Push-pull effects

Why is the complex OsHCl(CO)(P Pr3)2 stable, when it is unsaturated It has been argued that lone pairs on the alpha atom of a ligand M—X (M is a transition metal) can have a major influence on reactivity and structure. If M has empty orbitals of appropriate symmetry, X M tt donation creates an M—X multiple bond, with consequent transfer of electron density to M decreasing its Lewis acidity.23 The presence of a carbonyl ligand in OsHCl(CO)(P Pr3)2) increases the n-donor capacity of chloro by means of the push-pull effect making this molecule not a truly 16-valence electron species. 

IV. Strained Ethylenes without Push-Pull Effect. 160... 

However, given a sufficiently strong steric effect, a permanent twist can be induced in a carbon-carbon double bond even without a push-pull effect. This is a field that has been the subject of much interest, as exemplified by the intense but still unsuccessful search for tetra-/m-butylethylene, and by the still very active studies of trans-cyclooctenes. Besides the synthetic challenge, such compounds present interesting chiroptical and other physical properties, and a knowledge of their heats of formation presents crucial tests for current force fields. 

IV. STRAINED ETHYLENES WITHOUT PUSH-PULL EFFECT... 

The second chapter, by Jan Sandstrom, deals with stereochemical features of push-pull ethylenes. The focus is on rotational barriers, which span a large range of values. The ease of twisting is partly a matter of electron delocalization and partly a matter of steric and solvent effects. Electronic structure and such related items as dipole moments and photoelectron spectra for these systems are discussed. The chapter also deals with the structure and chiroptical properties of twisted ethylenes that do not have push-pull effects, such as frans-cyclooctene. 

A strong jr-donor therefore stabilizes the tram coordination of 7T-acceptors by a push-pull effect illustrated in transmission path F. Further evidence for such push-pull effects is found in the static or dynamic tram effects observed with metallopor-phyrins that carry dinitrogen, nitrosonium ion, or dioxygen. Table 11 displays (inter alia) some properties of dinitrogen or nitrosylosmium porphyrins. 

In Sect. 5.2, the strong 7r-donor ability of methoxide and fluoride has been elaborated. These two ligands effect a push-pull effect on the nitrosonium ion bound in Os(OEP)NO(OMe) [31c] and Os(OEP)NO(F) [3Id], as indicated by the low NO-stretching frequencies of the NO ion as compared with the dinitrosyl Os(OEP)(NO)2 [31e] and the perchlorato complex, 0s(0EP)N0(0C103) ([3If], Table 11). Thus, the a/7r-donor balance for the coordinated anions decreases in the series OMe > F > NO > OClOf. [31c] and [3Id] can be vaporized at 200°C/10-6 Torr in a mass spectrometer, while the dinitrosyl [31e] decomposes above 100 °C. This demonstrates the push-pull effect also in a chemical sense. 

The Os—O bond is reinforced by the push-pull effect in which the strong jr-acceptor, NO , participates. Thus, the trans effect obeys different transmission paths in [25c] and [57c]. Path C (Fig. 1) applies for the former, path F for the latter. 

When the effective on-site Coulomb repulsive energy (Geff) of the solid composed of Tt-radical molecules is smaller than the bandwidth (W), then the solid becomes a half-filled metal provided that the molecules stack uniformly without dimerization and can be described by a band picture. So far, no such radical molecules have been prepared. In order to decrease Ues and stabilize radical molecules chemically, a push-pull effect and an extension of the re-system have been implemented, though a large U ff and high reactivity (polymerization) are stiU crucial for the metallic transport. Table 2 summarizes selected organic conductors of neutral 7t-radical molecules. 

In the NMR spectra, a push-pull effect of octahydropyrido[2,3- pyrimidine 28 was observed in the olefinic C-4a and G-8a due to the electronic behavior of their substituents. Thus, C a appears at low 6 values (88-93 ppm) and C-8a gives peaks at lower field (146.0-148.5 ppm) <1996JHC45>. 

Proton NMR studies of 2-thiopheneamines bearing only an electron-withdrawing substituent in the ortAo-position, that is, the ester 261, indicate the existence of the imino form <2001M279, 2006T11311, 2006MI139>. Due to the push-pull effect, the imino form 262 can be generated (Scheme 24). 

Intermediate V thus formed would allow for the transfer of the dimethylamino group to the alkyne by virtue of a push-pull effect of coordination of the triple bond with the catalytic proton and the electron releasing effect of the two ether oxygens. The discoverers of this reaction assumed that it proceeds by way of carbonium ions (VI and VII) which subsequently eliminate or add ethanol to give III and IV, respectively. 

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