Cp2M + complexes, reactivity

There are important reactivity differences between CP2ML2 systems, depending upon whether the 1 orbital is filled or not. Consider reductive elimination in a dialkyl substituted complex. In 20.22 the orbitals of a CP2MR2 complex are listed. A correlation is drawn for reductive elimination to a CP2M complex along with the a and levels of an alkane in 20.23 where a least-motion pathway that conserves C2V symmetry is followed. The reaction is symmetry forbidden, a non-least-motioTi pathway must be followed, not unlike reductive elimination from an... 

Hydrogen chloride attacks the Mn—Pb bonds to form lead chloride486. To compare the reactivity of the M—H bond with the M—Pb bond in the [(Cp2MH)2(Ai-Pb(02CMe)2] (M=Mo, W), the complexes were reacted with cyanoalkyne, HC=C—CN however, the only isolated product was Cp2M C(CN)=CH2 2515. 

Since the mid-1980s the chemistry of related cationic 16-electron Cp2M(R)(L) complexes (1) and base-free 14-electron Cp2M(R) complexes (2) (M = Ti, Zr) has been developed (/O). These complexes are considerably more reactive than their neutral counterparts as a result of the increased Lewis acidity of the cationic metal center, as well as the presence of the labile ligand L in 1 and the increased unsaturation of 2. These features promote coordination and activation of olefins, acetylenes, H, C— H bonds, and other substrates, and they open reaction pathways which are unavailable to neutral 16-electron analogs. Cationic complexes 1 and 2 are thus classified as highly electrophilic metal alkyls. 

A 16-electron, d° Cp2M(R)X species contains one low-lying metal-centered LUMO localized in the equatorial plane between the Cp ligands which is utilized for substrate coordination (Fig. 1) (11). Zr—X r-bonding (X = OR, halide, etc.) utilizes this orbital and generally results in decreased reactivity (5). The electronic structure of a Cp2Zr(R)(L) complex is similar to that of neutral analogs, but the LUMO is stabilized by the metal... 

In general, d silyl complexes are reactive toward primary and secondary silanes. We have found that a number of metallocene derivatives of the type Cp2M(SiR3)R (Cp = Cp, Cp s TlS-CsMes M = Zr, Hf R = Me, Ph, SiMes R = Cl, alkyl, silyl) are catalyst precursors for the dehydrogenative polymerization of silanes. An example of such a polymerization is shown in equation 3. The polymer s molecular weight... 

Titanium and zirconium chemistry is conveniently divided between simple complexes and those based on the metallocene imit Cp2M (Cp = CsHs = cyclopentadienyl). Most simple complexes are oligomeric, insoluble, and difficult to characterise, although alkyl titanium complexes such as X3TiR have found some use as non-basic Grignard equivalents. The dicyclopenta-dienyl metal moiety, Cp2M, renders complexes monomeric, soluble, and easily characterised by NMR spectroscopy, and thus many applications based on these systems have been devised. The most stable electronic configuration of titanocene and zirconocene complexes has only 16 electrons in the valence shell, not the 18 electrons common in most of the rest of the transition metal series. The empty orbital this leaves on the metal is crucial for reactivity. 

Kool LB, Rausch MD, Alt HG et al (1987) Preparation, characterization and reactivity of Cp2M(PMe3)2 complexes (M = Ti, Zr) the molecular structure of Cp2Zr(PMe3)2. J Organomet Chem 320 37 5... 

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