Rotation about metal-ligand bond

Ligand Rotations about Metal-Ligand Bonds. 324... 

In Section IV,C,2 octa- and pentaphenyl-, as well as penta(isopropyl)-and penta(neopentyl)-Cp, were cited as examples for hindered rotations about substituent-C5 bonds. Half-sandwich metal complexes or sym-metallocenes with these ligands will be chiral when there is a barrrier for the interconversion of the enantiomers created by the phenyl canting or the isopropyl and neopentyl directionalities of the substituents (clockwise or counterclockwise) around the C5 ring. For penta(isopropyl)- and penta(neopentyl)-Cp this difference in directionalities is commonly sketched (24) by showing the methine proton only and omitting the methyl or ethyl groups for clarity (14). 

Three types of stereochemical lability have been observed (1) rotations about M-M bonds, (2) cis—r- trans isomerizations at each metal center and (3) bridge = terminal ligand exchange processes. 

Bipyridines without a chiral center have axial chirality if they exist as enantiomers due to the inability to rotate about a single bond (e.g., the 2,2 bond). These molecules are typically A-oxides (32),128,129 exist in the trans conformation (33),130,131 or have extremely bulky substituents at the 6 and 6 or 3 and 3 positions (34)132 and have limited utility as transition metal ligands. 

An extremely rare one-coordinate complex. Rotation about the single bonds linking the aromatic rings relieves the steric clashing of wo-propyl substituents (apparent in this drawing), and potentially opens up the other side of the metal for addition of another ligand. 

Ligand Rotations about the Metal-Ligand Bond. 341... 

In this complex, the molecule of coordinated ethylene occupies a single coordination site and is oriented perpendicularly to the plane of the square anion. In both complexes mentioned above (i.e., the complexes with two-center ligands and the olefin rr complexes), the bonding of ligands to the central atom occurs via the dihapto type. Moreover, the rotation of olefin about the metal-ligand bond was observed both in Zeise s salt and in some other ir complexes. Thus, the free energies for the activation of olefin (L) rotation in complexes (PR3)PtCl2 -L and PtCl(acac) - L depend on the nature of R and L, and vary within 44.4-67 kJ mol (10.6 16 kcal mol ) [80]. 

Ligand Rotation.—Rotation about metal-car bene bonds in cu-[M(CO)4(L )g]... 

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