Back-bonding model

The pi back-bonding model of Dewar (29), Chatt, and Duncanson (30) has been widely invoked as an explanation of a variety of features of transition metal complexes. While extended Huckel theory clearly shows such mixing of orbitals, ab initio calculations have found them more elusive (31,32). 

Figure lA shows a simphfied picture of the arrangement of MOs in 6C MNO hnear complexes. The d Jt— Jt No back-bonding model describes the hnearity in the MNO units and the multiple character of the Fe—NO and N—O bonds in terms of the orbital mixing of two fiaUy occupied (hc-bonding (dxz.dy ) and vacant antibonding orbitals, with a minor... 

Back-bonding has usually been discussed in terms of the orbital model (Chapter 3), and we will revisit it again in later chapters. For the moment we need only emphasize that since the apparently short bond lengths can be accounted for in terms of the polarity of the bonds. Bond lengths do not provide any compelling evidence for the concept of back-bonding. 

Experimental data from Bech Nielsen s study is shown in Fig. 6 and Fig. 7. The data show that implanted 2H is found predominantly in bond-center sites. This qualitative conclusion can be drawn immediately from the raw channeling data, especially the 111 planar scans, and does not depend on the details of the model used to subsequently analyze the data in greater detail. Si—Si bonds run perpendicularly across the 111 planar channel. At zero tilt, a strong flux peak of planar channeled ions is focused on the bond centered site and causes the peak seen in the data at this angle. However, back-bonded sites are hidden in the wall of this channel, which is unusually thick and consists of two planes of atoms close together. Thus, the ion flux near the back-bonded sites is low when the tilt angle is small, hence the dip in nuclear reaction yield calculated for this site. Bech Nielsen (1988) found that this data pointed to there being a minority of the 2H... 

The picture sketched above disagrees with the frequently used donation/back donation model of adsorbed CO. This model describes the chemisorption bond in terms of donation of electrons from the CO 5a orbital into empty orbitals at the surface of the metal, and back donation of d-electrons from the metal to the unoccupied 2n level of CO. The back donation is essentially correct, but the donation is not. 

Having generated suitable (partially) cationic, Lewis acidic metal centers, several factors need to be considered to understand the progress of the alkene polymerisation reaction the coordination of the monomer, and the role (if any) of the counteranion on catalyst activity and, possibly, on the stereoselectivity of monomer enchainment. Since in d° metal systems there is no back-bonding, the formation of alkene complexes relies entirely on the rather weak donor properties of these ligands. In catalytic systems complexes of the type [L2M(R) (alkene)] cannot be detected and constitute structures more closely related to the transition state rather than intermediates or resting states. Information about metal-alkene interactions, bond distances and energetics comes from model studies and a combination of spectroscopic and kinetic techniques. 

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