7r-back-donation

Two possible reasons may be noted by which just the coordinatively insufficient ions of the low oxidation state are necessary to provide the catalytic activity in olefin polymerization. First, the formation of the transition metal-carbon bond in the case of one-component catalysts seems to be realized through the oxidative addition of olefin to the transition metal ion that should possess the ability for a concurrent increase of degree of oxidation and coordination number (177). Second, a strong enough interaction of the monomer with the propagation center resulting in monomer activation is possible by 7r-back-donation of electrons into the antibonding orbitals of olefin that may take place only with the participation of low-valency ions of the transition metal in the formation of intermediate 71-complexes. 

IR spectroscopy of adsorbed carbon monoxide has been used extensively to characterize the diluted, reduced Cr/silica system [48-54,60,76,77]. CO is an excellent probe molecule for Cr(ll) sites because its interaction is normally rather strong. The interaction of CO with a transition metal ion can be separated into electrostatic, covalent a-dative, and 7r-back donation contributions. The first two cause a blue shift of the vco (with respect to that of the molecule in the gas phase, 2143 cm ), while the last causes a red shift [83-89]. From a measurement of the vco of a given Cr(II) carbonyl complex, information is thus obtained on the nature of the Cr(II)- CO bond. 

The 7r-back donation stabilizes the alkene-metal 7c-bonding and therefore this is the reason why alkene complexes of the low-valent early transition metals so far isolated did not catalyze any polymerization. Some of them catalyze the oligomerization of olefins via metallocyclic mechanism [25,30,37-39]. For example, a zirconium-alkyl complex, CpZrn(CH2CH3)(7/4-butadiene)(dmpe) (dmpe = l,2-bis(dimethylphosphino)ethane) (24), catalyzed the selective dimerization of ethylene to 1-butene (Scheme I) [37, 38]. 

Bader population analysis has shown (90JA1707) that the complex (115) is primarily formed through the tr-donation from the nonbonding cr-orbital of N2 to the low-lying vacant cr -orbital of borabenzene supplemented by the 7r-back-donation from the bt ir-orbital of borabenzene to the vacant 7r -orbital of N2. 

Comparison of the orbital populations with the idealized ionic states (Table 10.8) shows reasonable agreement with the 5Eg state. The depopulation of the dxzyz orbitals and an excess population of dxi y2 relative to the ion is as expected from the c-donation, 7r-back-donation concept applied to a high spin complex. 

As earlier reported for Ru (577), [Os(NH3)5(dmso)]2+ preferentially binds the ligand via the S atom (67,120,200), an action that allows for considerable 7r back-donation. Upon oxidation of this species (0.36 V, NHE) a linkage isomerization ensues with a specific rate of =s0.1 sec-1, in which the sulfoxide ligand shifts from sulfur to oxygen coordination. The reduction potential of [Os(NH3)5(0-dmso)]3+ (-0.90 V, NHE) is dramatically shifted due both to stabilization of Os(III) and to destabilization of Os(II), as compared to sulfur coordination. The cycle is completed by an 0 — S isomerization on Os(II) at a specific rate of >100 sec-1. 

It is evident that the electron density on the olefin and the resultant changes in bond length, nuclear shielding, and vibrational modes arc related to the ionization potential of the metal atoms. It is not clear whether this effect is caused by inhibition of the a forward donation or enhancement of the 7r back donation as the ionization potential is lowered. When electronegative substituents are placed on the olefin to activate it, there is also the question of the extent to which the backbonded electron density remains localized on the double bond or is delocalized to the substituent groups. Any means of probing the electron density on the metal or olefin would be useful in ascertaining the relative importance of the [Pg.44]

Figure 15. Resonance effect in heteroanalogs of cyclopropabenzene involving 7r-back donation provided by lone pair(s), where X= NH and O.

The situation changes, however, if the electronegativity of the group X increases drastically as in the series X = NH2, OH, F. 7r-Back donation is stepwise decreased to zero and 71-complexes are formed. This has been demonstrated via the electron density analy-sis,which reveals that convex bent bonds change into concave (inwardly curved) bent... 

In the synthetic iron porphyrin, O2 affinity mainly depends on the strengths of the <7-donation from the lone pair of O2 to the heme-iron dz orbital and 7r-back donation from the d/r orbital on the iron to the Jt orbital of 2. To evaluate the O2 affinity and/or O2 binding dynamics in myoglobin and hemoglobin, O2 - protein interaction is a further important factor. For example, the O2 dissociation rate constant for oxymyoglobin is relatively smaller than those of O2 complexes... 

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