Stability of Compounds Containing le Ligands

The transition metal homoleptic complexes containing alkyl ligands are kinetically and thermally less stable than analogous compounds of the main group metals. For some time, it was believed that this is a result of the diminished strength of the carbon-transition metal bond. However, thermodynamic studies showed that the M —C(sp ) bond energy for the transition metals and the main group metals is the same (Table 4.2). 

In order to calculate Z)(M —L) values, experimentally determined energies of dissociation of alkanes are used. Instead of Z (M —L) values, equivalent (M —L) values are sometimes utilized (Table 4.3) which are calculated from the assumed constant value for (C —H) modified by introducing corrections for steric and electronic effects. 

In order to determine the energies of dissociation of the metal-ligand bond, the following methods are commonly utilized calorimetric combustion, solution calorimetry, ion cyclotron resonance, mass spectrometry, ion beam technique, equilibrium constant measurements, kinetic methods, and photochemical determination of the threshold wavelength of the photolytic dissociation of the metal-ligand bond.  

The energy of the M — C bond of homoleptic transition metal compounds increases as the atomic number of the element in the group becomes greater. In the case of main group complexes, this trend is opposite. The stability of the M—C bond also increases 

The knowledge of enthalpies of dissociations for M —H, M —R, M —C(0)R, and Y —C(0)R (Y = H, R) bonds allows the estimation of enthalpies of various organome-tallic reactions (Table 4.4). This table shows that the following reactions are thermodynamically favorable oxidative addition of hydrogen, CO insertion into the M —R bond, olefin insertion into the M —H bond, and olefin insertion into the M —R bond. Oxidative addition of aldehydes to the transition metal complexes is possible. However, due to the near-zero value of the enthalpy of this reaction, oxidative addition of aldehydes does not occur easily in the above-mentioned processes. Thermodynamically unfavorable are oxidative addition of the unstrained C —C bonds, oxidative addition of C — H bonds, and CO insertion into the M — H bonds. 

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