Fajan's Rules and Electronegativity

[Ashu2912]

Hey friends, I am having some confusion regarding covalent character of ionic bonds (Fajan's rules) and polar character of covalent bond (electronegativity and dipole moments). We apply Fajan's rules when we start a bond to be purely ionic, as in NaCl, and derive it's partial covalent character. While applying electronegativity and dipole moments, we start with purely covalent bonds, as in HCl and explain their polarity. However, I come across certain compounds such as BF3 or AlCl3 and get confused about what to apply, Fajan's rules or electronegativity. My book says that these compounds are covalent (sp2 and sp2 hybridized respectively), so we apply 'polar character of covalent bonds'. However, at the first look these compounds seem to be ionic as B(3+) has a duplet in outermost shell and 3 F atoms gain one electron each to form 3 F(-1), forming an ionic bond. So, therefore my basic question is:
(1) What to do in case of componds which seem to be ionic?
(2) Are the Fajan's rules applicable only while comparing the covalent character of two bonds considered to be purely ionic?
(3) If we consider BF3 and AlF3 to be purely covalent, how can we say that BF3 is more covalent than AlF3 (a result of Fajan's rule : The smaller the cation the more covalent is the bond)?
There's a lot of confusion... Please help me out...

 

[gerrardz]

Chemistry is to explain the behaviour of material based on experimental findings. Hence those rules eg Fajan rule and electronegativity are just only general rules try to clarify. Hence if we are given a substance without prior knowledge of it, there is a high risk predicting its nature by using those fundamental rules. Hopefully i have given a satisfactory reply.

 

[Ashu2912]

Ya, well, i too believe that science is built by the aim of explaining things, and can predict only when the fact is universal...
However, for example, i have found a compound BF3 or AlCl3. From the first look, I personally feel that these are ionic, due to their charge configuration. Then, I find that they are primarily covalent in nature. So, that's the entire dilemma in my mind...
Now, if I start with covalent, I'll have to apply the dipole moment and electronegativity concept...
My post "Can hybridization be applied to ionic compounds is related to that". Now, if I find a compound MgCl2, it can be ionic due to the charge configuration. However, I can also say that it is sp2 hybridized (which I now know I can't)...
Guess Fajan's Rules and dipole moments are only used to predict the relative covalent character of ionic molecules and polarity of covalent bonds respectively...