Ligands rotation

A new generation coordination catalysts are metallocenes. The chiral form of metallocene produces isotactic polypropylene, whereas the achiral form produces atactic polypropylene. As the ligands rotate, the catalyst produces alternating blocks of isotactic and atactic polymer much like a miniature sewing machine which switches back and forth between two different kinds of stitches. 

The same holds tme for all conformations between these two extremes meaning that the activation energy for ligand rotation is very low (ca. 0.9 0.3 kcal/mol) [10, 11]. In the gas phase the eclipsed conformation is preferred, while for solid state structures of substituted derivatives, preference for one conformation is often due to packing forces or interactions of the various substituents. 

Ferrocen-l,l -diylbismetallacycles are conceptually attractive for the development of bimetal-catalyzed processes for one particular reason the distance between the reactive centers in a coordinated electrophile and a coordinated nucleophile is self-adjustable for specific tasks, because the activation energy for Cp ligand rotation is very low. In 2008, Peters and Jautze reported the application of the bis-palladacycle complex 56a to the enantioselective conjugate addition of a-cyanoacetates to enones (Fig. 31) [74—76] based on the idea that a soft bimetallic complex capable of simultaneously activating both Michael donor and acceptor would not only lead to superior catalytic activity, but also to an enhanced level of stereocontrol due to a highly organized transition state [77]. An a-cyanoacetate should be activated by enolization promoted by coordination of the nitrile moiety to one Pd(II)-center, while the enone should be activated as an electrophile by coordination of the olefinic double bond to the carbophilic Lewis acid [78],... 

Activation Parameters for 4-Diene Ligand Rotation in [( 5-C,H5)W(CO)(COCHj)( 4-... 

Fig. 36 Top Depiction of the co angle used in [330], and how it reduces D3A symmetry to D3. Starting with parallel ligands, rotation of L about its C2 axis by co / 0/90/180° will result in a chiral structure. Bottom Depiction of how the s, h, and

Kinetics of Iron(III) Porphyrin Axial-ligand Rotation and Porphyrin Ring Inversion... 

Normally, an H2 ligand rotates too fast about the M-(H2) bond for separate rotamers to be observable. In d complexes, however, where only one d orbital is available, back donation is only possible in one plane but not at right angles to that plane. This means that the H2 loses all the back donation stabilization when it has rotated 90° from its preferred conformation. This in turn provides a significant rotation barrier leading to the phenomenon of blocked rotation, detected by Chaudret and Sabo-Etiemie in compounds of the type [Cp2TaH2L]+ (3), where barriers of up to 11 kcalmoR were found. 

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