Rr-acceptor properties

In order to distinguish between kinetic and thermodynamic phenomena it is convenient to refer to the former as the 7tr/ i-effect and the latter as the tra/u-influence or static /ra/u-effect". though this nomenclature is by no means universally accepted. However, it appears that to account satisfactorily for the kinetic /rau.s-effect , both it (kinetic) and a (thermodynamic) effects must be invoked to greater or les.ser extents. Thus, for ligands which are low in the Trans series (e.g. halides), the order can be explained on the basis of a u effect whereas for ligands which arc high in the series the order is best interpreted on the basis of a jt effect. Even so, the relatively high position of H , which can have no rr-acceptor properties, seems to be a result of a a mechanism or some other interaction. 

Rhenium complexes containing the metal in the oxidation state +III are comparatively numerous. This may be ascribed to the fact that the d configuration of the rhenium center can readily be stabilized by ligands with pronounced donor and rr-acceptor properties. Most of the rhenium(III) compounds are stable against hydrolysis rendering them suitable for nuclear medical applications. 

Bis(t-butylisocyanide)tetraphenylchlorinatoiron(III) (chlorin = dihydroporphyrin) is a low-spin iron(III) complex with the unusual ground state (d z, dy dxy). This deviation from normal behavior, viz (d z, dy dxyf, is ascribed to the strong rr-acceptor properties of the axial Bu NC ligands." ... 

The fraM.s -dichloro and dithiocyanate complexes show MLCT transitions in the entire visible and near-IR region. The lowest-energy MLCT transition band of the /razi.s-di chloro complexes is around 700 nm in DMF solution and they show weak and broad emission signals with onset at above 950 nm. The absorption and emission maxima of the tran -dithiocyanate complexes are blue-shifted compared to its tran.s-dichloro analogs due to the strong rr acceptor property of the NCS compared to the CI , which is consistent with the elctrochemical data of these complexes. 

From i CO) spectra of [M(CO)5 E=C(Aryl)H ] (M = Cr, W E = S, Se) it follows that the heteroaldehyde ligand acts essentially as a coordination mode. The acceptor properties and thus v(CO) spectra are similar to those of [M(CO)s(thioether)] and [M(CO)s (phosphine)] complexes. In the 7/2-bonding mode the heteroaldehyde ligands are strong rr acceptors and their IR spectra resemble those of, e.g., alkyne and olefin complexes. Because [W(CO)5 Se = C(Aryl)H ] complexes are present in solution as rapidly interconverting mixtures of the 771 isomers and the if isomer their IR spectra are composed of both types of i CO) spectra. The intensity ratio of the v(CO) absorptions depends on the temperature due to the temperature dependence of the 17V172 equilibrium. 

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]. 

Modification of the acceptor properties of the metal atom may be achieved by using complexes containing rr-cyclopentadienyl ligands. 

In his papers, Marder et al., have considered the relation between bond-legth alternation (BLA) and non-linear optical properties of rr-conjugated systems [22, 33, 68, 69, 70, 71, 72, 73, 79, 80]. As it was shown, the NLO properties strongly depend on the BLA parameter. One can distinguish three forms of donor-rr-acceptor system neutral, polymethine-like and zwitterionic. The variations of 3 and y for different forms can be substantial. It is well know, that solvent molecules generate... 

Phosphines are cr-donor and w-acceptor ligands (see Section 20.4) and related to them are arsines (ASR3), stibines (SbR3) and phosphites (P(OR)3). The extent of cr-donation and TT-acceptance depends on the substituents, e.g. PR3 (R = alkyl) is a poor w-acceptor, whereas PF3 is a poor cr-donor and as strong a rr-acceptor as CO. The rr-accepting properties of some PR3 ligands follow the ordering ... 

Therefore, rr-acceptor versus (T-donor properties of CS and CSe change more drastically than those of CO. In thiocarbonyl complexes additional bonding contribution comes from In orbitals. 

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