Back-bonding model complexes

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... 

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. 

The first stable actinide complex of carbon monoxide was prepared by reaction of (Me3SiC5H4)3U (Cp sU) with CO in solution or in the solid state to produce Cp 3U(CO), which reversibly dissociates CO. Cp 3U(CO) exhibits a carbonyl stretch in the IR spectrum at 1976 cm, or approximately 170 cm lower than the uco for free carbon monoxide. This is taken as an indication of uranium-to-carbonyl jr-back bonding see Back Bonding). Electronic stracture calculations on the model complex CpsUCO indicated a significant U 5f-CO 2n... 

This assumes the sign of A to be positive. This is so for the model described but is not necessarily so for complexes where ligand-to-metal back bonding might occur [e.g., in hexa-cyanide complexes). Such situations have not yet been investigated by neutron diffraction. 

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