The Early Transition Metals

A model olefin polymerization catalyst previously characterized as [(C5H5)2TiAlEt2]2 has been studied by X-ray diffraction, H n.m.r., and mass spectral techniques. The compound contains (l- -CjHj) and/i(l- t-C5H5) rings and is the first well-characterized dimeric titanium-aluminium hydride structure (3) has been suggested. Alkyl exchange between a polymeric alkyl-titanium compound and alkylaluminium compound, present in excess, is 

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This chemical bond between the metal and the hydroxyl group of ahyl alcohol has an important effect on stereoselectivity. Asymmetric epoxidation is weU-known. The most stereoselective catalyst is Ti(OR) which is one of the early transition metal compounds and has no 0x0 group (28). Epoxidation of isopropylvinylcarbinol [4798-45-2] (1-isopropylaHyl alcohol) using a combined chiral catalyst of Ti(OR)4 and L-(+)-diethyl tartrate and (CH2)3COOH as the oxidant, stops at 50% conversion, and the erythro threo ratio of the product is 97 3. The reason for the reaction stopping at 50% conversion is that only one enantiomer can react and the unreacted enantiomer is recovered in optically pure form (28). 

Extended metal-metal bonding in halides of the early transition metals. J. D. Corbett, Acc. Chem. Res., 1981,14, 239-246(48). 

B. Amidinate and guanidinate complexes of the early transition metals... 

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,... 

Keppert, D. L., The Early Transition Metals, Academic Press, New York, 1972. 

Bis(cyclopentadienyl) complexes are central to the organometallic chemistry of the early transition metals and feature in applications such as alkene polymerization chemistry. Parallels can be drawn between a porphyrin ligand and two cyclopentadienyl ligands, in that they both contribute a 2— formal charge and exert a considerable steric influence on other ligands in the same molecule. Several of the metalloporphyrin complexes discussed below have bis(cyclopentadienyl) counterparts, and authors in some ca.ses have drawn quite detailed comparisons, although these discussions will not be repeated here. 

The last decade has seen the development of a rich and varied chemistry for or-ganometallic porphyrin complexes of the early transition metals (groups 3 and 4). However, there have been many fewer developments in the organometallic chemistry of the middle transition elements. Despite the paucity of its organometallic porphyrin compounds, molybdenum has played a very important role in... 

Perhaps the most important chemical property of these complexes is their potential as catalysts, particularly of the early transition metal isoleptic compounds for a-olefin polymerization. This arises because unlike the methyls, they are sufficiently stable to be used at temperatures where polymerization rates are adequate. Some data are summarized in Table VIII ( 9) TT-acceptor ligands are clearly disadvantageous. It will be seen that some of the systems are more active than Ziegler types, although stereoselectivity is poorer. 

Two principle strategies have been employed for the synthesis of siloxide-containing molecular precursors. The first involves a silanolysis, or condensation, reaction of the Si - OH groups with a metal amido, alkyl, hahde, or alkoxide complex. The second method involves salt metathesis reactions of an alkali metal siloxide with a metal hahde. Much of our work has been focused on formation of tris(tert-butoxy)siloxide derivatives of the early transition metals and main group elements. The largely imexplored regions of the periodic table include the lanthanides and later transition metals. 

Terminal methylene complexes are relatively rare—less than 10 such compounds have been isolated and about as many again have been characterized by spectroscopic techniques only. The methylene complexes previously reported fall into two groups, (i) neutral complexes of the early transition metals (e.g., Ti, Ta) and (ii) cationic complexes of the later transition metals (e.g., Re, Fe). The osmium complex 47 is important, then, as it is a new example extending the neutral group to the later transition metals. Compound 47 is the prototype for the series Os(=CHR)Cl(NO)(PPh3)2 and is one of only three terminal methylene complexes to be structurally characterized by X-ray crystallography (see Section IV,B). 

The oxophilic character of the early transition metals may provide such stabilization in the form of dihapto binding (I). This unusual donor behavior was first... 

The formation of metal-oxygen bonds has previously been found to occur for the stoichiometric hydrogenation of CO to methanol with metal hydrides of the early transition metals (20). Moreover, in ruthenium-phosphine catalyzed hydrogenation (with H2) of aldehydes and ketones, metal-oxygen bonded catalytic intermediates have been proposed for the catalytic cycle and in one case isolated (21,22). 

Although orbital hybridizations and molecular shapes for hypovalent metal hydrides of the early transition metals and the normal-valent later transition metals are similar, the M—H bonds of the early metals are distinctly more polar. For example, metal-atom natural charges for YH3 (+1.70), HfH4 (+1.75), and TaHs (+1.23) are all significantly more positive than those (ranging from +0.352 to —0.178) for the homoleptic hydrides from groups 6-10. Indeed, the empirical chemistry of early transition-metal hydrides commonly reveals greater hydricity than does that of the later transition-metal hydrides. 

Values for the enthalpy of solution of hydrogen in transition metals at infinite dilution shown in Figure 7.22 are more negative for the early transition metals. It should be noted that the enthalpies of solution in general are functions of the concentration of the solute. Still, the values at infinite dilution are useful when looking for systematic variations, particularly since changes with composition are often limited. 

Compounds and complexes of the early transition metals are oxophilic because the low d-electron count invites the stabilization of metal-oxo bonds by 7T-bond formation. To a substantial extent, their reactivity is typical of complexes of metals other than rhenium. That is particularly the case insofar as activation of hydrogen peroxide is concerned. Catalysis by d° metals - not only Revn, but also CrVI, WVI, MoVI, Vv, ZrIV and HfIV - has been noted. The parent forms of these compounds have at least one oxo group. Again the issue is the coordination of the oxygen donating substrate, HOOH, to the metal, usually by condensation ... 

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