Electron density donation

The ligands are referred to as ir acceptors because of their receiving electron density donated from the metal to 7rg orbitals. Back donation results in increasing the bond order between the metal and ligand, so it results in additional bonding. 

In the second case, the phosphorus atom in the phosphine also has empty d orbitals that can accept electron density donated from the Pt2+. In fact, it is more effective in this regard than is the sulfur atom in SCN-. This results in the more stable bonding to SCN- being to the nitrogen atom when PR3 is in the trans position. In essence, the presence of tt bonding ligands in trans positions that compete for back donation leads to a complex of lower stability. As will be discussed in Chapter 20, this phenomenon (known as the tram effect) has a profound effect on the rates of substitution reactions in such complexes. 

Very cold. Immersing a clean iron surface into liquid nitrogen at 77 K (—196 °C) yields a weak physisorptive bond. The N=N molecule is probably aligned parallel to the metal surface, with electron density donating from the centroid of the triple bond directly to iron atoms on the surface of the metal via a van der Waals type of interaction. The experimental value of A //fadsj is small at about 1.5 kJ mol-1. 

The CO trans to Br is held more tightly than the other four because Br does not compete effectively with CO for n bonding electron density donated from Mn. The other four CO groups, which are all good n acceptors, cause the CO groups trans to each other to be replaced more easily. 

CO acts as both a a-donor (via the lone pair of electrons on carbon) and a 7i-acceptor ligand in transition metal complexes. CO is usually depicted as having a triple bond (one a- and two ti-) between the C and the O as well as lone pairs on both the C and the O. The lone pair on C is used for donation into a suitable metal centred o-orbital. However, the strongest M-CO bonds are formed (in simple terms) when some of the electron density donated by the carbon to the metal is directed back from a filled metal (i-orbital of the correct symmetry into an antibonding ti of the CO. Thus the M-CO bond has two parts, the forward (C M) donation, and the (M C) back donation (Figure 1). 

This dj —p delocalization more than compensates for the electron density donated to the metal (o — rfo bond) by the lone pair on one of the N-atoms or the C-atom of CO, respectively. This delocalization is called back donation or back bonding . An orbital representation for the bonding in end-on complexes of N2 is given in Figure 3-2. This description is supported by MO calculations performed by the groups of Hoffmann and Fukui (Hoffmann et al., 1977 Yamabe et al., 1980). 

This is shown on the right-hand side of Figure 2.2. The interactions and orbitals that result from the metal acting as a base are indicated by lines of lighter shade. The phenomenon of electron density donation by the metal is often referred to as back donation. 

The reaction of NO3 with phenolic compounds is believed to proceed via an overall H-atom abstraction mechanism preceded by electrophilic addition of NO3 to the aromatic ring (Calvert et al., 2002). The presence of the electron density donating CH3 groups has an important effect on the reactivity. For example, the increase in the reactivity observed for the methylated 1,2-dihydroxybenzenes compared to nonmethylated 1,2-dihydroxybenzenes has been attributed to the positive inductive effect of the methyl group (e.g., Calvert et al., 2002 Olariu et al., 2002 Thuner et al., 2004a). 

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