Back-donation of electrons

No new pnnciples ate involved in describing the bonding in these complexes and appropriate combinations of ihe 4p ottoilals on the diene system can be used lo construct MOs with the metal-based orbitals for donation and back donation of electron density.As with ethene, two limiting cases can be envisaged wbicb can be represented schematically as in Fig. 19.25. Consistent with... 

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. 

The data given in Table 13.3 show that the extent of bond shortening is greatest for B-F bonds. This is to be expected because back donation of electron density from F to B is more effective when the donor and acceptor atoms are of comparable size. The following resonance structures are used to represent the multiple bonding between B and F ... 

I FIGURE 16.9 Back donation of electron density from metal d orbitals to ligand tv orbitals. 

Due to its importance in many industrial processes, the prototypical reaction of CO binding to metal surfaces has received much attention. Using Hiickel molecular orbital theory, Blyholder showed that CO bonding at top sites consists of the donation of electrons from the filled CO 5a HOMO to the metal d 2 orbitals with a back-donation of electrons from the metal dxz caddy orbitals to the CO 2n LUMO. Consequently,... 

Models of CO adsorption show that top site binding is governed by the CO HOMO (5cr orbital) donating electrons into the metal unoccupied states, with simultaneous back-donation of electrons from the metal s occupied dxz and dyz states into the CO LUMO 2tt orbital). Therefore, it follows that the standard chemisorption model, which considers shifts in the total d-band center, can be inaccurate for systems in which individual molecular orbitals, involved in bonding with the adsorbate, shift differently due to external interactions. In particular, we have shown that the formation of hybrid orbitals with the support material can lead both to downward shifts in the metal d-band center, which do not affect the adsorption of molecules to the metal surface, and to upward shifts that are vitally important. 

According to Dewar (41) the metal-to-olefin bond in such complexes consists in part of the overlap of the 7T-electron density of the olefin with a cr-type acceptor orbital of the metal atom and in part of the back-donation of electrons from filled metal or other d-n-pn hybrid oribitals into the antibonding orbitals on the carbon atoms. 

Similar high rate enhancements on coordination are observed with the analogous Rh(NH3) +- and Ru(NH3)j - but not with Ru(NH3)5 -coordinated nitriles. This is an important finding. With ruthenium(II) complexes considerable metal-ligand rc-bonding occurs. This results in back donation of electron density from the metal center to the C = N bond. The polarization by the metal of the nitrile, which is the basis for the enhanced effects with Co(lll), Rh(lll) and Ru(III), is therefore lost with Ru(II). ... 

An understanding of the mechanism [10] for rhodium-mediated intramolecular C-H insertion begins with the recognition that these a-diazo carbonyl derivatives can also be seen as stabilized ylides, such as 15 (Scheme 16.4). The catalytic rhodium(II) car-boxylate 16 is Lewis acidic, with vacant coordination sites at the apical positions, as shown. The first step in the mechanism, carbene transfer from the diazo ester to the rhodium, begins with complexation of the electron density at the diazo carbon with an open rhodium coordination site, to give 17. Back-donation of electron density from the proximal rhodium to the carbene carbon, with concomitant loss of N2, then gives the intermediate rhodium carbene complex 18. 

Many metals bind to the sulfur of sulfhydryl groups in proteins. Metals that bind sulfur in preference to oxygen not only form strong cr bonds with the readily polarizable ligands, but also tt bonds by back-donation of electrons from metal drr to ligand dir orpir orbitals. The electronegativity... 

Palladium, platinum and iridium complexes of thiirene 1,1-dioxides (10) show upfield shifts of about 5 p.p.m. for vinyl protons and 1 p.p.m. for methyl protons, the shifts being attributed to back-donation of electrons by the metal and its associated ligands <73JOM(57)403>. 

The obvious decrease in the number of electron-acceptor sites with palladium deposition on silica-alumina strongly suggests an interaction between the metal and these sites. Turkevich (28) first demonstrated that palladium behaves like an electron-donor toward tetracyanoethylene we suppose that it can be the same toward an electron-acceptor site of a solid support. In that hypothesis, palladium should have a partial positive charge on the second class of supports. This is actually observed by the adsorption of CO. This adsorbate can be considered as a detector of the electronic state of palladium. The shift toward higher frequencies of the CO band reflects a decrease in the back donation of electrons from palladium to CO. Thus, palladium on silica-alumina or HY is electron-deficient compared with the silica- or magnesia-supported metal. Moreover, the shift of CO vibration frequency is roughly parallel to the increase of activity thus, these two phenomena are connected. We propose that the high activity of palladium on acidic oxides is related to its partial electron deficiency. 

INDO calculations on these complexes257 gave a net stabilization of the planar geometry over the tetrahedral one. The nickel-dioxygen bond was formed primarily through the donation and back-donation of electrons using the nu and Jt oxygen orbitals parallel to the molecular plane. 

The nature of the Niu—P bond in pseudotetrahedral phosphine complexes has received much attention. With the AOM Gerloch et al. showed that phosphine bonds are characterized by a large o basicity and large it acidity corresponding to a back-donation of electrons from the nickel to the phosphorus atom in both mono and bis phosphine complexes.549,550... 

With the exception of the Ru1 complex, a variety of metal ions activate the hydration reaction to a very consistent extent (106-108-fold). The relative constancy of the rate acceleration and the anomaly of the Ru11 complex are consistent with the view that it is the Lewis acidity of the metal ion which is essential for activation. In the Ru11 complex, considerable metal h-ligand -bonding is expected, resulting in back donation of electron density from the metal centre into the C=N bond. This view is supported by measurements of the C=N stretching frequency of free nitriles and of their complexes with Ru11 and the other metal ions.317... 

In their metal complexes, bonding of either species to the metal atom is via a ligand - > metal a donor bond and a metal - >ligand n bond, enabling back donation of electron density to the n orbitals of the C-C multiple bond system to take place. Vinylidene is one of the best 7t-acceptors known, and is exceeded only by S02 and CS in this respect the relationship between phenylvinylidene and other common ligands has been determined (18) from the CO force constants exhibited by a series of Mn(CO)2(q-C5H5) complexes, which increase in this order ... 

In principle, the effects of oxygen substituents are consistent with CF2 being a relatively stable carbene despite the corresponding carbocation being quite unstable. This is understandable if the dipolar structure produced by resonance interaction in the carbene (52) is compensated by an inductive back donation of electrons in the cr-bonds. In the cation (53), back donation accentuates rather than compensates charge separation arising from resonance... 

The position of the equilibrium between imine and carbonyl may be perturbed by interaction with a metal ion. We saw in Chapter 2 how back-donation of electrons from suitable orbitals of a metal ion may stabilise an imine by occupancy of the jc level. It is possible to form very simple imines which cannot usually be obtained as the free ligands by conducting the condensation of amine and carbonyl compounds in the presence of a metal ion. Reactions which result in the formation of imines are considered in this chapter even in cases where there is no evidence for prior co-ordination of the amine nucleophile to a metal centre. Although low yields of the free ligand may be obtained from the metal-free reaction, the ease of isolation of the metal complex, combined with the higher yields, make the metal-directed procedure the method of choice in many cases. An example is presented in Fig. 5-47. In the absence of a metal ion, only low yields of the diimine are obtained from the reaction of diacetyl with methylamine. When the reaction is conducted in the presence of iron(n) salts, the iron(n) complex of the diimine (5.23) is obtained in good yield. 

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