expansion of review article 37.1. 

Baik and Friesner used an SCRF procedure in conjunction with B3LYP density functional calculations to obtain electrode potentials for groups of aromatic molecules, metallocenes and transition metal complexes in four different solvents 132 the average absolute deviation was about 0.15 volts for a range of values of 3.82 volts. Saracino et al. computed pKa for a series of carboxylic acids with an average absolute deviation of 0.41 for pKa between 1.23 and 5.03.133... 

As mentioned in the introduction, early transition metal complexes are also able to catalyze hydroboration reactions. Reported examples include mainly metallocene complexes of lanthanide, titanium and niobium metals [8, 15, 29]. Unlike the Wilkinson catalysts, these early transition metal catalysts have been reported to give exclusively anti-Markonikov products. The unique feature in giving exclusively anti-Markonikov products has been attributed to the different reaction mechanism associated with these catalysts. The hydroboration reactions catalyzed by these early transition metal complexes are believed to proceed with a o-bond metathesis mechanism (Figure 2). In contrast to the associative and dissociative mechanisms discussed for the Wilkinson catalysts in which HBR2 is oxidatively added to the metal center, the reaction mechanism associated with the early transition metal complexes involves a a-bond metathesis step between the coordinated olefin ligand and the incoming borane (Figure 2). The preference for a o-bond metathesis instead of an oxidative addition can be traced to the difficulty of further oxidation at the metal center because early transition metals have fewer d electrons. 

An exhaustive treatment of the electrochemical behaviour of transition metal complexes is beyond the scope of this book, because the enormous number of ligands available, combined with the possibility to prepare mono- and/or polynuclear complexes using identical or mixed ligands, would render such a task almost impossible. Therefore, the discussion is limited to some aspects associated with the redox properties of (essentially) mononuclear metal complexes. In particular, we will concentrate representatively on the redox changes of first row transition metal complexes (excluding the metallocene complexes, as they have been already discussed in Chapter 4) that give stable, or relatively stable products. A systematic and useful examination of the redox activity of organometallic complexes of transition metals dated to 1984 has appeared.1... 

One of the most important carbon 7i-donors are the cyclopentadienes and their heteroanalogues, for instance 15 and 16. In general, cyclopentadiene itself forms three general types of mononuclear Cp transition metal complexes Cp2M 3 (symmetric molecules with mutually parallel Cp rings examples M = Fe, Cr, Ni), Cp2ML 124 [bent metallocenes, L = H, R, CO, etc. (n=l-3)], and half-sandwich compounds CpML 125 (n — 1 4) [21b], In the bent sandwich complexes... 

The electronic spectrum is yet another property which illustrates the similarities between the metallocenes and (7r-ollyl) metal compounds. In Table VI are listed some data for a series of Coin(absorption bands with the small extinction coefficients are probably two of the spin-allowed d-d transitions. Scott (34) has developed an approximate axial ligand field model for the carborane-transition metal complexes and has discussed the optical spectra in relation to this bonding theory. The actual assessment of bonding in the (7r-ollyl) metal compound as well as the metallocenes would be greatly aided by accurate assignments of the electronic spectra. 

Michalak A, Ziegler T, Theoretical Studies on the Polymerization and Copolymerization Processes Catalyzed by the Late-Transition Metal Complexes, in Beyond Metallocenes Next-Generation Polymerization Catalysts ACS Symp Series 857, edited by AO Patil, GG Hladky (American Chemical Society, Washington 2003), pp 154-172... 

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