Anionic ligands mechanism

The specific role of the electrolyte composition at a given pH is an extremely common experience. The contribution of anions other than hydroxyl is expected through the acid-catalyzed or anion-ligand mechanism [24,25]. It must be emphasized that the role of pH in polyacids, such as H2SO4, may be obscured by the correlated changes of electrolyte composition depending on acidity constants [86],... 

Several authors have suggested that the pathway may prove to be the most common mechanism in substitution reactions of octahedral complexes generally. However, the D path can be clearly demonstrated in some cases including at least two examples from Co(III) chemistry. The path (I - III - IV, Fig. 7) through the fivecoordinate intermediate would lead, in the case of rate studies in the presence of excess anionic ligand, to observed first-order rate constants governed by equation (13)... 

The mechanism of substitution of anionic ligands on [Cr(NH3)5H20]3+ has received both Id and Ia assignments from various authors (23). This dichotomy of viewpoint comes about substantially... 

Transmetalation, though, requires enhanced electrophilicity of the Pd. Additionally a free coordination site may be required, which may be freed by dissociation of either a neutral or an anionic ligand. The involvement of five-coordinate species and association-dissociation ligand-exchange mechanisms in the individual steps of Pd-catalyzed reactions also cannot be neglected (Scheme 3).384... 

Both five-coordinate and four-coordinate pathways have been proposed for these reactions. The associative (five-coordinate) mechanism involves the formation of a trigonal bipyramidal or square pyramidal intermediate, which can revert back to tetracoordination by alkene insertion into the Pt—H bond.151 The dissociative (four-coordinate) mechanism involves initial substitution of a ligand other than hydride by alkene, followed by insertion to form the alkyl product. The ligand which is substituted is usually the anionic ligand, and if this group is trans to hydride an isomerization will need to occur prior to insertion of the coordinated alkene into the Pt—H bond. 

A key feature of the cationic mechanism is that removal (or dissociation) of an anionic ligand from the palladium coordination sphere allows alkene complexation to occur while maintaining coordination of both phosphines of a bisphosphine ligand. That both phosphines can be accommodated in a square-planar four-coordinate intermediate during the insertion step has provided a simple rationalization for the higher enantioselectivities often observed for the cationic pathway. Concrete information on the enantioselective step of asymmetric Heck reactions proceeding by the cationic pathway has not been reported to date. It is likely to be either coordination of the alkene to generate 20.S or the insertion step (20.5 —> 20.6, Scheme 8G.20). 

The Mechanism of Nitrite Ion Binding to Metmyoglobin Under some physiological conditions, NO can be rapidly metabolized to nitrogen species in the + 3 (NO2 ) and + 5 (NO, ) oxidation states.319 It is of chemical and biological interest to establish the similarities or differences of the interaction of NO and the nitrite ion with metMb. Earlier kinetics and thermodynamics studies of these reactions were undertaken but no firm mechanistic conclusions were reached.320 Two anionic ligands, azide and cyanide, exhibit different reactivities toward metMb, and... 

Despite the differences in mechanism, the expectation that thiolate ligands will act as nucleophiles appears to be a feature of the oxidation of [Ni(L)CN] by 02. The strong tendency of Ni thiolates to form dimers and higher polymers (80, 92-94) and the fact that the presence of a tightly bound anionic ligand appears to be required to cleave the di-nuclear complexes (e.g., CN or thiolate (85)) is evidence of the nu-cleophilicity of terminal thiolate ligands in planar Ni(II) complexes. 

Oxalate (ox, 204 ) complexes of Cr° have been known since the very beginning of coordination chemistry. Thus, the resolution of chiral [Cr(ox)3] with strychninium counterion by Werner in 1912 prodnced the first optically active anionic coordination compound. There also exists a series of bis(oxalato) complexes of the type [Cr(ox)2X2] + , where X can be any of a variety of nentral donors (e g. H2O, NH3, etc.) or anionic ligands (e g. SCN, N3, etc.). These compounds have been used to study the mechanism of cis/trans isomerization and racemization of optically active octahedral coordination compounds. 

---