Molecular transition metal complexes

All mechanisms proposed in Scheme 7 start from the common hypotheses that the coordinatively unsaturated Cr(II) site initially adsorbs one, two, or three ethylene molecules via a coordinative d-7r bond (left column in Scheme 7). Supporting considerations about the possibility of coordinating up to three ethylene molecules come from Zecchina et al. [118], who recently showed that Cr(II) is able to adsorb and trimerize acetylene, giving benzene. Concerning the oxidation state of the active chromium sites, it is important to notice that, although the Cr(II) form of the catalyst can be considered as active , in all the proposed reactions the metal formally becomes Cr(IV) as it is converted into the active site. These hypotheses are supported by studies of the interaction of molecular transition metal complexes with ethylene [119,120]. Groppo et al. [66] have recently reported that the XANES feature at 5996 eV typical of Cr(II) species is progressively eroded upon in situ ethylene polymerization. 

Some molecular transition-metal complexes of P , As3-, and Sb3 have been isolated as salts of cryptated alkali metal ions. The structures of the complex anions are shown in Fig. 15.3.9. The bond valence b and bond number of these complex anions are as follows ... 

The detailed description of all the proposed mechanisms is not the aim of this work (see Reference (i) for more details), but a few concepts are briefly discussed in the following (a) Scheme 11 may be read in two dimensions in the vertical direction, the evolution of the initial species upon addition of one ethene molecule is represented, whereas, in the horizontal direction, all the possible isomeric structures characterized by an average C fCv ratio equal to 2, 3, and 4 are reported, (b) In all the proposed reactions, the metal formally becomes Cr(IV) as it is converted into the active site. This hypothesis is supported by investigations of the interaction of molecular transition metal complexes with ethene (226,227). Furthermore, it has... 

Because this chapter focuses on molecular transition metal complexes that catalyze the formation of polyolefins, an extensive description has not been included of the heterogeneous titanium systems of Ziegler and the supported chromium oxide catalysts that form HDPE. However, a brief description of these catalysts is warranted because of their commercial importance. The "Ziegler" catalysts are typically prepared by combining titanium chlorides with an aluminum-alkyl co-catalyst. The structural features of these catalysts have been studied extensively, but it remains challenging to understand the details of how polymer architecture is controlled by the surface-bound titanium. This chapter does, however, include an extensive discussion of how group(IV) complexes that are soluble, molecular species polymerize alkenes to form many different types of polyolefins. 

A number of oxidation catalysts have been investigated in the context of solar water splitting, from crystalline metal oxides, to molecular transition metal complexes. Common to most of these is the presence of one or multiple transition metals centres, as in the OEC in PSII. 

Aminoboranes have been used as ligands in complexes with transition metals (66) in one instance giving a rare example of two-coordinate, non-t/ transition-metal complexes. The molecular stmcture of the iron complex Fe[N(Mes)B(Mes)2]2 where Mes = is shown in Figure 1. The... 

W. A. Nugent and J. M. Mayer, Metal-Eigand Multiple Bonds The Chemistry of Transition Metal Complexes Containing Oxo, Nitrido, Imido, Jilkylidene, orJilkylidyne Eigands,Jolm. Wiley Sons, Inc., New York, 1988. Contains electronic and molecular stmcture, nmr, and ir spectroscopy, reactions, and catalysis. 

Construction of one-dimensional multicomponent molecular arrays, transition metal complexes with terpyridines and/or porphyrins as ligands 98EJI1. 

Many transition metal-catalyzed reactions have already been studied in ionic liquids. In several cases, significant differences in activity and selectivity from their counterparts in conventional organic media have been observed (see Section 5.2.4). However, almost all attempts so far to explain the special reactivity of catalysts in ionic liquids have been based on product analysis. Even if it is correct to argue that a catalyst is more active because it produces more product, this is not the type of explanation that can help in the development of a more general understanding of what happens to a transition metal complex under catalytic conditions in a certain ionic liquid. Clearly, much more spectroscopic and analytical work is needed to provide better understanding of the nature of an active catalytic species in ionic liquids and to explain some of the observed ionic liquid effects on a rational, molecular level. 

An example of a serendipitous discovery in a field related to diazo chemistry is the first in vitro product of a reaction of molecular nitrogen with a transition metal complex (Allen and Senoff, 1965). As discussed in the context of diazo-metal complexes (Zollinger, 1995, Sec. 3.3), the metal —N2 bonds are similar to C —N2 bonds in organic diazo compounds. The paradigm that N2 is (almost) inert in chemical reactions probably explains why it took so long for N2 complexes to be discovered. ... 

Dioxygen activation in transition metal complexes in the light of molecular orbital calculations. R. Boca, Coord. Chem. Rev., 1983, 50,1-72 (245). 

Activation of molecular oxygen on interaction with transition metal complexes. A. V. Savitskii and V. I. Nelyubin, Russ. Chem. Rev. (Engl. Transl.), 1975,44,110-121 (124). 

Molecular orbital theory of transition metal complexes. D. A. Brown, W. J. Chambers and N. J. Fitzpatrick, Inorg. Chim. Acta, Rev., 1972, 6, 7-30 (193). 

Inclusion complexes of molecular transition metal hosts. T. J. Meade and D. H. Busch, Prog. Inorg. Chem., 1985,33, 59(331). 

C.J. Ballhausen, Molecular Electronic Structures of Transition Metal Complexes, McGraw-Hill, New York, 1979. 

Similar films are obtained from powdered molecular sieves loaded with organic molecules Zeolite Y microparticles embedded into a polystyrene film and loaded with appropriately sized transition metal complexes allow selective electron exchange reactions between trapped and mobile species in the film... 

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