Stability of the Carbon-Metal Bond

It is desirable to examine in greater detail the reasons for the thermodynamic instability (small dissociation energy) of the alkyl carbon-transition metal a bond, which appears to be so much less than the carbon-metal a bond of the nontransition metals. The reasons for the instability are (a) the very small covalent energy of the metal-carbon bond and (6) the relatively small difference in electronegativities between the trairsition metal and the carbon atom, which accounts for the small ionic resonance energy contribution to the total energy of the bond. 

One of the early efforts to evaluate quantitatively the bond dissociation energy of particular bonds in a compound was the work initiated by Mulliken (-3) in his so-called Magic Formula. Although this formula contains five terms, the two most important for the evaluation of a bond dissociation energy, Dq (uncorrected for zero-point vibrational energy), between two atoms i and j, are the covalent bond energy, Xjj, and the ionic resonance energy, IRE. The evaluation of Ay takes the form  

The importance of the other major contributor to the bond energy, the IRE, can be illustrated in Pauling s (4) original terms. He showed that the reaction 

The energies are usually expressed as electron volts. The IRE for the bond in ethane is zero and for CHgNa it is 2.56 ev. The stability of alkyl carbon-metal bonds for a variety of metals has been evaluated by Jaffe and Doak (5). They point out that not only is the (the measure of covalent energy) for the C—M bonds of transition metals appreciably smaller (perhaps one-half) than the corresponding values for other elements, but the ionic resonance energy of the alkyl-transition metal bonds is also appreciably smaller (perhaps one-third) than that of alkyl-alkali or alkyl-alkaline earth metal bonds. 

It is well known that, although CH3Co(CO)4 is unstable, the acylcobalt 

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