Rr-Donor ability

Axial Ligands with Combined a- and rr-Donor Ability.94... 

There are also some studies concerning the kinetics of the displacement of amine and halide ligands by the R2S ligands of Pd11, Pt11 and AuIU complexes.3 The results again suggest that the coordination properties of the thioether are dominated by its rr-donor ability. 

Only a few examples have been reported of cycloaddition reactions of azodienophiles and linear (T/iO-dienes bearing an allylic stereogenic center. The diastereoselectivity in these reactions is dependent on several factors (1) the nature of the dienophilc, i.e., A -phenylmaleimide, benzo-quinone, and 4-phenyl-3//-l,2,4-triazole-3,5(4f/)-dione (2) the nature of the allylic substituent, i.e., the steric properties and/or the rr-donor ability (3) the electronic properties of the substituent at C-l of the diene, as electron-donating groups in particular can affect the mechanism and the transition state of the reaction40 42,50. 

A review of the complexes of 1,5-disubstituted tetrazoles, which covers the earlier literature and contains tables of comparative physical constants for the various complexes, has been published.10 Recent studies have been concerned mainly with determining the nature of the bonding and the structure in tetrazole complexes and with assessing the rr-donor ability of the tetrazole ring. 

In ligands such as CO that can interact with metal atoms in several ways, the number of electrons counted is usually based on o- donation. For example, although CO is a tt acceptor and (weak) tt donor, its electron-donating count of 2 is based on its cr donor ability alone. However, the rr-acceptor and rr-donor abilities of ligands have significant effects on the degree to which the 18-electron rule is likely to be obeyed. Linear and cyclic organic pi systems interact with metals in more complicated ways, discussed later in this chapter. 

X-ray single crystal structure study of the complex 2 revealed that the C—C bond of the ethylene ligand in 2 (1.366 A) was much shorter than those in analogous ethylene complexes of Ni(0) and Pt(0).t The shormess of the ethylene carbon-carton bond in 2 accords with the relatively poor rr-donor ability of the d ° palladium species. The weaker rr-donation from the palladium center in 2 results in the weaker Pd-ethylene bond. Indeed, exchange between the coordinating and free ethylenes was observed by NMR. In the NMR spectrum of 2, signals due to the pairs of diastereotopic coordinated ethylene proton are detected in the absence of free ethylene. 

Phosphites, such as triisopropyl and triphenyl phosphite, are weaker electron donors than the corresponding phosphines, but they are used in some reactions because of their greater rr-accepting ability. The cyclic phosphite trimethylol-propane phosphite (TMPP) or 4-ethyl-2,6,7-trioxa-l-phosphabicyclo[2.2.2]oc-tane (8), which has a small cone angle and small steric hindrance, shows high catalytic activity in some reactions It is not commercially available, but can be prepared easily[27]. 

Axial Ligands with Combined o-Donor-rr-Acceptor Ability.97... 

Monocyclic azines are very weak rr-donors and behave mostly as n-donors on interaction with electrophiles. However, ir-donor character is significantly increased in their benzo-derivatives. For instance, acridine forms with chloranil a highly colored 1 1 molecular complex. Perimidine is one of the strongest heterocyclic ir-donors which gives deeply colored molecular complexes with a variety of organic electron acceptors. On the other hand, the rr-acceptor ability of perimidine is moderate. 

The TT-electron excess of the five-membered rings is accompanied by a high rr-donor character. The best measure of rr-donation is the value of first ionization potential, IP, which for all aromatic heterocycles with one heteroatom of pyrrole type reflects the energy of highest occupied rr-orbital. IP, values decrease in the sequence pyrrole > indole > carbazole furan > benzo[/ ]furan > dibenzofuran thiophene > benzo[/ ]thiophene (Section 2.3.3.9, Tables 21 and 23). Thus, the more extensive the rr-system, the stronger is its electron donor ability. Furan and thiophene possess almost equal rr-donation, which is considerably lower than that of pyrrole. 

A number of Group VIB pentacarbonyl compounds of the general type LM(CO)s, L = amine (173), pyridine (277), phosphine (173, 278), sulfide (278), or isonitrile (173), have been investigated in an effort to probe the relative o-donor or rr-acceptor abilities of the various ligands. 

The cyclooctadiene ligand is easily replaced by two molecules of CO. The vCO stretching frequencies of the carbonyl ligands can be used to estimate the electronic properties of the l,3,7,9-tetramethylxanthine-8-ylidene ligand. The electron donor ability is found to be less than for pyrimidine based carbenes [99] or imidazol-2-ylidenes [100]. It is also one of the strongest NHC rr-acceptor ligands known [98]. 

A physical property that reflects the electronic character of the carbonyl substituent is the NMR chemical shift. Although the relation of the chemical shift to electronic properties is complex, there is a correlation with the resonance electron-donating ability of the substituent. The rr-donor substituents cause large downfield shifts in the order CK F < OCH3 < NH2. Nonconjugating substituents, such as cyano, trifluoromethyl, and methyl, have much smaller effects (Table 3.23). 

The higher activity of Fe(II)(acac)2) DMF complexes as compared with Fe(II)(acac)2) HMPA complexes at the initiation (Wj) and propagation (Wp ) steps seems to be to rr-donor properties of DMF and its ability to form H-bonds [140]. The coordination of DMF may increase the probability of formation of the primary 0 complexes 02-Fe(ll)(acac)2)-DMF [141, 142] and, presumably, enhance the activity of the nascent superoxide complexes [DMF Fe(ll)(acac)2) O2 ] at the radical generation step w). As above the the schemes of radical-chain oxidation including intermediate formation [LM-OOR] peroxo complexes at the propagation step followed by the homolytic dissociation of the peroxo complexes ([LM-OOR]—>R C=0 (ROH) + R ) are a likely explanation of the observed increase in the rate and MPC selectivity of the Fe(ll)(acac)2)-DMF catalyzed ethylbenzene oxidation in the initial stages of the reaction 58%. It is quite like-... 

Taking into account the above described and other similar empirical rules, as well as equations (3.1) and (3.2), one can design and prepare new rr-donors, for instance by binary, ternary etc. combinations of (1)— (60), with the required polarity, solubility, rr-donating ability, etc. 

Hydroxide attack on complexes of the type cis-[(NO)L(bpy)2Ru ] also yields the corresponding nitro complex [118]. The equilibrium constants for these reactions are a strong function of the cis-ligand and increased by 10 on changing L from a jr-donor (Cl ) to a rr-acceptor (py) [98]. The ability of the nitro complexes to transfer an oxygen atom to a reductant such as triphenyl-phosphine is also a function of the cis-ligand [98]. 

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