71-Back donation

The filled molecular orbitals beyond the crg and au that arise from combining the 2s atomic functions are (tcJ2 (-,J2 (.-r,]2. However, there is an empty antibonding orbital (designated as 7r or zr, ) on CN that can be shown as 

Because the electron density is flowing from the metal onto the ligands, this donation is known as back donation. It is in the reverse direction to that in the normal donation of electrons in forming coordinate bonds. The term back bonding is sometimes used instead of back donation, but it is not as descriptive because the ligands are functioning as acceptors of electron density from the metal. The essential feature of electron donation is that there must be an acceptor, which in this case is the ligand. 

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

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. 

Comparison of the C-O stretching frequencies for a series of metal carbonyl complexes can reveal interesting trends. The complexes listed below all obey the 18-electron rule, but with different numbers of CO ligands attached, the metal atoms do not have the same increase in electron density on them because the coordination numbers are different. 

In previous sections, CN- has been used as an example of a ligand that forces electron pairing (a strong field ligand) in complexes of metal ions containing four or more electrons in the d orbitals. What characteristics of CN- make it able to force such pairing when Cl- 

The back donation of electron density from the metal to the CN- results in a weakening of the C-N bond, but it gives additional strength to the Fe-C bond. In fact, the C-N bond is lengthened and weakened, but the Fe-C bond is shortened and strengthened. The partial back donation can be shown in terms of the resonance structures (only one ligand is shown)  

Although some double bonding between Fe3+ and CN- occurs, the two structures do not contribute equally, and the actual structure more closely resembles the structure on the left. Also, the back donation is spread among all six CN- ligands. 

A one-electron function used in the expansion of the molecular orbital function. The basis functions are commonly represented by atomic orbitals centered on each atom of the molecule. 

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Figure A3.10.23 Schematic diagram of molecular CO chemisorption on a metal surface. The model is based on a donor-acceptor scheme where the CO 5 a FIOMO donates charge to surface unoccupied states and the surface back-donates charge to the CO 2 71 LUMO [58].

The rate of the Nf -catalysed Diels-Alder reaction is barely sensitive to the presence of ligands. Apparently no significant effect due to -back donation is observed, in contrast to the effect of aromatic diamines on the metal-ion catalysed decarboxylation reaction of oxaloacetate (see Section 3.1.1). 

Figure Schematic representation of the two components of the ij -Hi-metal bond (a) donation from the filled (hatched) CT-H2 bonding orbital into a vacant hybrid orbital on M (b) jr-back donation from a filled d orbital (or hybrid) on M into the vacant a antibonding orbital of Hj.

In many of their complexes PF3 and PPI13 (for example) resemble CO (p. 926) and this at one time encouraged the belief that their bonding capabilities were influenced not only by the factors (p. 198) which affect the stability of the a P M interaction which uses the lone-pair of elecU"ons on p and a vacant orbital on M, but also by the possibility of synergic n back-donation from a nonbonding d , pair of electrons on the metal into a vacant 3d , orbital on P. It is, however, not clear to what extent, if any, the a and n bonds reinforce each other, and more recent descriptions are based on an MO approach which uses all (cr and n) orbitals of appropriate symmeU"y on both the phosphine and the metal-containing moiety. To the extent that a and n bonding effects on the stability of metal-phosphorus bonds can be isolated from each otlier and from steric factors (see below) the accepted sequence of effects is as follows ... 

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... 

DET calculations on the hyperfine coupling constants of ethyl imidazole as a model for histidine support experimental results that the preferred histidine radical is formed by OH addition at the C5 position [00JPC(A)9144]. The reaction mechanism of compound I formation in heme peroxidases has been investigated at the B3-LYP level [99JA10178]. The reaction starts with a proton transfer from the peroxide to the distal histidine and a subsequent proton back donation from the histidine to the second oxygen of the peroxide (Scheme 8). 

The stability of carbonyls of a metal in the +3 oxidation state is unusual because d7r-p7r back donation is necessary to stabilize the Ir-C bond, generally only one CO group can be bonded to the electron-deficient Ir3+. 

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. 

CS, N2, NO+, CN-, NC-, HCCH, CCH2, CH2, CF2, H2), which showed that the metal—>ligand back-donation correlates well with the change of the M-COtrans bond length, while the ligand metal donation does not [88]. 

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