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Higher the positive oxidation state of the central atom, more will be its polarising power which, in turn, increases the covalent character of bond formed between the central atom and the other atom.

This seems counter-intuitive to me. When the polarising power of the central atom increases, shouldn't the bond be more ionic instead of covalent? If not, can you tell me where I am wrong?

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I believe it is about perspective. Note that "covalent molecule with ionic character" and "ionic compound with covalent character" are different.

If we are looking at a covalent molecule and assessing its ionic character, we cannot talk about polarising power, because in a covalent molecule, there would be no cation and anion to polarise in the first place. Thus, we would go by the rule: the bigger the electronegativity difference, the higher the ionic character and therefore, the more polarised the bond.

If we are looking at a ionic compound and assessing its covalent character, then we will go by the rule: the more polarising the central atom, the higher the covalent character.

Now, from the way you phrased your question, I believe that you are conflating the concepts "big electronegativity difference" and "polarising power". They are distinct concepts and should be thought of separately.

Although bond polarity is associated with electronegativity difference, "polarising power" is about the polarisation of the electron cloud of the anion by the cation and not the polarisation of the bond. This is also why "polarising power" is a concept associated with ionic compounds with covalent character.

Hope this clarifies your doubt! If you are still unclear and require some examples for clearer understanding, please leave a comment on my post.

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Fajans' Rules help to explain the covalent character of ionic compounds. According to these rules, as charge on ion increases, the covalent character of the bond increases.
For example, between $\ce{NaF, MgF_2, AlF_3}$ and $\ce{SiF_4}$, order of covalent nature of bond would be as follows:
$\ce{NaF < MgF_2< AlF_3 < SiF_4}$

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Kindly note, we are talking about polarising power of 'cation'. This means there already exists an ionic bond and electron transfer has already taken place and the ions are formed. Now that cation again attracts the electron cloud of anion. So more polarising power means that the among ions of ionic bond, the cation is attracting electron cloud of anion and hence inducing a 'covalent character'.

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  • $\begingroup$ No one mentioned anything about a cation in the question or any of the answers, as far as I can see. $\endgroup$ – jonsca Jan 22 '18 at 21:08

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