Anion neutral donors

Both electrophilic and nucleophilic reactions can generate halogenopur-ines with differences in regioselectivity dependent on substituents and on the nature of the substrate (anion, neutral molecule, or cation). In the neutral molecule nucleophilic displacements occur in the order 2 > 4 > 6 in the anion the imidazole ring may be sufficiently 7r-excessive for attack to occur at C-2, and the nucleophilic substitution order becomes 4 > 6 > 2. Strong electron donors are usually necessary to promote 2-halogenation by electrophilic halogen sources. 

Structural types for organometallic rhodium and iridium porphyrins mostly comprise five- or six-coordinate complexes (Por)M(R) or (Por)M(R)(L), where R is a (T-bonded alkyl, aryl, or other organic fragment, and Lisa neutral donor. Most examples contain rhodium, and the chemistry of the corresponding iridium porphyrins is much more scarce. The classical methods of preparation of these complexes involves either reaction of Rh(III) halides Rh(Por)X with organolithium or Grignard reagents, or reaction of Rh(I) anions [Rh(Por)] with alkyl or aryl halides. In this sense the chemistry parallels that of iron and cobalt porphyrins. 

Much of the current research into halogen bonding involves organohalogen or inorganic halide acceptors. However, the dihalogens (X2) and inter halogens (XY) continue to attract attention. This review will focus specifically on structural and theoretical studies of such systems over the past decade. While polyhalide anions are closely related to the neutral donor-acceptor concepts that are the focus here, they will be discussed only when necessary to elaborate on the central theme of the review. 

The cited reactions may be considered as ligand exchange reactions, in which either a neutral donor D replaces an anion donor X at M or a neutral acceptor A replaces the cation at X. 

Since the reactions may be described as ligand exchange reactions, the competition for coordination between neutral donor molecules and (ionized) anions... 

In Table 5 the values for the free standard enthalpies for the reactions of neutral donors and anion donors with vanadyl acetylacetonate are listed. It can be seen that towards the reference molecule iodide ion is a somewhat weaker ligand than propanediol carbonate, whereas the bromide ion is between tri-methylphosphate and acetone, and the chloride ion between DMF and DMSO 22>. The fluoride ion and the NCS -ion are stronger donors than all neutral donors but are somewhat weaker than the azide and the cyanide ion. 

The ionization of trimethyltin iodide by neutral donors is an example of a heterolytic fission of a covalent bond. The ionization process is more complicated if the substrate contains more than one ionizable bond, in particular, if the anions formed are capable of competing successfully with the donor molecules for coordination at the substrate. If they are successful both complex cations and complex anions are formed and this process is known as autocomplex formation or ligand disproportionation ... 

Normally, the reaction partners in PET reactions are neutral molecules. That is why a donor radical cation—acceptor radical anion pair is obtained by the PET step. These highly reactive intermediates can be used for triggering interesting reactions. Since the PET is not restricted to neutral molecules PET reactions of donor anions and neutral acceptors or neutral donors and acceptor cations resulting in radical—radical anion (cation) pairs are known as well. These reactions are also called charge shift reactions due to the fact that the overall number of charged species is kept constant throughout the PET step. Finally, a PET process of a donor anion and a acceptor cation is possible as well (Scheme 2). 

The use of several new types of amido ligands that are based on a multiplicity of anionic and/ or neutral donor sites has been a major feature of transition metal amide chemistry since around 1930 5-9.17,18.22.23,26,30,31.33.36.37.40.44.46 8,52.53 develop-... 

Scheme 19 shows a general mechanism for C—H bond activation. In principle, any donor groups, including olefinic bonds, carbanions, heteroatom anions, neutral heteroatoms, for example, can activate their adjacent C—H bonds through coordination with appropriate transition metal centers. The metal hydride complexes formed by oxidative addition or /3-elimination, undergo unique chemical transformations. 

Anions such as sulfate and nitrate bond weakly to palladium, forming complexes which dissociate to a significant extent in water or any donor solvent. They may be isolated from concentrated aqueous solutions following reaction of [PdC Lj (L = neutral donor) with AgX (X = S04 or NO3).126-128 A variety of complexes containing triphenylphosphine may be prepared by reactions of [Pd(02)(PPh3)2] with the appropriate oxide (equations 12-14). 

Excited states of carbanions are extraordinarily good electron donors (238,239). They offer an additional advantage compared with neutral donors in that the redox partners of the radical-radical anion pair formed by donation from a carbanion to a neutral acceptor are not electrostatically attracted. In a transfer between an excited carbanion and a neutral acceptor, eq. 77,... 

The existence of these dielectrically different phases can also cause concentration gradients within the micelle so that effective concentrations of reagents substantially different from the bulk concentrations can be attained, Scheme 5. This concentration effect can be utilized to control bimolecular quenching events, especially when short-lived intermediates are involved. In an anionic micelle, for example, a neutral donor may dissolve in the hydrophobic core, while the electron deficient acceptor (either as a neutral molecule or as a cation) may associate with the surface head... 

Pronounced tendency of tetravalent actinides, especially of Np(IV) and Pu(IV) to undergo hydrolysis and polymerization and their instability of oxidation state in low acidic media, make it necessary to carry out their extraction studies from fairly high aqueous acidic media. Depending on the aqueous media and the neutral donor used, several species, involving the anion present in the aqueous media, can participate in the extraction. This, therefore, calls for a careful interpretation of extraction data. 

Different types of migratory insertions are exemplified by reactions 10, 11, and 12 in the Table II. An evaluation scheme is given in Matrix 6. In the present study we consider this reaction according to its general mechanism, shown in Eq. (6), where L is a neutral ligand (olefin, carbon monoxide, carbene, etc.) which becomes X (anionic c-donor ligand). X is also an anionic a-donor ligand. 

The phenomenon was explained by Mulliken1 in 1952 [1] these 1 1 "charge-transfer" solution complexes have ground-state wavefunctions ij/G and excited-state wavefunctions i// which are (a) linear combinations of the unperturbed wavefunctions (j)o of the neutral donor D and the neutral acceptor A and (b) linear combinations of the ionic state wavefunctions 4>ct of the donor cation D+ and the acceptor anion A-, as follows ... 

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