Ligands attack

Chromium(II) has been extensively used in investigations of inner-sphere electron transfe because inert chromium(lll) captures the ligands attacked by chromium(II).70... 

Nucleophiles partition between the two mechanisms based on their hard-soft characteristics, with soft nucleophiles undergoing ligand attack and hard nucleophiles attacking at the metal. A limited class of nucleophiles appear capable of adding by either mechanism, with secondary factors controlling their choice of mode of addition. 

Alkenylzirconiums react with either preformed allylpalladium complexes155 or allyl acetates and Pd° catalysts156-395 via addition directly to the palladium, followed by reductive elimination. In the reaction with steroidal iT-allylpalladium complexes (equation 174) the natural configuration at C-20 is now obtained, in contrast with the addition of malonates (ligand attack) which yielded the unnatural configuration.74... 

Since enolates also add via a ligand attack process, the regioselectivity that they exhibit is quite comparable to soft caibon nucleophiles. Alkyl or aiyl substituents at the allyl termini direct attack to the less substituted terminus (equations 228-232) functional groups such as COaMe and halogen at one allyl terminus direct attack to the remote terminus (equations 233 and 234). Remote functionalities such as —OR also direct addition to the allyl terminus more removed from the substituent (equations 23S and 236). 

The addition of Grignards and organolithium reagents proceeds by attack at the metal center in ir-allylpalladium complexes. The regiochemical selectivity exhibited by these hard carbon nucleophiles with ir-allyl complexes substituted at the termini with alkyl or aryl groups is comparable to the soft carbon nucleophiles (ligand attack) in most cases, with addition occurring predominantly at the less substituted terminus (equations 248 and 249).1591387... 

Tor example, from Lever s formalism it can be directly calculated that Ru(ll/lll) redox potentials for derivatives formed by neutral ligand attack on the chlorobridges in ([Ru(CO)3Qj2) are similar to that of molecular fluorine ( ). 

The picture is more complicated when we apply Rule 3 to odd, open ligands. Attack may occur at the termini or internal positions depending on the electron richness of the attached metal fragment. Usually attack occurs at the terminal carbons of irf-allyl ligands, but for relatively electron-rich metal complexes such as [Cp2Mo(in3-allyl)]+ (equation 8.3551) and [Cp (PMe3)Rh(iq3-allyl)]+ (equation 8.3652), addition occurs at C-2 to form metallacyclic products. 

A more recent experimental and computational study of rf-allyl-Pd cationic complexes confirmed the tendency of nucleophiles to attack the terminal positions of the allyl ligand as long as jx-accepting ligands are present.54 If a-donating ligands are attached to palladium, which would pump electron density into the allyl ligand, attack occurs mainly at the C-2 position. Equations 8.37 and 8.38 summarize these results. 

Attack by different enantiofaces of prochiral nucleophiles if the nucleophile is an enolate ion, it has two prochiral faces that could attack the i(]3-allyl ligand attack from the top face will lead to one enantiomer and attack from the bottom face will lead to the other (step c, Scheme 12.10a or b). 

Two types of mineral dissolution (hydrolysis and ligand attack) give rise to mixed kinetics overall. These kinetics are (Furrer and Stumm, 1983)... 

Nucleophilic Attack. There are innumerable reactions involving anions or bases such as OH, OR", OCOR, N, R, NR3, N2H4, etc. and the ligands attacked may be CO, NO, RCN, RNC, alkenes, etc. It is not always certain that attack is direct, and prior coordination may well occur in some cases, so that then the reactions could be considered as intramolecular transfers. 

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