Ligand properties aryl ligands

The series of wide-bite-angle, bulky ligands derived from a cyclobutene scaffold gave Pd complexes (117) showing appreciable activity in the cross-coupling of reactive aryl bromides with trimethylsilylacetylene. A considerable shift of electron density to the phosphorus atoms, probably arising from alternative aromatic canonical structures, may account for the ligand properties.422... 

As a ligand class, the aryls have usually been treated in the same context as the alkyls. In 1991, however, a review,8 which dealt exclusively with aryl transition metal complexes, drew attention to their often unique structures and reactivity. It was also apparent that the influence of steric properties of aryl ligands had not been extensively investigated for transition-metal complexes. The bulkiest aryl ligand to be widely employed with these metals... 

The ylides and imides are present as monomers, and the bismuth center adopts a distorted tetrahedral geometry. In contrast, the structural properties of the bismuth oxides vary widely depending on the aryl ligands attached to the bismuth center the oxides exist as hydrates, dimers, or polymers in solution and in the solid state. X-ray structural analysis of an oxide dimer revealed that the bismuth center has a distorted, trigonal bipyramidal geometry with the two oxygen atoms at the apical and equatorial positions [47, 48]. 

The electronic properties of both alkyl [5] and aryl alcohols [6] play a clearly definable role in ester formation, with formation constants decreasing with increase in electron withdrawing ability of the ligand. For both types of ligands, the electronic influences are quite small, but the resonance effects found with the aromatic ligands indicate there are 71-electron contributions to the empty d orbitals of vanadate [6], The influences of the electronic properties of ligands on coordination mode and geometry are discussed in detail in Chapter 9. 

Arylated cyclopentadienes, especially pentaarylcyclopentadienes 6, have interesting properties as ligands for transition metals [1] or as electroluminescent materials [2], The classical methods for their preparation are multistep procedures [3], which are somewhat tedious and usually less suitable for sterically demanding aryl groups. In contrast, the palladium-catalyzed arylation of either metallocenes such as zirconocene dichloride (1) or simply cyclopentadiene (3) with aryl halides leads to the target compounds 4—6 in a single preparative step (Scheme 1) [4—6]. 

The superior donor properties of amino groups over alkoxy substituents causes a higher electron density at the metal centre resulting in an increased M-CO bond strength in aminocarbene complexes. Therefore, the primary decarbo-nylation step requires harsher conditions moreover, the CO insertion generating the ketene intermediate cannot compete successfully with a direct electro-cyclisation of the alkyne insertion product, as shown in Scheme 9 for the formation of indenes. Due to that experience amino(aryl)carbene complexes are prone to undergo cyclopentannulation. If, however, the donor capacity of the aminocarbene ligand is reduced by N-acylation, benzannulation becomes feasible [22]. 

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