D-n backbonding

Regardless of the system there is little electron density removed from the metal because the ligands do not exhibit strong d n backbonding. The molecular orbital diagram (117, 170a, 207, 302) for these materials is [Pg.90]

A well-developed series of complexes with rich MLCT excited-state behaviour are Re(I)-diimine complexes. [Re(bpy)(CO)3Cl] was the first transition metal complex used as a catalyst for CO2 reduction to CO, proposed by Lehn and Ziessel [41]. This series of complexes is particularly amenable to study of the excited state by time-resolved infrared spectroscopy. Formation of the MLCT Re bpy excited state leads to a reduction of the electron density on the metal centre. Consequently, d n backbonding from Re ti-orbitals to the antibonding n orbitals of CO ligands is reduced, resulting in an increase of the energy of the stretching vibrations, v(CO), by several tens of wavenumbers in the excited state if... 

In this section the preparation and uses of heteroatom-substituted carbene complexes L M=C(Xn)R,2 n) (n = 1, 2 X NRj, OR, SR) will be discussed. In these complexes the electron deficit at the carbene carbon atom is compensated both by electron-donation from the lone pairs on the heteroatom and by d-electron backbonding from the metal (Figure 2.1). 

Several effects can influence the electronic structure of Cjq upon metal complex formation. One is the removal of one double bond from the remaining 29 fullerene double bonds. As in any polyene system, this decreased conjugation is expected to raise the energy of the LUMO and therefore decreases the electron affinity of the system. Conversely, the d-orbital backbonding transfers electron density from the metal into n orbitals of the remaining double bonds, which also decreases the electron affinity. 

It is important to emphasize that the n backbonding is of course only possible if the transition metal has available d electrons in the complex, i.e. if the NNBE is not... 

Group (5) Acids that are a-acceptors but capable of n-donation in backbonding. This group includes cations with mobile d electrons e.g. Cu +, Co +, Fe"-". 

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