Pi bonds strength

The pi bond strength in the case of ethene (and most other olefins) is 64 kcal/mol and is determined by several methods. We will discuss some of the most common and evaluate their prospects for being meaningful in Group 13-Group 15 pi bonds. The conceptually simplest is to twist the molecule so that the 2p orbitals no longer overlap. This has the advantage that the number obtained... 

First, the rotational barriers for the monoamides are close to those obtained previously by other groups. McKee (15) and Allen and Fink (16) computed the B-N pi bond to be 32 and 38 kcal/mol, respectively. Fink et al. (17) computed the pi bond strength to be 11.2 kcal/mol for Al-N and 12.8 for Ga-N. Davy and Jaffrey obtained 10.7 kcal/mol for Al-N earlier (18). These values are quite close, although it should be noted that Fink et al. used the energy difference between the planar and perpendicular forms, which exaggerates the difference between Al-N and Ga-N. This also explains why the previously published value of 38 kcal/mol for B-N is higher than the value from the rotational barrier. 

If one interprets heats of addition to be measures of pi bond strengths, then the trends shown below are also explicable, in part, in terms of the concept of sigma-pi hybridization. 

We have just seen that, in making a transition from ethylene to perfluoro-ethylene, the increased ionicity of the sigma bonds causes a decreased ionicity of the pi bond with accompanying decrease of pi bond strength. In other words, sigma... 

In summary, transifion-metal-catalyzed alkene-polymerization reactions highlight the metal-induced electrophilic activation of C—C n bonds to form carbo-cation-like alkene complexes. Considerations involving substituent pi-donor or pi-acceptor strength (i.e., tendency toward carbocation formation) will be useful in similarly rationalizing polymerization reactions (4.105) for more general alkenes. 

Anions with larger bonding strengths are better able to bind so it is no surprise that there is a correlation between bonding strength and pi a, the equilibrium constant that measures the ability of an anion A to attach an ion in the reaction ... 

The pi bond between the metal and carbon atom arises as a result of a backbonding interaction between a filled metal rf-orbital of the appropriate symmetry and the empty pi star orbital of the carbonyl. Any factor that increases the electron density at the metal will increase the strength of the M C pi bond, and thus the contribution from resonance form b. The result is a... 

The work on cobalt compounds raised two questions which could best be answered by studying a smaller model compound. One was the role of dispersion forces in metal ligand binding. The other was the elusive role of pi back bonding which is supposed to account for a large fraction of the metal carbonyl bond strength. 

Dissociative adsorption of CO has been found on a variety of transition metal surfaces. Broden el al. (17) and Nieuwenhuys (14) correlated the tendency for CO, N2, and NO to dissociate with the position of the transition metal in the periodic table the tendency for dissociation increases the further to the left the metal appears in the table, and it decreases from 3d to 5d metals. Furthermore, the borderline for dissociative or molecular adsorption moves to the right in the sequence CO, N2, NO to O2, being the same order as the bond strength in the free molecules. There is sufficient evidence for the proposed correlation. For example, W and Mo surfaces dissociate CO easily al room temperature dissociative adsorption has not been reported lor Pi, Ir, and Pd(III) surfaces, and CO dissociation has been reported to occur on Ni, Co, and Ru at elevated temperatures. Ben-zinger (IS) suggested that the state of adsorption (molecular or dissociative)... 

The average bond strength of a triple bond is 200 kcal/mol (837 kJ/mol) therefore each pi bond is worth about 59 kcal/mol (247 kJ/mol). If placed in a ring, the triple bond can be bent from colinearity and become weaker and much more reactive. Cyclooctyne is the smallest ring that contains a triple bond and is still stable enough to be isolated. 

These interactions are synergicin increading the M-CO bond strength. In fact, the second interaction, as known as pi backbonding increases the available electron density on the CO. 

The strength of the pi bond in H2MEH2 ethene analogs is strongest when E =... 

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