Electronegativity bond shortening

A well recognized effect, often referred to as the fluorine effect, is related to bond shortening upon progressive substitution of one center by electronegative groups such as fluorine316). The LCFC approach leads to a satisfactory understanding of this trend. 

Another factor that affects bond length is electronegativity. Bonds lend to be shortened, relative to the expectations for nonpolar bonds, in proportion to the electronegativity difference of the component atoms. Thus the experimental bond length in HF is 91.8 pm versus an expected value of 108 pm. The quantitative shortening of bonds because of electronegativity differences and multiple bonding in elements other than carbon will be discussed in Chapter 8. 

Similar trends can also be observed for X = AIH149, SiH232b-47-, 24, PH47-124 and S47-124 where, however, CC bond shortening and the increase in y are somewhat smaller because of the decrease in electronegativity when going up in the columns of the periodic table. 

In 1937 Brockway reported that C—F bond distances in the fluoromethanes and in the fluorochloromethanes were significantly shorter in those compounds where several fluorine atoms are attached to the same carbon atom than in the monofluorides. Since then many proposals have been presented to account for this progressive bond shortening with increasing fluorination. In terms of valence bond theory the effect was correlated with changes in hybridization and electronegativity. One of the earliest explanations was formulated as double bond-no bond resonance. ... 

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