[Brian Lin]

Hi!

I have a question about the trends between high oxidative numbers and covalent character. In bonds where the transition metal has a high oxidative number like 7+ or 8+, the bonds are supposed to be covalent, instead of ionic. Lower oxidative numbers tend to be ionic.

However, I am not sure about the reason of these trends. Is it because high oxidative numbers have smaller atomic radii, resulting in shorter bonds, and hence, a covalent bond?

For smaller oxidative charge, the radii is larger, resulting in farther bonds and ionic character. However, what does not make sense is that the change in oxidative numbers in the transition metals result in slight change in atomic radii, so radii can't be a significant contributor to change in bond character (Am I wrong about this?)

What is the cause to this change in bonding character from low oxidation number to oxidation number?

[mjc123]

What else changes with oxidation number, in an ionic model?

[Brian Lin]

Zeff charge increases so Pauling electrongetivity difference is smaller? Is that a correct explanation then?

[mjc123]

No need to get so complicated. Charge. M⁺, M²⁺, M³⁺ etc. Suppose MnO₄^- consisted of a bare Mn⁷⁺ ion surrounded by four O^2- ions. What would you expect to happen?

[Brian Lin]

Mn 7+ and the O atoms will not dissociate of course since they are bonded together by covalent bonds. But even if you consider charge, it seems as if the bond is ionic (I know that it's not) because a high charge seems as if A LOT of electrons are donated to the O atoms by Mn.

Im not sure how charge can affect it, I think I am missing something else about the charge.

[mjc123]

Mn 7+ and the O atoms will not dissociate of course since they are bonded together by covalent bonds

No they aren't. I'm asking you what would happen in the ionic case - Mn⁷⁺ surrounded by four O^2-, not covalently bonded. What would happen to the outer electrons of the O^2- ions?