Coordination compounds back donation

The transfer of charge from the metal to the ligand caused by back-donation can also be seen from a comparison of the ionization potentials of chromium in different complexes. The ionization potential of complex compounds is higher than that of the uncomplexed coordination center (6.76 eV), e.g. for dibenzenechromium ) 7.07 eV, and for hexacarbonyl-chromium 8.03 eV 43). It may be expected that a decrease in the net positive charge at the coordination center will give rise to an increase in ti-EPD properties. Hence the Fe-C distance will be shorter in [Fe(CO) 4] than in Fe(CO) 5. Likewise iron is more strongly coordinated in ferrocene than in the ferrocinium ion. 

In many cases the M—S bond has been found to be normal in length, i.e. the bond distances agree fairly well with the sum of the covalent radii (Pauling s scale, data appropriate to coordination number and oxidation state). However, in some complexes the M—S distances are significantly shorter than the calculated ones, for example in thioether compounds with Cr°, Pd11, Pt, Cu1 and Au1. This observation has often led to the suggestion that the shortening is due to some Jt-back-donation from the metal to the sulfur. 

The reactions of olefins and related unsaturated compounds play key roles in many of the reactions described in this book. Such reactions proceed via metal-olefin complexes, the bonding in which is related to that in the carbonyls here the forward donation is from the filled olefin C-C 71-orbital, while the back donation is from metal d-orbitals of the correct symmetry into the empty 71 -orbital of the olefin (Figure 2). The geometric constraints mean that ethylene for example lies perpendicular to the coordination plane of, and binds v - to, the metal. The classic example of this is found in Zeise s anion, Pt(C2Fl4)Cl3 . 

The dipole moments of a variety of coordination compounds show that the bond dipole moments of the M-L bonds of most a-donor ligands are about 4 D, with the donor atom positive. In contrast, metal carbonyls show an M-C bond moment which is essentially zero because the M L back donation compensates for L M direct donation plus the enhanced polarization of CO on binding. Formation of the M-CO bond weakens the C-O bond relative to free CO because a n orbital on CO is now partly filled by back donation. This will still lead to a stable complex as long as the energy gained from the bond exceeds the loss in C-O. Bond weakening within L on binding to a metal is a common feature in many M L systems. 

In the context of alternative substrates for nitrogenases, in particular alkynes, [Equation (4.27)], model compounds are of interest in which the alkyne is coordinated side-on to vanadium. The side-on or rr coordination implies a weakening of the triple bond by TT-back-donation from vanadium-d into the tt orbitals of the ligand, and hence an activation. The activated siloxyacetylene in the complex [ClVdmpe)2 i7 -(Me3SiOC= COSiMe3)] [dmpe = bis(dimethylphosphino)ethane] is reduced by hydrogen to the respective ethene, [Equation (4.39)]. The precursor acetylene complex is formed by... 

Mechanistic and kinetic studies on the activation of metal-alkyl bonds by entering olefins are carried out by Yamamoto, Yamamoto, and Ikeda 132>. For example, diethyldipyridylnickel is a quite stable complex up to 100 °C. However, the formation of butane by ligand coupling, even at low temperature, was observed when the complex was treated with olefinic compounds such as acrylonitrile. Blue shift in a spectrum was observed when acrylonitrile was added to the diethyl complex. This change was explained by back donation from the nickel to the olefins, and it was concluded that the stronger the coordination of the entering olefins, the more the alkyl-metal is activated. 

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