Ionic-covalent resonance electronegativity

Strengthening and hence shortening of the B—X bonds by ionic-covalent resonance, especially for B—F and B—O bonds because of the large electronegativity differences. Evidence that this is important, in addition... 

Whether a carbon-metal bond is ionic or polar-covalent is determined chiefly by the electronegativity of the metal and the structure of the organic part of the molecule. Ionic bonds become more likely as the negative charge on the metalbearing carbon is decreased by resonance or field effects. Thus the sodium salt of acetoacetic ester has a more ionic carbon-sodium bond than methylsodium. 

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. 

Resonance with the ionic structures and A B+ also occurs for unlike atoms as well as for like atoms, and is, indeed, of great importance in case that the atoms A and B differ greatly in electronegativity, the contribution of the favored ionic structure then being large. This aspect of the covalent bond is treated in Chapter 3. 

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