Fluorine effect bond length

The influence of fluorine on the physical, chemical and biological properties of molecules is described. A discussion of the effect of fluorination on bond lengths and bond strengths is presented in the course of an analysis of the steric demand of fluorinated substituents. These steric effects as well as electronic interactions are illustrated in a discussion of fluorinated vitamin D3 and prostaglandin and thromboxane analogs. 

The effect of successive fluorination of hydrocarbons on bond lengths has been extensively discussed in a recent review article which summarizes the most important experimental trends316). These trends are nicely accounted for by the LCFC approach. 

The weakness of the covalent bond in dilithium is understandable in terms of the low effective nuclear charge, which allows the 2s orbital to be very diffuse. The addition of an electron to the lithium atom is exothermic only to the extent of 59.8 kJ mol-1, which indicates the weakness of the attraction for the extra electron. By comparison, the exother-micity of electron attachment to the fluorine atom is 333 kJ mol-1. The diffuseness of the 2s orbital of lithium is indicated by the large bond length (267 pm) in the dilithium molecule. The metal exists in the form of a body-centred cubic lattice in which the radius of the lithium atoms is 152 pm again a very high value, indicative of the low cohesiveness of the metallic structure. The metallic lattice is preferred to the diatomic molecule as the more stable state of lithium. 

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