Push-pull bonding

A simple view of the mode of bonding of the carbon monoxide molecule to the surfaces of the transition metals and in their carbonyl complexes is well known and is thoroughly described in the literature.4 These metals have unfilled d-orbitals (or holes in their d-band), and the molecule in the linear form is held by a push-pull bond in which charge is transferred from the 5a orbital of the molecule into the metal s d-band, while there is back-donation of charge from the top of the d-band into the molecule s vacant... 

Compare the geometry of para-nitroaniline to those of both aniline and nitrobenzene. Is there any evidence for push-pull resonance contributors Is there shortening of bonds to the amino and nitro groups Are the bonds in the ring localized Is the dipole moment for para-nitroaniline smaller, larger or about the same as the sum of the dipole moments for aniline and nitrobenzene What does your result say about the importance of push-pull resonance contributors ... 

The mechanistic interpretation of the acid-catalyzed ring opening reaction of thiirane oxides125 is based on the push-pull mechanism162 with a transition state in which the bonded hydrogen atom plays a major role (see equation 59). 

Conversely, there are compounds in which nearly free rotation is possible around what are formally C=C double bonds. These compounds, called push-pull or captodative ethylenes, have two electron-withdrawing groups on one carbon and two electron-donating groups on the other (66). The contribution of di-ionic... 

The HIV-1 protease, like other retroviral proteases, is a homodimeric aspartyl protease (see Fig. 1). The active site is formed at the dimer interface, with the two aspartic acids located at the base of the active site. The enzymatic mechanism is thought to be a classic acid-base catalysis involving a water molecule and what is called a push-pull mechanism. The water molecule is thought to transfer a proton to the dyad of the carboxyl groups of the aspartic acids, and then a proton from the dyad is transferred to the peptide bond that is being cleaved. In this mechanism, a tetrahedral intermediate transiently exists, which is nonconvalent and which is mimicked in most of the currently used FDA approved inhibitors. 

Fig. 4. Push-pull substituted diphosphanes, bonding parameters in Angstroms and degrees...

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. 

The STRAIN AND DISTORTION model for catalysis involves pushing, pulling, or twisting a bond that is to be made or broken during the reaction. Parts of the substrate not involved directly in the chemical reaction are required to hold the substrate on the enzyme in the distorted form. The distortion and strain make it easier to reach the transition state. 

Ros-Lis JV, Martmez-Manez R, Sancenon F et al (2007) Signalling mechanisms in anion-responsive push-pull chromophores the hydrogen-bonding, deprotonation and anion exchange chemistry of functionalized azo dyes. Eur J Org Chem 2449-2458... 

Low torsional barriers in combination with strong steric interactions between donor and acceptor groups in push-pull ethylenes have in several cases been demonstrated to cause permanently twisted double bonds, in which a planar arrangement of substituents at the double bond may represent an energy maximum. 

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. 

The stereochemistry of the interesting group of compounds known as push-pull dienes has not been much studied, although a fair number are available. The earliest studies stem from the interest in the influence of the number of intervening double bonds in vinylogous amides (76) on the barrier to C—N rotation. It was... 

The change in potential energy accompanying torsion about die C,=C2 bond in a push-pull ethylene has an important component which depends on the overlap between the p2 orbitals on C] and C2, and which we may call E , which has maxima at torsion angles of 90° and 270°. In many of the molecules discussed... 

The separation of formal charges in a polar limiting structure like 2b creates a dipole moment of ca. 20 D. Therefore, if such structures were of great importance, quite high dipole moments should be expected for push-pull ethylenes. Data for a reasonable number of mostly symmetrical and rather rigid compounds are known (Table 20). Several high dipole moments are observed, though not in the vicinity of those required for a complete transfer of the double-bond it... 

To date, the (phosphino)(silyl)- and (phosphino)(phosphonio)carbenes are the only stable carbenes that feature a 77-donor and a 77-type-withdrawing substituent. They have a singlet ground state with a planar environment at phosphorus and a short phosphorus-carbon bond distance. However, because of the reluctance of phosphorus to keep this planar geometry, these push-pull carbenes behave in a manner very close to that of most of the transient carbenes. 

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. 

The concept of captodative substitution implies the simultaneous action of a captor (acceptor) and a donor substituent on a molecule. Furthermore, in the definition of Viehe et al. (1979), which was given for free radicals, both substituents are bonded to the same or to two vinylogous carbon atoms, i.e. 1,1- and 1,3-substitution, and so forth is considered. One might, however, also include 1,2-, 1,4-,. .. disubstitution, a situation which is more often referred to as push-pull substitution. Before discussing captodative substituent effects it might be helpful to analyse the terms capto and dative in more detail. 

The cleavage of the 0-0 bond of the hydroperoxide is promoted by the push-pull mechanism shown in Fig. 4, in which the native HRP reacts with the unionized form of the hydroperoxide. Thus, the latter is converted into a much better nucleophile upon transfer of its proton to the distal basic group (His 42). 

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